Beetle versus bergamot: Evidence for intraspecific chemical specialization

Background/Question/Methods

Specialist insects comprise a continuum from oligophagous to monophagous feeders, and unlike generalists, often prefer hosts with high levels of secondary compounds. While many plant species exhibit large amounts of secondary chemical diversity, few studies have examined the performance or survival of specialists on different chemotypes of the same host species. The tortoise beetle Physonota unipunctata uses wild bergamot Monarda fistulosa as its only host. Like most labiates, M. fistulosa produces mono- and sesquiterpenes in trichomes on leaves and reproductive parts. Throughout its range, plants are one of two chemotypes, producing either of the phenolic monoterpenes thymol (T) or carvacrol (C) as the dominant terpenes of their essential oil. This study examined whether polymorphic host chemistry differentially affected a monophagous herbivore and/or influenced patterns of herbivory. In a field experiment, I subjected plants of the two chemotypes to controlled levels of herbivory and assessed the effects of larval herbivory on M. fistulosa growth and fitness and tested P. unipunctata larval growth and survival. In a separate growth chamber experiment, I measured differences in larval performance due to host chemistry by separating broods initially hatched on a particular chemotype and then rearing them on foliage from plants of either chemotype.

Results/Conclusions

In the field trials, host plant chemotype did not affect larval weight or length. However, host chemistry did marginally influence larval survival, with a slightly higher number surviving on T plants. While there were no herbivory differences for plant seed head area, stem mass, or stem height due to chemotype, there were large differences for these variables between the no herbivore and herbivore treatments. Larvae greatly reduced seed head area and stem mass, and had a modest effect lowering stem height. In the growth chamber experiments, larvae taken from T chemotype natal plants and reared on T host plants had the highest growth rates, which differed from larvae taken from C plants and reared on T. Larvae from either T or C natal plants reared on C host plants performed at intermediate levels. These results revealed that the most important factor in larval performance was the chemotype identity of the natal plant. Overall, larvae that originally emerged on T plants demonstrated higher relative growth rates than if they originated from a C natal plant. These combined results demonstrated that insects can show intraspecific chemical specialization, which has important implications for the evolutionary trajectory of both host and herbivore.