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Chemical control of nematodes: efficiency and side-effects

S.R. Gowen


Introduction
Chemical control of nematodes
References


Introduction

Plant-parasitic nematodes are at their most vulnerable during their active phase in soil when searching for the roots of host plants. Once endoparasitic species have penetrated a root, control with chemicals is more difficult as nematicidal compounds have to be non-phytotoxic and preferably systemic.

A nematicide that can be safely applied to growing plants and is translocated to the roots in sufficiently large amounts to kill endoparasitic or ectoparasitic nematodes has not been discovered. Oxamyl, a systemic compound that is translocated basipetally, is the only commercial product that is used as a foliar treatment, but its use as a liquid formulation is restricted in many countries for toxicological reasons.

There are several nematicides that can be used effectively for nematode pests of annual crops (van Berkum and Hoestra,1979), but there appears to be little prospect for management of nematodes in many susceptible perennial crops without repeated application of nematicides (Tables 1 and 2). Only in certain cases will such treatments be justified economically. Since the discovery and wide-scale use of fumigant nematicides 50 to 60 years ago, a number of products and formulations (Table 1) have been developed for use against several nematode pests, and these are available in most regions of the world (Hague and Gowen, 1987). Only in comparatively recent times have the dangers associated with the manufacture and use of these products become apparent. This has resulted in restrictions on use and sometimes withdrawal from the market. It seems that the age of the traditional fumigants and nematicides has passed, and the opportunity for managing nematodes with synthetic chemicals with broad biocidal activity is declining.

The development of new classes of nematicides with novel activity, that are effective when used in soil or applied directly to crops and are environmentally benign and specific to target pests, is perhaps an idealistic hope. Such compounds will, by definition, require considerable research effort, and if they are specific only to certain nematodes are unlikely to be considered an economic proposition for the traditional agrochemical company.

Chemical control of nematodes

The demand

The elimination of nematodes from some crops is essential for certain export requirements, particularly of high-value horticultural products. Chemical treatment with fumigants or nematicides may be the only technique available, and from the plant quarantine standpoint it is important that their use is retained.

The use of chemicals in protected cropping may still be preferable to other techniques such as steam treatment for economic and practical reasons. The use of soil-less growing media in some north European countries has resulted in a decreased demand for chemical treatments. In southern Europe, the Mediterranean region and North Africa, many horticultural and salad crops are grown in soil under polythene and soil treatment with methyl bromide, dazomet or non-fumigant nematicides is widely practised. The cost of such treatments may be as much as US$500 per hectare per year and can only be justified if the crops are of high market value.

Efficacy

Fumigants. Several general purpose fumigants give excellent control of nematodes in soil. The efficacy is related to their high volatility at ambient temperatures. All fumigants have low molecular weights and occur as gases or liquids. As they volatilize, the gas diffuses through the spaces between soil particles; nematodes living in these spaces are killed.

TABLE 1
Nematicides currently available on world markets

Chemical name

Trade name

Formulation

Fumigants

Methyl bromide

Dowfume

Gas

1,3 dichloropropene

Telone/DD-95

Liquid

Ethylene dibromide1

Dowfume W-85

Liquid

Metam-sodium

Vapam

Liquid

Dazomet

Basamid

Dust (prill)

Methyl isothiocyanate

Di-Trapex

Liquid

Chloropicrin1

Larvacide

Liquid

Organophosphates

Thionazin

Nemafos

Granular or emulsifiable liquid

Ethoprophos

Mocap

Granular or emulsifiable liquid

Fenamiphos

Nemacur

Granular or emulsifiable liquid

Fensulfothion

Dasanit

Granular

Terbufos

Counter

Granular

Isazofos

Miral

Granular or emulsifiable liquid

Ebufos

Rugby

Granular or emulsifiable liquid

Carbamates

Aldicarb

Temik

Granular

Aldoxycarb

Standak

Flowable

OxamyI

Vydate

Granular or emulsifiable liquid

Carbofuran

Furadan/Curaterr

Granular or flowable

Cleothocarb

Lance

Granular

1 Use restricted.

Fumigants perform best in soils that do not have high levels of organic matter (which deactivates the toxicant) and that are free-draining but have adequate moisture. In general, fumigants are most effective in warm soils (12° to 15°C) as dispersion is temperature related.

Methyl bromide, the most dangerous of the fumigants still in common use, has to be applied beneath a polyethylene sheet. In some countries this is done with specialized machinery that treats and covers the soil in one operation. The cover is removed some days later and the crop is sown or planted when all traces of the fumigant have dispersed.

TABLE 2
Examples of recommended nematicidal dosages and treatments for some important crops1

Crop

Nematode pest

Nematicide

Application rate2

Application techniques

Potato

 

Globodera spp.

Aldicarb

2.24-3.36

Incorporated in row

Oxamyl

4.0-5.5


Carbofuran

4.0-5.5


Tomato, cucurbits

 

Meloidogyne spp.

 

Aldicarb

3.36

Incorporated in 30-cm bands

Ethoprophos

0.9-2.9

Incorporated in bands

OxamyI

0.6-1.2

Incorporated in bands

Fenamiphos

1.6-3.3

Incorporated in bands

Dazomet

30-50 g/m2

Incorporated in bands and irrigated Time interval before planting

Citrus

Tylenchulus semipenetrans

Fenamiphos

10.8-21.6

Annual treatment applied along drip-line

Aldicarb

5.5-11.0

Annual treatment applied along drip-line

Grape

Meloidogyne spp.

Fenamiphos
(e.c. formulation)

10.0

In bands for nursery use

Xiphinema index

Aldicarb

5-10

In bands for nursery use

Banana

Radopholus similis and/or

Carbofuran

2-4 ga.i. per plant

Applied around plant 2-3 times per year

Helicotylenchus multicinctus and/or

Ethoprophos

2-4 ga.i. per plant

Applied around plant 2-3 times per year

Pratylenchus spp. and/or

Fenamiphos

2-4 ga.i. per plant

Applied around plant 2-3 times per year

Meloidogyne spp.

Isazofos

2-4 ga.i. per plant

Applied around plant 2-3 times per year

Ebufos

2-4 ga.i. per plant

Applied around plant 2-3 times per year

1 Information taken from literature. Products may be unavailable for use in some countries. Economic and environmental justification should be evaluated before use. The omission of compounds does not imply that they are not equally suitable for nematode control.

2 kg a.i./ha unless otherwise stated.

Liquid fumigants EDB, metam-sodium and 1,3-D are applied to soil that has been prepared for planting. The soil surface is compacted with a roller after treatment which helps to seal the fumigant in the soil. Compounds releasing methyl isothiocyanate (dazomet, metam-sodium) work best in soils at >15°C. In cooler soils, the period between treatment and planting may have to be extended to allow sufficient time for the product to disperse.

The liquid fumigant DBCP is the only volatile compound that can be applied to growing plants without causing phytotoxicity. However, its manufacture has now ceased for toxicological reasons and its use is banned in many countries.

Non-volatile nematicides. A number of organophosphate and oximecarbamate nematicides were developed in the 1960s, which had the advantage that application was relatively simple (Wright, 1981). As a consequence nematicide use became more widely practised. These compounds (Table 1) are active at dosages of 2 to 10 kg a.i./ha which are smaller than the 200 to 300 litres/ha required for treatment with liquid fumigants. Most of the early formulations of these products were as granules that, when applied to the soil surface (or preferably incorporated in the top 10 cm of soil), release the active ingredient, which is spread through the soil by rainfall or irrigation. The efficacy of soil penetration depends on the amount of moisture, organic matter and soil structure. Heavy soils with relatively small pore spaces are more difficult to treat than sandy soils which have larger pore sizes. Some chemicals, particularly the organophosphates, are absorbed in organic matter, in which case efficacy may be impaired (Bromilow, 1980).

In general, distribution of the active ingredient or its toxic degradation products is less efficient than that of fumigants and results with granular nematicides have sometimes been inconsistent. To be effective, nematicides have to persist long enough for nematodes to be exposed to lethal concentrations, which may be as low as 1 to 2m g/litre. Extended persistence is, however, not desirable if there is a risk of residues in the crop or the active compounds contaminating groundwater.

Persistence of soil-applied nematicides depends on the soil characteristics. In warm countries, relatively high soil temperatures may accelerate the natural degradation of nematicides, and in protected crops where even higher soil temperatures than out-of-doors may occur, the effective life of a nematicide might be as short as one to three weeks (Bromilow, 1980). The repeated use of products of similar structure can lead to the selection of a soil microflora that metabolizes these compounds and decreases their persistence.

Side-effects

All nematicides are eventually degraded if they remain in the topsoil where there is greatest microbial activity. Once nematicides or their degradation products are flushed through the upper soil layers their persistence may be extended. It is the problem of toxic products in groundwater that has led to the prohibition of fumigant and non-fumigant nematicides in some countries. The permitted level of pesticide residues in drinking-water in the European Union is 0.1m g/litre. In regions of intensive agricultural production these tolerance levels may be exceeded at certain times of the year.

Nematicides are highly toxic compounds that have very low LD50 values. This is particularly important for operators of application machinery and people at risk from exposure to the chemicals during their application. The liquid formulations of some of the non-fumigant nematicides are emulsifiable concentrates. Their use should therefore be restricted to skilled operators who take adequate safety precautions. This may not always be the case where basic levels of education are poor or where operators cannot read the instructions on the labels of the products. The application of nematicides to crops too near to harvest is another risk which pesticide residue monitoring may not be sufficiently well coordinated to prevent.

The incidence of pesticide poisoning and mortality in some countries (Kottegoda, 1985) serves as a grim warning of the risks that arise when pesticides are widely used under poor management.

Future developments

The development cost of new products is more than US$20 million and the costs for registering these products are increasing as the criteria for their use are tightened. Conventional compounds (organophosphate or oximecarbamates) are unlikely to be developed if their toxicities are high.

New classes of nematicidal compounds are constantly being sought but there are currently no promising materials close to commercial development. Avermectins, which are of microbial origin, have been developed for veterinary use and are powerful anthelmintics. Their efficacy against plant-parasitic nematodes is well established, however, because the compounds are complex they cannot be used successfully as soil treatments.

References

Bromilow, R.H. 1980. Behavior of nematicides in soil and plants, p. 87-107. In Factors affecting the application and use of nematicides in Western Europe. Workshop, Nematology Group Association of Applied Biologists.

Hague, N.G.M. & Gowen, S.R. 1987. Chemical control of nematodes, p.131-178. In R.H. Brown & B.R. Kerry, eds. Principles and practice of nematode control in crops. Academic Press.

Kottegoda, M.B. 1985. Safety in use of pesticides and medical treatment. Chemistry and Industry, (16 September): 623-625.

van Berkum, J.A. & Hoestra, H. 1979. Practical aspects of the chemical control of nematodes in soil, p. 53-154. In D. Mulder, ed. Soil disinfestation. Amsterdam, the Netherlands, Elsevier.

Wright, D.J. 1981. Nematicides: mode of action and new approaches to chemical control, p. 421-449, In B.M. Zuckerman & R.A. Rohde, eds. Plant-parasitic nematodes. London and New York, Academic Press.


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