Nematicidal efficacy, enhanced degradation and cross adaptation of carbosulfan, cadusafos and triazophos under tropical conditions.
Nematicides need to be applied in each cropping season but repetitive applications can reduce their persistence and efficacy due to the unpredictable phenomena of enhanced biotransformation and cross adaptation. Experiments were conducted to ascertain the number of times the nematicides carbosulfan, cadusafos and triazophos can be applied effectively in the field. Incubation studies checked their degradation rates and cross adaptation and bioassays assessed their efficacy against Meloidogyne incognita infecting Solanum lycopersicum. Monitoring nematode populations at the middle of seven consecutive tomato crops following nematicidal treatments at a recommendable rate of 1.0 kg a.s. ha-1 revealed a linear decrease in efficacy with successive seasons. The chemicals remained effective up to the fourth application when 53-62% reduction of M. incognita in soil was still achievable, which decreased significantly to 14-33% by the seventh application. The nematicides were more effective against endoparasitic (M. incognita and Rotylenchulus reniformis) than ectoparasitic (Helicotylenchus dihystera, Hoplolaimus indicus and Tylenchorhynchus vulgaris) nematodes. Bioassays revealed 13-18% more invasions of second-stage juveniles of M. incognita into roots of tomato grown in soil pre-treated seven times with nematicide than in similar soil with no history of nematicide use; invasion and soil population were positively correlated. Root galling of field-grown tomato increased from the first to the seventh application. In bioassays, tomato root galling was greater in unsterilised field soil (1.4-2.1) than in similar soil/sterilised soil (1.0) compared with 2.4-2.9 in untreated control soil. The decrease in efficacy was attributable to accelerated microbial degradation of nematicides due to repeated use in each cropping season. Carbosulfan, cadusafos and triazophos exhibited a half-life (t1/2) of 14, 20 and 27 days in soil with no history of nematicide use, whereas the t1/2 was 6, 13 and 9 days in soil pre-treated seven times with nematicides and 28, 28 and 32 days in unsterilised soil, respectively. In cross adaptation studies, carbosulfan exhibited a t1/2 of 7-9 days in soil pre-treated seven times with cadusafos and triazophos. The t1/2 of cadusafos (22 days) was not affected in carbosulfan-treated soil but was affected (13 days) in triazophos-treated soil. Triazophos had a t1/2 of 20 days in carbosulfan-treated soil and a t1/2 of 8 days in cadusafos-treated soil. These results indicate that carbosulfan, cadusafos and triazophos can be applied effectively to the same field at least four times without decrease in efficacy due to accelerated biotransformation. Rotation of nematicide from different groups can be used in long-term nematode management strategies to avoid accelerated degradation and/or cross adaptation.