Efficacy of the newly discovered entomopathogenic nematode Steinernema adamsi against Helicoverpa zea: life stage susceptibility, UV tolerance, and field performance.

Helicoverpa zea is a major agricultural pest, particularly in cotton, and poses significant challenges due to its ability to develop resistance to chemical insecticides. This study evaluates the efficacy of the entomopathogenic nematode (Steinernema adamsi) and its mutualistic bacteria (Xenorhabdus) as biological control agents against H. zea larvae in both laboratory and field settings. In laboratory assays, mortality rates for 1(st) to 4(th) instars were high, ranging from 74.2% to 100%, while 5(th) instars exhibited significantly lower susceptibility (<37% mortality). Pupae were completely resistant to nematode infection. The impact of UV radiation on nematode efficacy was assessed, with mortality decreasing from 100% in control conditions (0 hours of UV exposure) to 71.8% after 5 hours of UV exposure, highlighting the vulnerability of S. adamsi to UV degradation. In addition, Xenorhabdus caused 100% mortality in H. zea larvae when injected directly into the hemocoel, but oral toxicity was significantly lower, with 36% mortality in 7 days post-exposure. Field experiments demonstrated that the combination of S. adamsi with 0.05% sodium alginate (hygroscopic agent) and 0.02% Congo red (UV protectant) resulted in a significant increase in larval mortality. In field test A, where S. adamsi was applied in water, mortality averaged 56% with 82% EPN infection. In field test B, the combined treatment of sodium alginate and Congo red led to 98% larval mortality, although infection rates were lower and statistically non-significant. The addition of these protective agents likely enhanced the environmental stability and efficacy of the nematodes under field conditions. These findings suggest that S. adamsi can be an effective biological control agent for H. zea, particularly when combined with formulations that protect against UV radiation and desiccation. Future research should focus on optimizing nematode delivery systems to improve field efficacy under diverse environmental conditions.