Abstract
A large proportion of phytophagous insects show host plant specificity (monophagy or oligophagy), often determined by host secondary chemistry. Yet, even specialists can be negatively affected by host chemistry at high levels or with novel compounds, which may manifest itself if their host species is chemically variable. This study tested for reciprocal effects of a specialist tortoise beetle (Physonota unipunctata) feeding on a host plant (Monarda fistulosa) with two monoterpene chemotypes [thymol (T) and carvacrol (C)] using a controlled field experiment where larvae fed on caged plants of both chemotypes, haphazardly collected natural plants with and without beetle damage, and growth chamber experiments where larvae that hatched and briefly fed on one chemotype were reared on either chemotype. In the field experiment, plant chemotype did not affect larval weight or length, but did influence larval survival with almost 8.3 % more surviving on T plants. Herbivores reduced seed head area (86.5 % decrease), stem mass (41.2 %) and stem height (21.1 %) of caged plants, but this was independent of host chemotype. Natural plants experienced similar reductions in these variables (74.0, 41.4 and 8.7 %) and T chemotypes were more frequently damaged. In the growth chamber, larval relative growth rate (RGR) differed for both feeding history and year. Larvae from T natal plants reared on T hosts grew at almost twice the rate of those from C and reared on T. Larvae from either T or C natal plants reared on C plants showed intermediate growth rates. Additional analyses revealed natal plant chemotype as the most important factor, with the RGR of larvae from T natal plants almost one-third higher than that of those from C natal plants. These cumulative results demonstrate intraspecific variation in plant resistance that may lead to herbivore specialization on distinct host chemistry, which has implications for the evolutionary trajectory of both the insect and plant species.
Highlights
IntroductionA large proportion of the millions of insect species eat plants (phytophagous) and a majority of these show high host specificity (monophagous or oligophagous, Jaenike 1990; Ali and Agrawal 2012)
A large proportion of the millions of insect species eat plants and a majority of these show high host specificity
Herbivore effects on host plant The field experiment portion of this study showed that, while levels of herbivory did not differ due to plant chemistry, P. unipunctata larvae greatly reduced seed head area, mass and height of M. fistulosa stems
Summary
A large proportion of the millions of insect species eat plants (phytophagous) and a majority of these show high host specificity (monophagous or oligophagous, Jaenike 1990; Ali and Agrawal 2012). Plant chemistry has driven the evolution of this host specialization, with specialists often choosing hosts more on the basis of chemistry than on plant relatedness (Ehrlich and Raven 1964; Jaenike 1990; Rasmann and Agrawal 2011). Keefover-Ring — Bergamot versus beetle that many specialist insects use for defence against predators and parasites (Pasteels et al 1983; Weiss 2006). Despite these advantages, even specialists can be negatively affected by the secondary chemistry of their host, especially at high levels (Ali and Agrawal 2012). Novel compounds not usually encountered by specialists probably affect them to a greater or equal extent compared with generalists (Ali and Agrawal 2012)
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