Missing temporal effects of soil insecticides and entomopathogenic nematodes in reducing the maize pest Diabrotica virgifera virgifera

Control methods for the larvae of the maize pest Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) are known to produce inconsistent results under field conditions. To better understand the effects of plant protection products on the root-feeding larvae, we looked for changes in efficacy of the granular soil insecticides chlorpyrifos, cypermethrin, and tefluthrin during a cropping season, as well as a fluid-applied entomopathogenic nematode Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae). Twelve field-scale experiments carried out in Hungary between 2010 and 2018 revealed that treatments, whether chemical or biological, are able to reduce D. v. virgifera . However, results were variable with failures in about a quarter of the experiments. Unexpectedly, our findings indicated only limited effect of time on treatments, meaning all products appeared capable of continuously reducing larvae during their time in the soil. Only chlorpyrifos seemed to slightly lose and tefluthrin to slightly increase efficacy over time. Nevertheless, there is no major evidence that failure of treatments is due to temporal effects. Other factors may play a larger role and merit investigation under field conditions.