Limited Influence of Abiotic and Biotic Factors on the Efficacy of Soil Insecticides and Entomopathogenic Nematodes when Managing the Maize Pest Diabrotica v. virgifera (Coleoptera: Chrysomelidae)

Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) is a serious pest that infects maize. Insecticides or entomopathogenic nematodes are used to control the root-damaging larvae. However, such treatments are reportedly inconsistent in terms of efficacy under farming conditions. To better understand the reasons behind these inconsistencies, we studied the control efficacy of seed coatings, such as clothianidin: granular soil insecticides, such as cypermethrin and tefluthrin: and fluid-applied entomopathogenic nematodes, such as Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae). We assessed the influence of 12 biotic and 20 abiotic factors on the reduction of Diabrotica v. virgifera populations and on the prevention of root damage in 20 field-scale experiments in Hungary between 2010 and 2020. Results confirmed that all treatment types are able to control pest populations and prevent root damage, but with high variability. Our analyses showed that most investigated factors, for example, air temperature, most soil parameters, and pest infestation levels, did not influence the efficacy of the treatments. The efficacy of clothianidin in preventing root damage decreased slightly with increasing soil bulk density but improved with late maize sowing, and therefore late treatment, as well as with increasing soil moisture in July. The efficacy of cypermethrin in preventing damage improved slightly with increasing clay content in the soil. Tefluthrin was slightly less effective in reducing D. v. virgifera with increasing soil moisture in June. However, all these factorial influences were minor in their absolute effects. Surprisingly, none of the investigated factors seemed to influence the efficacy of H. bacteriophora. In conclusion, the efficacy of chemical and biological treatments against this soil pest remains difficult to predict under farming conditions.