What fraction of phytophagous insects drops off foliage after application of entomopathogenic fungi? Observations on Metarhizium brunneum against the potato psyllid, Bactericera cockerelli

Mortality of foliage insects after applications of entomopathogenic fungi on crops should consider that a fraction of live or dead, infected insects can fall off the aerial parts of plants before counts, resulting in biased estimates of insects killed. Although robust sampling should reduce this bias, it is important to estimate the numbers of infected insects lost to gravity. We applied conidia of Metarhizium brunneum against nymphs of the potato psyllid, Bactericera cockerelli, on foliage of pepper plants in the greenhouse, in order to determine the percentage of infection on Bactericera nymphs that fall to the ground after application of Metarhizium (4 x 10⁸ conidia/ml) with a motorized sprayer. Treatments were different application volumes, obtained by using ULV nozzles providing 0.10, 0.20, and 0.42 l/min. Clear plastic sheets were placed under the drip zone of 4 plants/treatment to collect dislodged nymphs and the semisolid honeydew from insects as an indicator of nymph population. Forty nymphs/treatment were collected from sheets eight days after application and incubated under high humidity. Metarhizium infection was confirmed on 5% of nymphs sprayed at 0.10 l/min, 7.5% at 0.20 l/min, and 12.5% at 0.42 l/min. Honeydew collected ranged from a 1.24 to 2.04 honeydew gr/plant, with Metarhizium treatments collecting the lowest amounts of honeydew. Dislodged infected insects should be considered on mortality estimates by entomopathogenic fungi on crops. Honeydew production could indicate population trends of treated insects.