Entomopathogenic Nematodes for Field Control of Onion Maggot (Delia antiqua) and Compatibility with Seed Treatments

Abstract

Onion maggot (Delia antiqua) is a prominent pest of allium crops in temperate zones worldwide. Management of this pest relies on prophylactic insecticide applications at planting that target the first generation. Because effective options are limited, growers are interested in novel tactics such as deployment of entomopathogenic nematodes. We surveyed muck soils where onions are typically grown to determine if entomopathogenic nematode species were present, and then evaluated the compatibility of entomopathogenic nematode species with the insecticides commonly used to manage D. antiqua. We also evaluated the efficacy of these entomopathogenic nematodes for reducing D. antiqua infestations in the field. No endemic entomopathogenic nematodes were detected in surveys of muck fields in New York. Compatibility assays indicated that, although insecticides such as spinosad and, to some extent, cyromazine did cause mortality of entomopathogenic nematodes, these insecticides did not affect infectivity of the entomopathogenic nematodes. Field trials indicated that applications of entomopathogenic nematodes can reduce the percentage of onion plants killed by D. antiqua from 6% to 30%. Entomopathogenic nematodes reduced D. antiqua damage and increased end of season yield over two field seasons. Applications of entomopathogenic nematodes may be a viable option for reducing D. antiqua populations in conventional and organic systems. Together with other management tactics, like insecticide seed treatments, applications of entomopathogenic nematodes can provide a yield boost and a commercially acceptable level of D. antiqua control.

Keywords: biological control; bulb yield; conventional management; soil pests.

Figure 1

Entomopathogenic nematodes (EPNs) detected from soil samples at 2019 and 2020 Field sites. (A) Results of surveys at the 2019 Field Trials location. (B) Results of surveys at the 2020 Field Trial Location (separate and independent location from the 2019 Field Trials). Blue bars indicate timing of soil surveys for entomopathogenic nematodes; bars above the horizontal indicate detection of entomopathogenic nematodes and bars below the horizontal indicate no entomopathogenic nematodes detected. Vertical grey lines indicate timing of application.

Figure 2

Mortality of entomopathogenic nematode cohorts of different species following exposure to solutions containing no pesticide (Control), spinosad, and cyromazine. Bar height and error bars denote mean mortality and standard deviation, respectively. Asterisks indicate a significant (p < 0.05) difference in mortality as compared to controls on a per-species basis.

Figure 3

Infectivity of entomopathogenic nematode cohorts previously exposed to either (A) spinosad or (B) cyromazine. Odds ratios measure the change in infective potential of nematode cohorts after exposure to compounds as compared to controls. An odds ratio of one denotes no change; an odds ratio of less than one denotes a decreased ability of entomopathogenic nematodes to infect insects. Points and error bars denote mean odds ratio and standard error respectively. Asterisk indicates that the odds ratio is significantly different from one at p < 0.05.

Figure 4

The effect of adding entomopathogenic nematodes on onion maggot damage. In 2019, in small plot trials without seed treatment, applications of entomopathogenic nematodes reduced cumulative onion loss (A) and resulted in an increase in end of season plant stand (B). In 2020, in larger plot trials with seed treatment, applications of entomopathogenic nematodes also reduced cumulative onion loss (C) and also resulted in an increase in end of season plant stand (D). In (A,C), solid lines and shaded region denote mean loss and standard error, respectively. In (B,D), points and error bars denote mean and standard error, respectively.