Effect and mode of action of some systemic nematicides

In this study, nematicidal effects, mode of action and specific characters of some systemic nematicides were studied, in search of substitutes for the widely used soil fumigants that require high dosages. The thesis comprises: - a review of literature, - development of techniques, - a test for nematicidal effectiveness of agricultural chemicals, - a detailed study of two systemic nematicides in vitro and in vivo, - a study of some important side-effects of the two systemic nematicides. Review of literature The main groups of test animals (nematodes and arthropods) and the systemic and fumigant categories of nematicides are briefly reviewed, and a scheme of the principal interactions between nematicide, fauna, flora and soil is given (Fig. 1). The literature review stresses: - the great increase in crop yields possible by soil disinfection, - the desirability of replacing currently used nematicides applied at dosages of 100-1000 kg a.i. per ha, - the effect of systemic nematicides on the relation plant/nematodes at dosages of 1-10 kg a.i. per ha, - the controversial opinions on mode of action of systemic nematicides, - the lack of information on residues and side-effects, - the selectivity of techniques for testing nematicides, which thus may not detect effective chemicals. Development of techniques Materials and methods are briefly indicated. Special attention was given to the development of new screen techniques. Three biological assays, called 'penetration inhibition' test (PI test), 'therapeutic' test (T test) and 'gall index' test (GI test) were developed or modified to meet the requirements for effective and rapid screening of all known types of nematicides, with emphasis on systemics (Fig. 2, 7 and 10). The PI test measures inhibition of invasion by Ditylenchus dipsaci in stem sections of Vicia faba (Fig. 3 - 6; Table 1). The T test can also be used like the PI test - to measure inhibition of invasion by D. dipsaci - but can be also used to measure the therapeutic effect of substances in Lycopersicum esculentum infested by D. dipsaci (Fig. 8 and 9; Tables 2 and 3). The GI test measures the effect on gall formation of L. esculentum by Meloidogyne incognita (Fig. 11; Table 4). Nematicidal effectiveness of some biocides This general screen as a basis for appraisal and choice between compounds for further study was made with the PI test. Tables 5 and 6 and Fig. 12 summarize details and results. Thirtyfour of 60 preparations tested were effective with an EC-50 (median effective concentration) of 50 mg/litre (ppm) or less. The very active materials were organophosphates and organocarbamates and are known to inhibit acetylcholinesterase; they comprise a group of 17 compounds with an EC-50 of 1 ppm or less. Of those materials, the following had not previously been recorded as nematicidal: dichlorvos, methiocarb, trichlorphon, pyrazophos, fenitrothion. However, three known nematicidal active compounds showed an EC-50 above 50 ppm: α-terthienyl, benomyl and dinitro-o-cresol (Table 5). Oxamyl, an organocarbamate, and phenamiphos, an organophosphate, were chosen on the basis of these results and of other properties for further study on their effects and mode of action. Effects of oxamyl and phenamiphos in vitro Both chemicals had direct effects on nematodes (contact action), including protrusion of stylets, and shortening, swelling and wrinkling of their body, resulting in aberrant undulations and reduced mobility (Fig. 13 -16). The symptoms of poisoning, however, were reversible for oxamyl, but less so for phenamiphos. When D. dipsaci was permanently exposed to solutions of the preparations, the nematode was initially affected but recovered gradually in oxamyl concentrations up to 64 ppm. Phenamiphos did not allow recovery - not even at 0.1 ppm - during 21 days' exposure (Fig. 17; Table 7). When D. dipsaci was washed with water, nematodes treated with 1000 ppm a.i. oxamyl for 24 hours recovered in 2 days; even after 4 days' treatment with oxamyl solutions at 10000 ppm a.i. nematodes in the L-4 stage could recover. The effects of phenamiphos were less reversible; after 24 hours' treatment in 100 ppm a.i. the nematodes did not regain normal behaviour, but nematodes did recover after treatment with 10 ppm a.i. Recovered nematodes could reproduce normally (Fig. 18-21; Tables 8 and 9). The effects observed and those noted in the literature indicate that the effects were caused by inhibition of acetylcholinesterase or other neuro-enzymes. Effects of oxamyl and phenamiphos in soil and plants Soil drenches with aqueous solutions of both preparations reduced Pratylenchus penetrans in plant roots or even eradicated them, and also reduced nematode densities in soil. Phenamiphos, particularly, is less effective in organic soils. Root dips and foliage sprays (without preventing soil contamination) were effective, but basipetal transport of the substances could not be demonstrated. Also within plants, the effects of oxamyl on nematodes were reversible and of phenamiphos irreversible. At low temperatures, at low dosages and in the first weeks after soil was drenched, oxamyl was superior as a nematicide to phenamiphos, either to control nematodes inside or outside the roots of the test plants. At high temperatures, for high dosages and longer periods, the reverse was true (Fig. 22 - 24; Tables 10-14). The effect of the two substances on nematodes in microplots sown with Lolium perenne (Fig. 25-27) were generally similar to those obtained in vitro and in drench treatments: this was true for several species of nematodes and microarthropods, although saprozoic mites were less susceptible (Fig. 28 and 29; Tables 17 and 18). Phenamiphos caused a greater and longer effect than oxamyl, although oxamyl too greatly reduced populations for several weeks after treatment. Plant growth was best on oxamyl plots as phenamiphos was apparently phytotoxic to L. perenne. The same treatments on fallow soil confirmed that the substances had an almost equal, direct effect on the nematodes as for nematodes in soil in which L. perenne was growing (Fig. 3 1 ; Table 20). Some specific effects of oxamyl and phenamiphos Oxamyl persisted less in soil than phenamiphos (Fig. 33; Table 22); in fact oxamyl was so transient that low-temperature application increased the nematicidal effect markedly. Experiments in vivo with sublethal concentrations of oxamyl and phenamiphos suggest that hardly any resistance could be expected with these systemics. About 9 successive generations of D. dipsaci were tested for resistance in one year. Sprays with oxamyl were not toxic to plants while phenamiphos was relatively toxic (Fig. 34), with differences from species to species of plant. A test in vitro on toxicity to fungi showed that oxamyl has no fungitoxic properties and phenamiphos has (Table 25). Both substances reduced Rhizobium trifolii nodulation on red clover plants infested by P. penetrans (Fig. 35). General conclusions Systemic nematicides are useful for preventing nematodes from attacking crops, by preventing penetration of nematodes or even by eradicating nematodes that have already entered roots, stems or leaves. The nematicidal effects and persistance is somewhat greater for phenamiphos than for oxamyl. Apparently they not only influence nematodes through the plant (by systemic action), but also in the soil (by contact). Systemic nematicides may also prevent damage to plants by microarthropods. As to residues, oxamyl seems less dangerous to the environment than phenamiphos, because oxamyl is quickly broken down to biologically inactive substances. The combination of short persistence and reversibility of the poisoning effect makes oxamyl - in contrary to phenamiphos - nematostatic rather than nematicidal.