Effect of Bait Substrate and Formulation on Infection of Grasshopper Nymphs by Beauveria bassiana

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Abstract
The effects of two formulations (oil and water) and two bait substrates (lettuce and bran) on infection of grasshopper nymphs (Melanoplus sanguinipes) by Beauveria bassiana were investigated. More nymphs died of mycosis after they had ingested substrates inoculated with conidia in oil than in water, but there was no difference between the lettuce and bran substrates. Of the conidia recovered in frass, most (95%) were recovered within 24 h of ingestion of the lettuce and bran substrates by nymphs. Significantly more conidia averaged over time were recovered in frass from the water than from the oil formulation. A higher incidence (33-82%) and more rapid onset of mycosis was observed in nymphs that were surface-sterilized before ingestion, compared with those surface-sterilized after ingestion of lettuce and bran substrates inoculated with B. bassiana in both formulations. A similar trend was observed in nymphs receiving the sterilization treatment before, rather than after, ingestion of wheat leaves sprayed with conidia in oil or water. Numerous conidia were observed on the heads, thoraxes and abdomens of nymphs that ingested treated lettuce or bran. Modest numbers of conidia were also recovered from the surfaces of nymphs, but no differences were observed between formulations or substrates. However, when nymphs ingested lettuce disks treated with fluorescent dye in either oil or water, more dye was observed on nymphs with the oil formulation. This laboratory study demonstrates that grasshopper nymphs are highly susceptible to infection by conidia formulated on bait substrates, and that the efficacy of the bait relies on the extent to which nymphs become surface-contaminated during ingestion.