The evolution of larval foraging behaviour in response to host plant variation in a leaf beetle

The evolutionary causes of variation in host specialization among phytophagous insects are still not well understood and identifying them is a central task in insect–host plant biology. Here we examine host utilization of the chrysomelid beetle <i>Oreina elongata</i> that shows interpopulation variation in the degree of specialization. We focus on larval behaviour and on what selection pressures may favour the use of two different larval host plants (<i>Adenostyles alliariae</i> and <i>Cirsium spinosissimum</i>) in one population as opposed to specialization onto one of them as is seen in other populations. The results suggest that the degree of exploratory foraging behaviour is higher in larvae from the two-host population than in single host populations, and a field survey of the two-host population also indicated that larvae do move between host species. A field experiment indicated that predation rates on <i>O. elongata</i> larvae in the two-host population are higher on one of the host species, <i>A. alliariae</i>, than on the alternative <i>C. spinosissimum</i>. In combination with earlier results this finding suggest that larvae move between hosts to obtain better food on one host, and to get better protection from predators on the other. It appears that in this two-host situation a single plant species does not provide the most beneficial conditions in all parts of <i>O. elongata</i> life cycle and individuals may obtain different plant-specific benefits by moving between host species. This heterogeneous host situation appears to have selected for the explorative larval foraging strategy seen in the in the two-host population. In general, the results support the notion that to understand patterns of host plant use in insects it is often vital to consider a range of host related selection pressures whose relative importance may vary between life stages of the insect.