A recently acquired host plant provides an oligophagous insect herbivore with enemy-free space

Enemy-free space (EFS) is a potentially important factor affecting host plant use by phytophagous insects. Yet only a few field studies have demonstrated that natural enemy activity is the sole mechanism underlying use of novel host plants by herbivorous insects. This may be due to the fact that in earlier studies, both herbivores and natural enemies had the opportunity to adapt to the new host plant. Here we studied the possibility that EFS underlies the recently recorded increase in Phthorimaea operculella densities on tomato plants in a few areas within its geographical range. Through field experiments in Ethiopia, we show that all three conditions proposed by Berdegue et al. to demonstrate EFS are fulfilled. First, a significantly higher proportion of larvae survive on caged than on exposed potato plants, showing that natural enemies are an important mortality factor on the original host, potato. Second, larval survival was significantly higher on exposed tomato than potato plants, implying greater protection for the herbivore from its natural enemies on tomato than on potato plants. Thus tomato plants provide P. operculella with an EFS. Finally, larval survival was significantly higher on caged potato than on caged tomato plants at the preblossom stage, indicating that, in the absence of natural enemies, there is a fitness cost when larvae feed on the sub-optimal tomato plants. Fulfillment of this third condition points to the importance of natural enemy activity relative to that of other unidentified factors, such as food quality and competition. An intensive field survey provides further support for this conclusion.