The use of chemicals to control pests of trees is an expensive and labour intensive exercise even in countries where agriculture and forestry are competitive and economically viable. To use the same techniques in arid or semi-arid zones where sparseness of vegetation would involve the movement of equipment over vast areas, would be uneconomic.
For the continuing success of any form of afforestation in arid areas, careful consideration must be given to the selection of the tree species, which in turn will centre on a thorough knowledge of the ecology of the area. When such trees reach the fruiting stage, providing seeds are unattacked by pests, regeneration will take place (assuming that other conditions are favourable).
The only circumstances in which consideration should be given to the use of chemicals is under the controlled situation of a research compound where trees are monitored for, among other things, specific seed production. Alternatively, in a small intensively planted area a vehicle mounted sprayer can be used for the application of pesticides. For larger scale spraying the cost becomes prohibitive. These two approaches will be dealt with under the respective sections.
It may be possible, once the capabilities of the natural enemies listed below have been evaluated and the biological data regarding the species have been amassed, to plan control programmes for defined areas using the integrated techniques of chemical and biological control. This should certainly be the aim if the natural balance is to be maintained.
The present position in respect of some Acacias in Africa, as pointed out by Lamprey et al. (1974), is such that further indiscriminate interference in the ecological system, which has already suffered considerably, could only aggravate the situation still further. Whilst this may be the only recorded case where insects are implicated as causative agents of damage in the arid zone ecology, it is equally probable that other regions will show similar instances needing to be dealt with in the same manner.
As indicated in the introduction to this section, chemicals as pesticides should only be resorted to as a means of control after a thorough study of the ecology of the insect has been made.
The main areas where control of tree pest species has been attempted are the coniferous forests of temperate regions. In this situation, not only are the trees at a high density but the insects to be controlled are reasonably exposed as they are foliage feeders. With the constraints imposed by conditions under which this project will operate, the situation becomes somewhat complex. Firstly, there are the severe environmental conditions in which the pesticide will have to be applied and these will test its residual effect to the extreme. Secondly, the particular aspect of the production of quantities of viable seed from gene pools imposes special problems. The majority of the insect species are, for the greater part of their lives, hidden within the seed and any control must therefore be aimed at the exposed phase - the adults. These have a limited period of activity and timing will therefore be critical.
As this form of control has not been undertaken in this situation previously, there is no background information on which to draw. It is possible to extrapolate from the type of equipment used for medical and veterinary vector control in tropical areas and for this purpose the apparatus described and figured in the World Health Organization publication “Equipment for Vector Control” will be invaluable.
Guidelines for control measures to be undertaken in connection with this project are suggested as follows.
The treatment with residual insecticides of:
Individual branches
Individual small trees
Individual large trees
Plantation areas raised for specialised seed production.
The equipment required for the above will be:
Small lever operated sprayers of between 7–9 litres capacity.
Hand compression sprayers of between 10–20 litres capacity.
Motorised knapsack mist blowers capable of reaching 6–9 metres into the tree canopy.
Vehicle mounted mist blowers capable of blanket coverage of large areas. These are of two types - one with centrifugal energy nozzles in the form of spinning discs or cups and the other, a thermal aerosol generator. The former are powered by electric motors or internal combustion engines, and the droplets produced from the nozzles of this type are dispersed by a current of air from a fan or blower. In the latter, the chemical, dissolved in an oil of suitably high flash-point, is vapourised by being injected into a high velocity stream of hot gas. The formulation emerging as a dense fog disperses among the branches of the trees. This treatment is indiscriminate in action and should only be employed when other methods fail. The severe ecological disruption which occurs takes a very long time to recover.
As with the techniques, care must be exercised in selecting the most appropriate insecticide and its carrier. In the little work carried out on bruchid control on grain legume crops, Endosulphan and Tetrachlorvinphos have given promising results but as these have not been used in the situation envisaged in this project, or against the beetle species concerned, it must be emphasized that it is imperative for tests to be carried out under laboratory and field conditions for the selection of the most promising formulation. The eventual choice will depend on the results of these trials.
The development of specifically oriented chemicals, selective against individual groups of insects, and which do not disrupt the effect of parasitoids or the use of pheromone traps, may be the ultimate goal. These sophisticated approaches can only be employed after the biological groundwork has been covered.
Subsequent studies on the biology of the insects associated with leguminous trees will reveal the role played by parasitoids and predators. Some parasitoids have already been shown to exert an influence on the populations of seed beetles in agricultural situations. A manipulation of their life cycle may enable them to be mass-reared and released in an attempt to control the seed beetles.
Parasitoids attacking the bruchids belong almost wholly to the Hymenoptera. They attack egg, larval and pupal stages.
Bruchid eggs, because of their position on the outside of a pod, are easily located by parasitoids. Of the Hymenoptera, the genus Uscana of the family Trichogrammatidae has several species associated with bruchids. In the literature these have been placed under one name U. semifumipennis but a recent paper by Steffan (1981) suggests a complex of species involved, one attacking a Bruchus sp., two associated with Callosobruchus and one with the genus Bruchidius and each of these appears to be host specific. The records in the literature of egg parasitoids using the name U. semifumipennis cannot therefore be relied upon.
Very little evidence of the effect of egg parasitoids on bruchid populations has been published. Bridwell (1918), in one of his early papers, gave very high mortality figures for the imported groundnut seed beetle (Caryedon serratus (Ol.) = C. gonagra (F.)) in Hawaii. He considered that mortality in this instance was due almost entirely to the egg parasitoids of the genus Uscana. In view of this, a detailed study of the biology of this group may suggest that the mass rearing of these insects could be used for biological control. This could be particularly appropriate if the parasitoids are shown to be all host specific.
The Hymenoptera associated with larval and pupal stages of field populations of bruchids attacking Acacia and related trees, do not cause any serious reduction in numbers. On average, pods will produce one bruchid per seed with only two parasitoids emerging for every twenty beetles. The reasons for the low incidence of parasitism is not clear but could be due to difficulty experienced by the wasp in locating or reaching the larva which is protected by a pod as well as the seed.
Parasitoids of the genus Bruchobius are the main species associated with Bruchidius on Acacia. The papers of Steffan (1981) and de Luca (1965) make a few observational remarks on these associations but no useful biological data is available.
The Hymenoptera do not appear to have exploited the adult stage of bruchids possibly due to the shortness of the life of the latter.
Pests of pods and seeds of legumes are noticeably free from predators with one exception. A proportion of the bruchids in samples of legumes collected in tropical areas fail to complete their cycle, due in part to the presence of mites (Acarina) of the genus Pymotes. These mites, about 0.2 mm long when fully grown, feed on the larvae, pupae and adults of bruchids. The effect which they exert on laboratory samples suggests that under field conditions beetle populations could be greatly reduced. No study, however, has been undertaken of their effect as a controlling agent although Moser (1975) hints at the possibility of a species of the P. ventricosus complex being a possible control agent for Scolytid sp. (Bark Beetle). He also states that this type of work has been hampered by lack of identification of the species involved and figures a bruchid adult with a number of female mites attached to the dorsal side of the abdomen. In a further paper Cross and Moser (1975) revised the taxonomy of Pymotes and showed the presence of a number of different species whereas workers had previously tended to place them all under the specific name P. ventricosus Newport. The species Pymotes which attack Bruchidae are most probably P. boylei.
Although no scientific data is available on these mites, the general biological cycle appears to follow conventional lines. The eggs are formed within the engorged body of the female and remain there until they receive the required stimulus to hatch, possibly contact with a developing bruchid larva. After several moults they reach maturity and the females attach themselves to adult bruchids that have survived attacks on the larval and pupal stages.
The females migrate to the dorsal side of the beetle's abdomen and settle down under the wings and elytra. They feed on the beetle and become engorged, the abdomen swelling up to several times its original size. Eggs develop within these enlarged abdomens and the mites eventually die. The dead mites presumably become detached and drop off when the beetles fly to a new oviposition site. Mites are sometimes so numerous under the elytra as to prevent these and the wings from being held in their normal resting position.
