In an effort to protect field corn (Zea mays L.) from wireworm (Agriotes obscurus L.) herbivory and yield loss, seeds were treated with conidia of Metarhizium anisopliae strain F52 alone or in combination with clothianidin or spinosad before planting at three farm fields in south coastal British Columbia, Canada. Corn seed treated with M. anisopliae conidia (main effect) resulted in significant increases in stand density (78% M. anisopliae treated vs. 67% no M. anisopliae) and stock and foliage area fresh wt. yield (9.6 Mg ha−1 M. anisopliae treated vs. 7.6 Mg ha−1 no M. anisopliae), and significantly increased plant (stock and foliage) fresh wt. when it was applied together with spinosad or with no additional agrichemical at one location. Spinosad had no effect on corn yield, whereas clothianidin caused a significant increase in plant stand density and yield. Wireworm cadavers showing M. anisopliae strain F52 growth were retrieved from treated plots, suggesting that the increase in yield may have been due to wireworm control. Laboratory experiments provided no evidence that the increase in stand density and yield from the M. anisopliae‐treated corn seed was attributable to an increase in germination rate or root growth. We concluded that seed treatment with this fungus may be a novel method to increase stand density and yield of corn.

We determined that spinosad interacts synergistically with the biocontrol agent Metarhizium anisopliae (Metch) Sorokin to increase the mortality of two wild-collected wireworm species, Agriotes lineatus (L.), and Agriotes obscurus (L.). Bioassays were performed using a M. anisopliae isolate originally acquired from a local wireworm cadaver. M. anisopliae was applied as a soil drench at 3.3 x 10(2) and 10(4) conidia per gram sand, respectively. Soil drenches also were prepared using a commercial formulation of the actinomycete toxins spinosyn-A and spinosyn-D (common name spinosad) at sublethal doses of 1.5, 3, and 6 ppm active ingredient per gram sand. Combined treatments of spinosad and M. anisopliae were synergistic in causing mortality for all spinosad concentrations. Wireworm feeding activity was reduced after exposure to both spinosad and M. anisopliae and was found to be concentration dependent. The high mortality and reduced rate of wireworm feeding suggest that spinosad and M. anisopliae treatment combinations should be tested in the field.

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Environmental and behavioral factors that affect the infection of wireworms [Agriotes obscurus L. (Coleoptera: Elateridae)] by a unique isolate of Metarhizium anisopliae Sorokin (Hypocreales: Clavicipitaceae) were studied. After wireworms were placed in soil containing 10(6) M. anisopliae conidia/g and incubated at 6, 12, or 18 degrees C, significant disease development and wireworm mortality occurred only in those wireworms incubated at 18 degrees C. At this temperature, mortality was found to be dependant on the time exposed to the contaminated soil, and a minimum exposure time of 48 h was required to cause significant levels of mortality. Despite the restrictive effect of cooler temperatures on disease development and mortality, infected wireworms did not choose temperatures that inhibited disease development when given the opportunity to do so in a separate experiment. Finally, wireworms were repelled by M. anisopliae-contaminated soil at a rate that increased with the soil conidia concentration, but the rate of emigration was reduced when a food source was present. The results of this study indicate that factors including temperature, time exposed to M. anisopliae, conidia soil concentration, and food availability will affect mortality rates of wireworms and are likely to affect field performance of M. anisopliae as a biological control.