Fall webworm host plant preferences create enemy-free space in its
interaction with parasitoids
Based on the “enemy-free space” hypothesis, in order to avoid natural
enemies, insects may prefer host plants that are nutritionally
suboptimal but are less visited by their natural enemies. The fall
webworm, Hyphantria cunea Drury has more than 600 reported hosts.
Chouioia cunea, a parasitoid wasp, is the main natural enemy of H.
cunea. We addressed the question whether the preference of H. cunea for
host plants correlate with attractiveness of the plants to C. cunea. H.
cunea larvae were reared on leaves of eight different host plants, and
the relationship between H. cunea host preference and preferences of the
parasitic wasp were evaluated. The preferred host plant of female H.
cunea was mulberry, Morus alba. Compared with other plants, M. alba was
a poor nutritional host for H. cunea. However, compared with other host
plants, M. alba attracted fewer natural enemies C. cunea. GC-MS combined
with GC-EAD analysis revealed six compounds in the volatiles of
different plant HIPVS after feeding by H. cunea. Behavioral assays
showed that tridecane alone did not elicit any attraction or repellency
responses of the host H. cunea or the parasitoid C. cunea. When
tridecane was mixed with other HIPVS, however, the mixture could
attracted C. cunea and repelled H. cunea. Notably, only M. alba HIPVS
did not contain tridecane. Thus, H. cunea exploits M. alba as an
enemy-free space, minimizing attacks by the parasitoid C. cunea. This
information underscores that adaptive responses of herbivores need to be
considered in the context of multi-trophic relationships rather than
optimizing herbivore growth on the most nutritionally adaptive plant