EFFECT OF BAIT SUBSTRATE AND FORMULATION ON INFECTION OF GRASSHOPPER NYMPHS BY BEAUVERIA-BASSIANA

The effects of two formulations (oil and water) and two bait substrate s (lettuce and bran) on infection of grasshopper nymphs (Melanoplus sa nguinipes) by Beauveria bassiana were investigated. More nymphs died o f mycosis after they had ingested substrates inoculated with conidia i n oil than in water, but there was no difference between the lettuce a nd 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 recover ed in frass from the water than from the oil formulation. A higher inc idence (33-82%) and more rapid onset of mycosis was observed in nymphs that were surface sterilized before ingestion, compared with those su rface-sterilized after ingestion of lettuce and bran substrates inocul ated with B. bassiana in both formulations. A similar trend was observ ed 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 o f nymphs that ingested treated lettuce or bran. Modest numbers of coni dia were also recovered from the surfaces of nymphs, but not differenc es were observed between formulations or substrates. However, when nym phs ingested lettuce disks treated with fluorescent dye in either oil or water, more dye was observed on nymphs with the oil formulation. Th is laboratory study demonstrates that grasshopper nymphs are highly su sceptible to infection by conidia formulated on bait substrates, and t hat the efficacy of the bait relies on the extent to which nymphs beco me surface-contaminated during ingestion.