Post-application Persistence of Entomopathogenic Nematodes

Abstract

Five phases can be distinguished in the post-application persistence of entomopathogenic nematodes and each phase is associated with a specific set of mortality factors. Pre-application factors associated with production, storage and transport conditions determine the survival rate and quality of nematodes at the time of application. The phase of tank mixing and application with a sprayer, hose or other equipment does not usually cause mortality as nematode dauer juveniles are quite tolerant of shear forces. The most critical periods for survival are the first few minutes and hours directly after application. High losses, in the order of 40-80%, often occur during this phase. Ultraviolet radiation and dehydration are probably the most important mortality factors. The remaining nematodes settle in the soil and their numbers gradually decrease at levels of 5-10% per day. Predation, infection by antagonists, depletion of energy and desiccation are probably the main mortality factors during this period. In most cases, after 2-6 weeks less than 1% of the applied population is still alive. Through recycling in host insects, nematodes may then persist for years at these levels. Thus, the pattern is a rapid decline in the first few days followed by a moderate decline over the next 2-6 weeks and then a long period of recycling at a low level. Some nematode species that normally occur in warmer climatic zones can also persist in colder climates. Major side-effects of applications of entomopathogenic nematodes are not likely to occur as the population density decreases to background levels within days or weeks after application. Furthermore, there is little or no migration of the nematodes to neighbouring fields. The relatively short period of persistence of entomopathogenic nematodes and the necessity of their populations to recycle frequently in hosts in order to survive make it unlikely that they could have major effects on non-target organisms. Their selectivity and beneficial traits as biological control agents outweigh the small risks of causing unwanted environmental disturbance in non-target populations.