The biology of some important primary, secondary and associated species of stored products coleoptera

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INTRODUCTION

The beetles comprise the largest natural order in the Animal Kingdom with a total of no less than 250,000 described species, of which more than 600 have been associated with stored food products throughout the world. Some, through the agency and dispersal by man in international trade have attained cosmopolitan distribution and constitute the major cereal pests that attack stored cereals and grain legumes.

Ever since man began storing food reserves in granaries during biblical times, various beetle species have become established pests and have continued till the present day as one of the most profound causes of grain loss in storage. Adults of the "cigarette beetle", "humpbacked spider beetle" and "weevils" were discovered in 1390 BC from the tombs of Tutankhamen, and no doubt the association of stored grain beetles with man extends even further back during the neolithic phase of human prehistory (earliest records are of flour beetles in an Egyptian tomb dating back to 2500 BC).

CHARACTERISTICS OF THE ORDER

Small to large insects, forewings modified to form horny wingcovers or elytra, which meet along a straight mid-dorsal line. The hind wings are membranous, folded away beneath the elytra, some times reduced or even absent in Sitophilus granarius. The mouthparts are adapted for biting and chewing. The prothorax is larger and more developed than the other thoracic segments extending dorsally as a single shield called the pronotum. Metamorphosis is complete, i.e. egg to larva to pupa to adult. The larvae, have well developed heads and mouthparts and are of three main types (a) campodeiform; elongate and active with well developed antennae and mouthparts, often carnivores e.g. Carabidae, Staphylinidae (b) scarabaeiform; crescentic, bulkier and less active, usually herbivores e.g. Anobiidae, Bruchidae and (c) apodous scarabaeiform; grub-like, with reduced antennae and mouthparts which live within the food source e.g. Curculionidae. The pupae of Coleoptera are mostly exarate but some are found within earthen cells and a few within cocoons (pharate) formed within the cast skin of the final larval instar. They are morphologically diverse and have invaded land, air and aquatic habitats with great success. Their hard cuticle is of particular value in surviving both biological and physical hardships as well as their ability to enter into facultative diapause under adverse conditions. Many are important to man as pests of growing crops, timber, skins and hides, textiles, furniture, structures and of stored foods and drugs. Some others, such as blister beetles (Meloidae) are directly injurious to mankind by their toxic secretions.

A diagrammatic representation of a typical beetle adult and the various larval forms is given in Figures 1 and 2.

The classification of stored products insects has been described elsewhere, especially with reference to primary and secondary grain pests. It has been established that Rhyzopertha dominica

Fig. 1. (A) Dorsal view of a beetle. (B) Ventral abdominal view of a Carabid beetle. Diagrammatic. After Hinton and Corbeth (1972)

Fig. 2. Three forms of insect larvae; (A) campodeiform; (B) scarabaeiform; (C) legless or apodous scarabaeiform; (D) cruciform. After Monro, J.W. "Pests of Stored Products."

(F) cannot penetrate sound paddy husks in the early larval stages, but gains access through the presence of minute cracks. Species such as R. dominica and Trogoderma granarium Everts can be considered intermediate between the two classifications, since only the late instars can penetrate undamaged grains (Evans, 1975). It also appears that both Sitophilus oryzae (L.) and Sitophilus zeamais Motschulsky are also not true primary pests of paddy due to the female's inability to penetrate sound husks, and since feeding is directly related to oviposition, egg-laying is inhibited unless some physical defect exists. However, these defects are apparently not uncommon an observation based simply on the frequency of occurrence of weevil infestations in paddy, especially after further husk damage has been inflicted by the feeding activity of the lesser grain borer or Sitotroga cerealella Oliver, the Angoumois grain moth.

Very little attention has been focussed on some secondary pests such as Lophocateres pusillus (Klug) and Cryptolestes spp (especially C. pusillus (Schon.) and C. ferrugineus (Stephens)), which are commonly found in moderate to large numbers on paddy in the humid tropics, sometimes even outnumbering the acknowledged primary pests. They are often assumed to cause little damage but it would seem apparent that such large and abundant populations maybe utilizing whole paddy, since the early instar larvae of these beetles would be certainly capable of gaining entry through defects necessary for the entrance of R. dominica.

Once husk and kernel damage has been initiated by the primary grain feeders, the extent of damage caused by these and other secondary pests of milled rice would be quite significant, especially in longterm storage.

The physical condition of a grain mass, its temperature and moisture content considerably affects its suitability as a habitat for a given species because each has its own particular temperature and humidity limits and optima. The activities of pests such as S. oryzae and R. dominica often change the temperature and moisture content of bulk grain causing heating and moisture migration and thereby initiating a succession of consumers within the storage ecosystem resulting in grain deterioration.

The major coleopteran families that we are concerned with in stored products entomology are listed in Table 1 (in alphabetical order).

MAJOR STORED PRODUCTS BEETLE FAMILIES

1. ANOBIIDAE

Adults are small, subcylindrical (wood boring) oval or nearly globular (general feeders) with the prothorax more or less covering the deflexed head. The stored products members of the family have 11-segmented antennae with a loose 3-segmented serrate club. The elytra completely cover the abdomen and there are 5 visible abdominal sternites. All tarsi are 5segmented with segments 1-4 decreasing in length.

A family which are damaging to wood i.e., Anobium puncatatum, the furniture beetle, and Xestobium rufillosum, the death watch beetle. The family is related to the Bostrychidae and the Ptinidae and although cosmopolitan, is best represented in temperate regions. The larvae are scarabaeiform, fleshy and have enlarged terminal segments; and can be disinguished from allied members of Ptinidae by possessing minute spinules on some of the folds of the thorax and abdomen. Two species Lasioderma serricorne and Stegobium paniceum are of importance as stored product pests.

(a) Lasioderma serricorne (F.). The tobacco or cigarette beetle.

Small (2-2.5 mm), oval, brown or dark brown beetle. Antennae serrate throughout; absence of line of punctures or striae on the elytra and lack of spinules on abdominal larval segments distinguish if from other anoblids infesting stored products.

The eggs are laid singly in crevices of folds in the substrate. The fleshy larvae, which are distinctly hairy, pass through four instars. Newly hatched larvae cannot attack undamaged grain but enter small holes in search of food thus gaining entry to packaged goods such as cigarettes. The larvae pupate, when mature, within a rather flimsy cell amongst the substrate of within a grain or bean. The adult, lives only about 25 days producing some 100 eggs at 30C and 70% R.H.

Table 1. Major coleopteran families.

FAMILY COMMON NAME

NUMBER OF DESCRIBED SPECIES

    TOTAL (WORLD) STORED PRODUCT
Anoblidae* Furniture, tobacco      
  and biscuit      
  beetles 1000 15 (2)
Anthicidae Incidental pests 1800 2  
Anthribidae Coffee bean borers 2400 1  
Bostrychidae* Wood borers 434   (3)
Bruchidae Bean weevils 1200   (11)
Carabidae Ground beetles      
  (predatory) 25000 (4)  
Cleridae Chequered beetles      
  (mainly predatory) 3400 10 (4)
Colydiidae Incidental pests:      
  biology unknown 1400 3  
Cryptophagidae Fungus beetles 800 25  
Cucujidae* Flat grain beetles 500 15 (4)
Curculionidae* "Weevils" 60000 30 (4)
Dermestidae* Hide and Leather      
  beetles 600 55 (13)
Histeridae Carrion feeders 2500   (1)
Languriidae Mexican grain beetle 400   (i)
Lathridiiae Plaster beetles 520 35 (7)
Mycetophagidae Fungus beetles 200 5 (2)
Nitidulidae Sap feeding beetles 2000 16 (4)
Ptinidae Spider beetles 500 24 (8)
Silvanidae* Secondary grain      
  feeders 400 15 (4)
Staphylinidae Rove or cocktail      
  beetles; Biology variable 27000    
Tenebrionidae Darkling beetles 15000 100 (14)
Trogossitidae Secondary grain      
  feeders (germ) 600   (2)
(Ostomatidae) Some predatory      

* Represents major stored products pests families. Numbers in parenthesis are the more commonly encountered species.

When humidity is not limiting the life-cycle can be completed in the range of 20C to 37.5C although the optimum is about 32.5C. High humidities retard development and increase mortality as do humidities lower than about 70% R.H.

By day, the adult beetle shuns light, hiding in crevices, but flies readily at dusk in warm conditions and is attracted to artificial light.

L. serricorne attacks an extremely wide range of stored products throughout the tropics and subtropics and is particularly damaging to tobacco, cigars, cigarettes, cocoa-beans, spices and various seeds such as coriander and caraway, cereals and cereal products.

(b) Stegobium paniceum (L.). The drugstore beetle

Small (2-3.5 mm), cylindrical, light brown beetle which resembles L. Serricome but whose last three anthennal segments are very long and elytra striated.

The life-cycle is similar to L. serricorne: the egg is followed by 4-6 larval instars, the last of which constructs a cocoon within which it pupates. After eclosion, the adult remains in the cocoon for several days before emerging and does not feed. At 20-25C and 60-80% R.H., each female lays about 75 eggs and has a life-span of about 40-90 days.

The life-cycle can be completed over 15-34C, development taking 40 days at about 30C and 60-90% R.H. At 20C development takes about 100 days at 90% R.H. and 130 days at 60% R.H.

The adult cannot fly and dispersal therefore depends on passive distribution during the movement of goods. S. paniceum is cold-hardy and can survive winter conditions in temperate regions.

S. paniceum is a cosmopolitan pest of more temperate than tropical climates infesting almost any dry animal or plant products, particularly in pharmacies where it eats drugs and in premises where it attacks breakfast foods, biscuits, etc.

2 ANTHRIBIDAE

(i) General characters

Adults infesting or associated with stored products resemble Bruchid bean weevils. The antennae are 11-segmented, thread like or filiform with a loose 3segmented club, rarely with a compact club. All tarsi are 5-segmented (the fourth segment being minute and more or less completely sealed in a dorsal groove (or apical emargination) of the third segment which is nearly always broadly dilated. Mostly of tropical distribution, associated generally with old wood dead branches and fungi.

(a) Araecerus fasctculatus (Degeer). The coffee bean weevil.

Small (2.5-4 mm), dark brown pubescence.

The eggs are laid within the endosperm. The larva is spodous scarabaeiform similar to Sitophilus. In maize of highs moisture content, the female lays about 80 eggs in a life-span of some 60 days.

The optimum range for development is 28-32C but the speed of development and mortality are much influenced by the moisture content of the grain. Thus, at 27C and 80% R.H. the life-cycle is completed in 43 days in maize and 66 days in cocoa. The lower limits for development are about 20C and 60% R.H.

A. fasciculatus, which is now more or less cosmopolitan, especially in the tropics where it attacks a wide range of stored commodities, such as coffee and cocoa beans, nutmegs, ginger, maize and dried fruit, particularly when such produce is not properly dried. It will also attack the fallen fruits of the custard apple, bananas and cotton balls.

It is a strong flier and is frequently seen in cornfields in Southern USA on exposed and damaged ears. It lays eggs in the soft kernels and continues to breed after the corn has been harvested and placed in storage. However, it is considered a minor pest since it does not cause significant damage once the corn becomes too hard to be attractive. In the humid tropics, it is considered a major pest of cassava, attacking whole tubers and chips while they are still being dried.

3. BOSTRYCHIDAE

(i) General characters

Prothorax coarsely tuberculate and concealing the deflexed head. Antennae terminate in a loose 3segmented club.

A cosmoplitan family of mainly wood-boring beetles which, particularly in the tropics and subtropics, attack felled timber or dry wood and, occasionally, unhealthy trees.

Besides Rhizopertha dominica, only Prostephanus truncatus (Horn), the larger grain-borer, which attacks corn, grain and tubers in the Southern states of USA, central and south America and Dinoderus minutus (F.), the bamboo borer, which attacks bamboo, grain, flour, tobacco and spices in the tropics and warm temperate areas are of any significance as pests of stored products.

P. truncatus previously regarded as having a restricted distribution to the Americas, has now been recorded in Africa, in the Tabora region of Tanzania and has now spread to Neighbouring

Tribolium castaneum (Herbst) The rust-red flour beatle

Rhyzopertha dominica (Fabricus) The lesser grain borer

Sitophilus oryzae (L.) The rice weevil

Acanthosselides obtectus (Say) Dried bean weevil

Cryptolestes pusillus (Schonherr) The flat grain beetle

Tenebroides mauritanicus (L.) The cadelle

Lasioderma sericorne (F.) The tobacco or cigarette beetle

Oryzaephilus surinamensis (L.) "The saw-toothed grain beatle"

Guinea, Togo, Kenya, Burundi, Benin and Ghana. The "greater grain borer" or "Scania beetle", is capable of boring through the husk sheath of maize, and losses reported in 3-6 months storage were 10% and sometimes as high as 30% in sample that had a moisture content of only 11% m.c.

Rhyzopertha dominica (F.). The lesser grainborer.

Small (2.5-3 mm) dark brown cylindrical beetle with head concealed by thorax which has many small tubercles. The elytra have clear rows of regular punctures.

The female lays her eggs in clusters on the grain in cracks and crevices or singly amongst the grain dust and frass produced by the adults. The newly hatched, white, cylindrical larva, may enter the grain directly or feed amongst the dust until it reaches the third of the four larval instar when it usually enters grain and eventually pupates. More than one larvae may be found within a single grain. Adults live for 2-3 months and females produce 200 to 400 eggs depending on conditions. Under optimum conditions of population density, temperature, moisture and presence of damaged and cracked grains, the adults can survive almost 5 months and produce approximately 600 eggs. The adults feeds voraciously, leaving characteristic holes with ragged edges in the grain, and produces floury dust and frass.

The life-cycle can be completed over 20-38C, although the optimum is between 32C and 35C. Development is completed in 25 days at 34C and 84 days at 22C (both at 70C R.H.) and ceases below 18C. Moisture levels have a considerable influence on the biology of the pest, where few eggs are laid in grain of less than 8% moisture content although, in contrast to grain weevils, oviposition is little inhibited until moisture content is less than 9%.

The adults are strong fliers and move freely from one store to another. Within a grain mass the species disperses rather slowly but is generally found in the driest part of the grain mass.

R. dominica is particularly well adapted to breeding in warm dry climates and is of particular importance in countries such as Australia and India. It is most important in wheat, rice, paddy, millet and of lesser importance in sorghum and is known from other cereals such as maize. Of little importance in milled products.

4. BRUCHIDAE

(a) General characters

Adults are small,- strongly convex beetles, densely clothed in hairs which are usually coloured and form distinctive patterns (generally variable and cannot be used for species differentiation). The last abdominal segment is large, more or less vertically inclined, and conspicuous beyond the shortened elytra. The eyes are large, and with exception of the genus Caryedon, the eyes are u-shaped or emarginate. The antennae are 11-segmented without a club, usually filiform with some segments compressed while in the males of some Callosobruchus, they are pectinate (comb-like) or flabellate (spoon-like). The tarsi are distinctly 4-segmented with the third segment broadly dilated and deeply emarginate. The hind femora are somewhat thickened or strongly enlarged and each bears one or more teeth. Bruchid larvae on hatching have three thoracic legs (a Chrysomeloid character) but after the first moult they become apodous (a Curculionoid character). Despite their common name, the bean weevils do not possess the elongate rostrum or snout of the true weevils.

Several species of bruchids are primary pests of stored beans, peas and other legumes.

Multivoltine species

(a) Acanthoscelides obtectus (Say), Dried bean weevil,

Adults are form 3.2 mm to 4.0 mm long and grey in colour with light and dark markings on the wing covers. The hind femur has one large and two small teeth which are of taxonomic importance in this genus.

This beetle is an important primary pest of pulses, especially of Phaseolus spp. beans. In all but the cool temperate regions of the world it attacks beans ripening in the field before harvest as well as in store. The larva develops entirely within a single grain, eventual,y forming a pupation chamber just under the seed coat, at which stage the infestation becomes evident as a characteristic circular 'window' on the surface of the grain. The adult which emerges from the chamber is short-lived and does not feed, and the female lays its its eggs rapidly; the eggs are laid freely among the grains, in contrast to other pest bruchids which cement their eggs to the seedcoat. A generation takes about 4 weeks at 30C and development is possible in temperatures down to 15C.

(b) Callosobruchus chinensis (L.), "Cowpea beetle".

Antennae of the males are pectinate to strongly pectinate, whlist on the female they are subserrate. The apical tooth on the inner carina of the hind femur (which are bicarinate in this genus) is long, narrow, parallel sided and blunt at the apex. Adults generally 2.5-3.5 mm in length with typical white markings on the scutellum.

This bruchid beetle is an important primary pest of pulses in tropical and subtropical climates, originating from the orient but now found in many other areas. Pulses are often infested in the field before harvest as well as in storage. The larva develops entirely within single grain, eventually forming a pupation chamber just under the seedcoat, at which stage the infestation becomes evident as a circular 'window' on the surface of the grain. The adult which emerges from the chamber (leaving a large round hole) is short-lived and does not feed, and the female lays its eggs rapidly; the eggs are cemented firmly to the outside of the grain and the larva hatches through the 'floor' of the egg straight into the grain. After the larva has hatched the eggcase is clearly visible as a small creamish-white spot on the surface of the infested grain. The other major pest species of Callosobruchus have a similar life-cycle and cause nearly identical damage. One generation is completed in 25 days at 30C, and females lay more than 100 eggs, usually in less than one week.

(c) Callosobruchus analis (F.)

C. analis differs from all other members of this genus by having a tooth on the inner carina of the hind femur which is relatively small and sometimes absent.

The eyes tend to be flattened and less prominent. Adults range in size from 2.5-3.5 mm. This bruchid is a primary pest of pulses in many parts of tropical Asia and has been reported on grain, soyabeans and groundnuts (probably incidental pest) from India, Burma, Hongkong, Indonesia but rarely E. Africa. Generally less common than C. chinensis, but causes identical damage. Optimum conditions for development are 30-32C and 70-90% R.H.

(d) Callosobruchus maculatus (F.), "Southern cowpea beetle"

The antennae of both sexes in this species are subserrate. The elytra are brown with two medio-lateral black spots (sometimes inconspicuous in males); apical tooth on inner carina of hind femur straight. Slightly larger varying from 3-4.5 mm. Adults live for 710 days only, the female lays about 80 eggs, and one generation takes 21 days (32.5C and 90% R.H.).

This bruchid beetle is an important primary pest of pulses in tropical and subtropical climates, probably originating from Africa but now nearly cosmopolitan in all warm regions of the world except India where it is not common. Pulses are often infested in the field before harvest as well as in store. Certain types of pulses, particularly cowpeas, are more susceptible to attack by this pest than are other pulses. The life-cycle is similar to that of C. chinensis, and the damage caused is identical.

(e) Callossobruchus rhodesianses (Pic), "Cowpea beetle".

Adults 2.5-3.5 mm in length; antennae of males serrate while on females sub-serrate. Colour of apical half of elytra black or nearly black in females, brown in males. Apical tooth on inner carina acute and curving towards apex.

This bruchid is a primary pest of pulses in central and southern Africa and has not been recorded outside this continent (before 1976). Pulses are often infested in the field before harvest as well as in pulses have dried. The life-cycle is similar to that of C. chinensis but is completed in 26-28 days at 30C and 70% R.H.

Zabrotes subfasciatus (Boheman).

This bruchid beetle is an important primary pest of several pulses but is particularly associated with the many varieties of common beans (Phaseolus vu/garis). It is particularly common in central and South America but is also found in several other tropical or sub-tropical regions (central Africa, Madagascar, the Mediterranean, and India). The pulses are often infested in the field before harvest as well as in store.

Smallish bruchids (2-2.5 mm long) with long, filiform antennae, black with basal segments reddish yellow. The dorsal surface of adults usually variegated dark and brown while the base of the pronotum has a large patch of whitish hairs also exist as a broad transverse patch on the elytra.

Caryedon serratus (= C. gonagra F.). "Groundnut borer".

The eyes of this species are large and not emarginate. The hind tibiae are strongly curved. Adults are larger (4-7 mm) and cosmopolitan in distribution.

This beetle is a primary pest of groundnuts in many parts of the tropics. It can be a serious pest, especially on groundnuts that are inshell (because of its size it cannot penetrate so easily between the grains of shelled groundnuts). Infestations normalty start in the groundnuts soon after harvest, often while they are being dried. The larvae spend most of their life within the pod but when they are fullygrown they leave the seeds and cut exit holes in the pods. Pupation occurs within a thin paper-like cocoon that is usually formed on the outside of the pod but occasionally protrudes through the exit hole. The adults generally live and lay eggs within the surface layers of a bulk of groundnuts: up to 25 cm into the bulk in groundnuts in shell, much less in shelled groundnuts.

5. CUCUJIDAE

(i) General characters

Small adults (1.5 to 2.5 mm) which are flattened, reddish-brown, Il-segmented filiform antennae which may display a thickening of apical segments in some species. The prothorax possesses an entire, sub-lateral ridge (carina) on both sides which are more or less parallel. Torsal segmentation are 5-segmented in females while in males the front and mid-tars) are 5segmented and the hind tarsi are 4-segmented. Some members are often predatory but in Cryptolestes, the genus of importance in stored products, predation and grain feeding both occur. They closely resemble each other in appearance, while determination of species is based on characteristics of the genitalia and antenna: body length ratios of the males. (C. turcicus are as long or longer than the body, C. pusillus are only 2/3 as long as the body while C. ferrugineus are only 1/2 as long as the body).

(1) Cryptolestes ferrugineus (Stephens). The flat or rusty grain beetle.

Very small (1.5 mm) reddish-brown flattened beetle with long filiform antennae.

The eggs are placed in crevices within the grain or dropped loosely. The elongate larva has pronounced tail-horns and feeds preferentially on the germ of the grain. It passes through four instars and pupates in a gelatinous cocoon which is usually covered in food particles. Cannibalism occurs under crowded conditions. After a pre-oviposition period of two or three days, the female lays approximately 400 eggs and lives for 6-9 months at 32C and 70% R.H.

The life-cycle can be completed over the range of 20C to 42.5C and the optimum for both development and population increase is about 35C, when development is completed in 21 days (about 100 days at 20C). Development is retarded and mortality increased at low humidities but the rate of population increase is still considerable at 40% R.H. This species is cold-tolerant and can overwinter without difficulty in very cold conditions.

The adult flies actively at temperatures above 21C and warm grain, i.e. above 30C is preferred.

By virtue of its wide tolerance of temperature and humidity, C. ferrugineus is a cosmoplitan and important secondary pest of cereals, cereal products and oilseeds in temperate, sub-tropical and tropical regions and is often associated with or follows infestations of primary pests.

(2) C. pusillus (Schonherr), The flat grain beetle.

The optimum conditions for increase are about 35C and 90% R.H., development being completed in about 27 days under such conditions. It is not tolerant of low temperatures or of low humidities less than 60% R.H.

A cosmopolitan pest attacking a similar range of commodities to C. ferrugineus but only abundant in the tropics. It is apparently unable to survive in sound uninjured grain, but is commonly associated with the more vigorous primary grain pests such as the rice weevil. It is also a scavenger, the larvae feeding on the remains of dead insects, and is most often associated with grain and meat that is in poor condition.

The larvae are particularly attracted to the wheat germ, and in infested grain, many kernels are found uninjured except for the removal of the germ.

(3) C. turcicus (Grouvelie)

Optimum conditions for increase are about 28C and R.H., when development takes about 38 days, but the life-cycle can be completed between 17C to 37C at 90% R.H. At lower humidities survival is reduced and approaches zero at 50% R.H. The species is cold tolerant and is most abundant in flour mills in temperate regions where it is often confused with C. ferrugineus which occupies the same niche in the sub-tropics and tropics.

(4) C. pusilloides (Steel & Howe).

Optimum conditions for increase are about 30C and 90% R.H., when development is completed is completed in about 23 days. Development is completed over 15-35C at 90% R.H. However, survival is reduced and approaches zero at 50% R.H. and is restricted to tropical and sub-tropical regions in the southern hemisphere where it attacks a similar range of products to C. ferrugineus.

6. CURCULIONIDAE

The Curculionidae or true weevils form the largest single family in the Animal Kingdom. Many species are pests of growing crops of many kinds throughout the world, and approximately 30 species have been recorded in stored food products, three of which are of extreme importance.

Adults may be distinguished from all other stored products species by having the head produced in front of the eyes to form a well defined snout, the antennae are geniculate (elbowed) and clubbed, and all tarsi are 4-segmented. The larvae are apodous scarabaeiform, stout and slightly curved, and are creamy white with a pale brown or yellowish head.

Three species of Sitophilus are important pests of whole cereals, however, they are of little significance as pests of milled cereals because the larvae require a hard substrate in which to develop. As primary pests, these weevils are of additional importance in that their entry into whole grains provides access for secondary pests such as Tribolium spp. The role of the immature stages in the formation of 'hotspots' and in moisturemigration is also noted.

(1) Sitophilus granarius (L.), The granary weevil.

Small (2-4 mm), polished, uniformly chestnut brown to black weevil. Thoracic punctures oval and rather shallow. Male distinguished by shorter and more deeply pitted rostrum than female.

The females bores a hole in a grain with her mandibles, lays her egg at the bottom of the hole which is then sealed with a gelatinous plug.

After four moults, the larva pupates within a single grain. If several eggs are laid within a single grain usually only one larva reaches the pupal stage due to cannibalism of the supernumerary eggs. Newly emerged adults spend some days within the grain before chewing their way out. Providing that temperature and moisture levels are favourable, the female lays about 250 (maximum 360) eggs during her life-span of about six months.

The life-cycle can be completed over the temperature range of 12-32C, the optimum lying between 26C and 30C. Development is completed in 26 days at 30C and 144 days at 15C (both at 70% R.H.). The moisture content of the grain greatly influences both oviposition and the development of the immature stages. Thus, eggs are seldom laid in grain of less 9% moisture content and are most numerous in grain of 14-16% moisture content.

The larvae produce much metabolic heat and moisture during their development where the heat generated may contribute to the formation of 'hotsports' with spot temperatures of 38-42C forcing the adults to migrate to cooler areas (generally towards the surface).

The adults have no functional wings under the elytra and their dispersal is therefore passive.

S. granaries is typically a pest in rather cool areas because the developmental threshold and temperature optima are lower than those of other Sitophilus spp. A wide range of cereals are attacked particularly wheat, oats, barley and rye.

(2) Sitophilus oryzae (L.), The rice weevil.

Small (2-3 mm), brown to black weevil with two paler reddishbrown patches on each elytra. Prothoracic punctures round and rather deep, except for a smooth narrow strip extending down the middle of the upper (dorsal) side. Punctures on the elytra are narrowly separate, as compared to S. granaries, which are widely separate.

Life-history and biology essentially as for S. granaries. However, the female is more fecund and lays about 380 eggs (maximum 576) during her lifespan of about five months. The life-cycle can be completed over the temperature range of 14-34C; the optimum for development and survival is 27-31 C i.e. somewhat higher than the granary weevil. Development takes 25 days at 29C and 220 days at 15C (both at 70% R.H.). Few eggs are laid in grain of less than 10.5% moisture content and mortality of the immature stages are increased in grain with less than about 13% moisture content.

The heat and moisture produced by the immature stages is of considerable importance in the development of 'hot-spots' and moisture migration.

The adults have functional wings and their dispersal depends on flight as well as passive dispersal in trade. In contrast to S. granarius, the rice weevil with its greater tolerance of temperature exceeding 30C, is typically a pest of warm temperate and tropical zones. It attacks wheat, barley, paddy, rice, millet, sorghum and maize amongst other grains but is of little importance in milled products.

(3) Sitophilus zeamais Motschulsky, The maize weeviI.

Small (3 mm) but somewhat larger than S. oryzae from which it cannot only be distinguished by examining the genitalia. Often confused with S. oryzae or referred to as S. oryzae 'large-strain'. Although generally darker than the rice weevil the variation that exists within each species makes separation on external characters alone rather inaccurate.

Life-history and biology essentially similar to S. oryzae but is about 1.5 times more fecund than S. oryzae with a life-span of five months. S. zeamais will oviposit in much moister grain, e.g. ripening maize, than S. oryzae and ovipostition is inhibited in grain of less 12.5% moisture content.

The life-cycle is completed in 37 days at 25C and 110 days at 18C (at 70% R.H.) which is somewhat longer than in S. oryzae.

S. zeamais is an active flier resulting in many field infestations in areas adjacent to infested storages.

S. zeamais occurs throughout the warm humid areas of the world and will attack a wide range of cereals although it is particularly a pest of maize. It will breed in rice and wheat but a true assessment of its pest status is confounded by the confusion that exists in early reports as regards to the separation of the species.

(4) Caulophilus latinasus (Say), The broadnosed weevil (= C. oryzae (Gyllenhal)).

Small (3 mm) dark-brown broad-nosed wevil. The life-cycle is essentially similar to the members of Sitophilus spp. The female lays an average 136 eggs over four months-the total life-span being about five months. Development is completed in 26 days at 28C and 76 days at 17C.

In warm weather, the adult may fly into maize fields where it attacks, damaged or exposed cods before the grains are hardened. C. Iatanasus breeds in stored maize, chick peas, millet and root crops such as sweet potatoes but is only of importance in the southern states of USA.

7. DERMESTIDAE

Adults are small to moderately large beetles densely covered with hairs or scales which are often conspicuously coloured. The head is small and somewhat deflexed and in some species (Anthreninae) bears a medium ocellus. The antennae are 5 to 11segmented, short with a distinct and often large club. The elytra completely cover the abdomen and there are five visible abdominal sternites. All tarsi are 5segmented.

Approximately 600 species have been described, and roughly 55 species have been reported as injurious to stored products. The larvae are general feeders and scavengers of dried organic matter of animal origin, particularly skins, furs, woollens and textiles, dried fish, dry carcasses and insect remains, while the adults are commonly found on flowers (necessary in the females of some species for egg maturation). Stored foodstuffs such as bacon and cheese are also infestible items to members of the genus, Dermestes, while Anthrenus spp., as well as Trogoderma versicolor Creutz., are troublesome as pests of insect collections in museums.

Certain species however, vary their diet by feeding on farinaceous materials, and may therefore be frequently found in flour mills, farm storage and central collection bulk storages, or any similar places where grain or grain products are being stored or processed. These include members of the genera Trogoderma, Anthrenus and Attagenus.

(1) Anthrenus flavipes (Le Conte), The "furniture carpet beetle".

Small (2-3.5 mm) oval and strongly convex beetle with characteristic but variable dark, yellow and white patterns on dorsal surface.

The active larvae, which vary in colour from white through yellow to dark red, are clothed in dark hairs and bristles and moult 6-12 times or more. The pupa is formed within the last larval skin and the young adult remains for up to two months before emerging. The female lays up to 100 eggs during a life-span of up to one year and the adults feed on pollen or nectar.

At temperatures between 25-26.7C the life-cycle is completed in 160-360 and in 70-80 days at 35C.

A. flavipes is a serious pest of woollen-fabrics, silks, furs, skins, leather and horn. It will also eat old grain, insect specimens, dried blood, glue and many other animal products or remains.

(2) Anthrenus and Anthrenocerus species.

A. verbasci (L.), the "varied carpet beetle" and A. museorum (L.), "the museum beetle", have similar biology and habits to A. flavipes but are temperate species. A. verbasci may be found as a scavenger in mills and is similar in appearance, biology and habits to A. scrophulariae (L.). Anthrenocerus australis (Hope) is indigenous to Australia which has been carried in trade to Britain where it attacks wollen goods, hides and food residues.

(3) Attagenus megatoma (F.), The black carpet beetle.

Small to medium (2.8-5.0 mm) beetle, head and thorax black, but the wing covers are black or dark reddish brown covered with short hairs. The larvae are characteristic and easily recognized by reddish or golden brown colour covered with short scale-like hairs with a tuft of long caudal hairs at the rear of the body.

The biology and life-history are similar to that of Anthrenus spp. Adults feed on pollen and nectar and produce 60-80 eggs over a lifespan of about one month. The number of larval instars and the speed of development depends greatly on temperature, humidity and diet. On fishmeal, the life-cycle is completed in 918 months at 26C but can extend for 3 years under cooler and less favourable conditions. The larvae shun light and tend to bore deeply into the food source.

A. megatoma is a cosmopolitan pest particularly in materials of high protein content such as bone and fish meals, grain residues, wool, hair, fur and dead insects. The borings of the pest frequently provide immediate access for more damaging pests.

The larvae are often found in abundance in cracks in the floors of warehouses where foodstuffs have accumulated.

(4) Dermestes maculates (Degeer), The hide or leather beetle.

Large (5.5-10 mm) black beetle with white markings on the side of the thorax and abdomen. Apices of elytra pointed.

At 23C and 70% R.H., there were 5-7 instars when the larvae were reared on fish-meal diet of 46% moisture content compared with 9-13 instars at 7.5% m.c. The larvae attack skins and hides from the flesh side burrowing actively and often cause much additional damage to timber structures when they seek pupation sites.

The optimum temperature range is 30-35C. At 90% R.H. development takes about 4 weeks at 32C and 14 weeks at 20C, but may sometimes extend over some years under field conditions. D. maculatus is cosmopolitan and serious pest of hides, skins, furs bones, horns and hooves and foodstuffs such as dried fish.

(5) Other Dermestes species.

D. ater may be found with D. maculatus and is more usually a scavenger on carrion and dead insects. D. Iardarius the "larder leetle" may be encountered as a domestic pest in temperate regious. D. frischli and D. carnivores have similar habits to D. maculatus.

(6) Trogoderma spp. complex.

Some of the members of the genus Trogoderma are highly important stored product pests.

The "Khapra bettle", Trogoderma granarium Everts., is one of the most damaging pests of stored products of agricultural origin throughout the world. Indigenous to the Indian sub-continent, Khapra beetle has become established in some Asian, Middle-East and African countries, being distributed mainly by shipping and international trade through the agency of man, since this beetle has limited mobility.

Species differentiation of Trogoderma; espeally larvae can be extremely difficult. For example T. variabile Ballion is difficult to recognize because of its variable appearance. T. glabrum (Herbest) and T. inclusum Le Conte are very similar and are often found in similar situations, the differences in the eyes and antennae can only be observed microscopically. All species of Trogoderma posses a median ocellus or simple eye on the forehead (of frons). Anthrenus species also posses a median ocellus, but differ from all other dermestids by possessing grooves in the prothorax in which the clubs of the antennae can be retracted which is characteristic of this genus.

(A) Trogoderma granarium Everts, "The khapra beetle".

i) Adults

Khapra beetles are small oblong oval insects appoximately 1.75 to 3.5 mm in length with the males tending to be slightly smaller than the females. The elytra are almost unicolorous, brown to brownish black with the body densely clothed in fine yellowish hairs and the eyes are evenly rounded.

The antennae club in T. granarium E. is 4-5 segmented in the males, while the female club is 3 segmented. In T. glabrum (H), the male club is 5-6 segmented and the females are 4 segmented.

Trogoderma species however, exhibit considerable variation in size, colouration and patterns, and therefore if confirmation of any diagnostic separation is required, morphology of the male or female genitalia must be used.

ii) Larvae

The separation of Trogoderma larvae based on morphology is also extremely difficult, and are often confused with one another or misidentified, due to broad variation in form. Members of Anthreninae (the sub-family to which Trogoderma and Anthrenus belongs) possess hastisetee, which are unique spearshaped hairs on the body, while native Trogoderma endemic to Australia, possess fiscasetae, which somewhat resemble a twig basket in shape. Anthrenus larvae have 3 short and dense tufts of spear-headed setae inserted on membranous areas behind the 5th to 7th tergites, coverging over the dorsum, while Trogoderma have spear-headed setae on the distinctly sclerotized parts of the tergites without obviously converging over the dorsum. Larvae of Attagenus spp. differ by having fine hairs extending posteriorly to form a caudal brush.

Larvae of Trogoderma are generally 6 mm long, and varies in colour from whitish-yellow, when young, to reddish-brown, when mature, and is clothed in transverse bands of hairs. The young larvae feeds mostly on grain debris or damaged grains but the large larva is able to bore into grains and therefore, is often regarded as a primary pest.

iii) Biology

Although adults possess functional wings they do not readily fly, and only live for a short period (5 to 10 days). During this period, adult females lay an average of 50 eggs loosely in the foodstuff but may lay over a hundred at temperatures exceeding 35C. On the other hand, larvae can live for several weeks, but are also capable of living for more than seven years, without food, and have been reported surviving in grain at near freezing temperatures. This ability to enter into a "facultative diapause" enabling them to survive unfavourable conditions, whether high or low temperatures, food or water shortages, is instrumental in their ability to "spontaneously reappear" after attempts to control them. During diapause, the larvae have also been reported as being able to rouse itself, search for better food and shelter, and then resume its diapause again.

The number of instars varies considerably according to the prevailing conditions and at 38C and 50% R.H., 4-5 instars are common but as many as 15 have been recorded under unfavourable conditions. Pupation occurs in the last larval skin, while the adult remains quiescent for some time before emerging, and requires neither food nor water to attain full fecundity and longivity.

The optimum temperature range is 33-37C; the lower and upper limits being about 22C and 43C. Development is completed in 25 days at 35C and 48 days at 25C. The most favourable humidity range is 45-75% R.H. but development can still be completed at only 2-3% R.H. However, as implied previously the developmental rate varies enormously according to prevailing conditions and consequently the life-cycle may extend over 2-3 years.

The habit of larvae seeking shelter often makes them difficult to detect, and they may burrow into materials that are not generally infested in search for pupation sites. They often damage sacking, weakening it and causing it to tear easily. Because of the long body hairs, they may be inadvertently carried on clothing or even adhere to rodent's fur, which aids in dispersal (a phenomenom coined phoresy).

Khapra beetle has been occasionally recorded in Great Britain, Ireland, Italy, Jamaica, Japan, Mexico, North Western Europe, Philippines and Thailand (up until 1979).

Wheat, barley, rice and spices seem to be the most attractive commodities, but the Khapra beetle also feeds on almonds, walnuts, spaghetti and egg noodles, tapioca, dried beans, powdered skim milk, oilseed cake, fishmeal, and to a lesser extent, dried fruits. The developing larvae are the damaging trophic phase, and causes damage which is essentially similar to Rhyzopertha dominica Fabricus, the "lesser grain borer"

(B) Trogoderma inclusum (LeConte), The larger cabinet beetle.

This species is somewhat larger than the khapra beetle and, as an adult, may be recognized by the notch in the inner margin of the eye. T. inclusum may be encountered in grain and cereal products but is seldom regarded as a pest. However, it and several other Trogoderma species may be confused with the khapra beetle. For conclusive identification, consultation with an expert taxonomist familiar with the genus is imperative.

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