Larval behavior in Lymantria dispar increases risk of fungal infection

Late-instar larvae of the forest defoliator Lymantria dispar display relatively unusual behavior for Lepidoptera. Late instars move down from the tree canopy and wander and rest in dark locations during daylight hours. When we sampled the area extending from below 3 m and within 200 cm of tree trunks during daylight hours, 71% of L. dispar late instar larvae were found at ground level. Providing dark resting locations on the soil surface where there was no litter resulted in rapid location and colonization of these sites by late instar larvae. L. dispar larvae were always more prevalent in leaf litter 0-50 cm from tree trunks compared with 50-200 cm away. In an area where the fungal insect pathogen Entomophaga maimaiga is established, larvae were caged on tree trunks, in the foliage, or on top of soil during photophase or scotophase to determine in which locations risk of infection was greatest. At both times of day, highest infection levels always occurred on the soil, with least infection among larvae caged in the foliage. Infection levels were greater during photophase than scotophase. When larvae were exposed to soil for shorter periods during daylight hours to mimic wandering, 4.7 and 6.1% became infected after 30- and 60-min exposure intervals, respectively, with increasing infection associated with longer exposure times. The high levels of infection by E. maimaiga that have been documented in L. dispar populations since this pathogen was first found in North America are consistent with the strong litter-dwelling behavior of late-instar L. dispar larvae. Rarity of other lepidopteran larvae at ground level could help to explain the host specificity of this pathogen in the field.