Host plant phenology drives risky larval dispersal in an outbreaking insect defoliator

Abstract Dispersal away from natal sites allows individuals to find suitable foraging sites to complete their development and successfully reproduce. Drivers of risky dispersal behaviour in forest landscapes by young larvae of outbreaking defoliators are not well understood. We assessed dispersal behaviour of young spruce budworm larvae in relation to spring budburst phenology of primary and secondary hosts, balsam fir and black spruce, respectively. We tested whether tree species and presence of suitable feeding sites influenced dispersal away from source branches and subsequent redistribution of insects. Laboratory experiments showed that dispersal is an active behaviour during which larvae disperse away from source trees without open buds, regardless of species. Establishment on sink branches was highest when they possessed open buds and the source did not. In the field, larval dispersal was higher from black spruce than from balsam fir and larval establishment was more persistent on balsam fir. The decision to disperse occurs before budburst of either host species. Larvae disperse preferentially away from black spruce whose old needles are too tough for larval mining, compared to balsam fir that can be mined for sustenance and refuge while larvae await budburst. While black spruce is a suitable host species after its buds expand, phenological defences drive larval dispersal away from this host plant. These findings are the first to show how risky dispersal behaviour in larval Lepidoptera is facultative and is determined by local food availability.