Abstract
Pre-release environmental safety assessments of weed biological control agents (BCA) heavily rely on choice and no-choice feeding and developmental tests. Assessments of behavioral responses of biocontrol agents to plant cues could provide vital information on their pre-alightment host recognition behavior, improving predictions of the environmental safety risk they pose to non-target plant species. I used Mogulones crucifer Pallas (Coleoptera: Curculionidae), a root-mining biocontrol agent for the Eurasian invasive weed Cynoglossum officinale L. (Boraginaceae) to study host-finding behavior of the target host and its confamilial nontarget plant species. Here, I investigated the behavioral response of M. crucifer to foliar and floral olfactory, visual, and a combination of olfactory and visual cues from C. officinale. Results show visual cues, in addition to olfactory ones, contribute to pre-contact host discrimination. Three electrophysiologically active volatile cues for M. crucifer were identified from floral cues from C. officinale using gas chromatography with simultaneous flame ionization and flame ionization detection (GC-EAD/FID) and gas chromatography-mass spectrometry (GC-MS). In addition, three electro-physiologically active wavelengths of light were identified for M. crucifer at 420 nm (violet), 520 nm (green), and 640 nm (red), using electroretinography (ERG). How M. crucifer differentiates C. officinale from nine Eurasian and one North American confamilial species using behavioral bioassays and electrophysiological methods was investigated. M. crucifer consistently discriminated C. officinale from non-target species based on foliar and floral olfactory and floral visual cues. Three electrophysiologically active volatile compounds were identified in three, and one was identified in one Eurasian confamilial plant species. Mean floral reflectance curves and relative reflectance differed between C. officinale and the five non-target plant species at each of the three bioactive wavelengths, except for two closely related congeners. I also conducted a meta-analysis using 31 studies that investigated the host-finding behavior of BCAs using various forms of plant volatile organic compounds (VOCs) and visual cues. These studies included a total of 526 plant cue–BCA interaction datasets, covering 24 different BCA species and using a range of methodological approaches and experimental outcomes. An extensive analysis showed that plant species, damaged conditions, and the concentration, number, and class of chemical compounds used significantly affected early host-finding behavior in BCA. The effect of VOCs emitted by host plants was large and statistically significant, indicating strong attraction and host recognition by BCAs. Overall, this study provides a detailed understanding of non-target plant host discrimination based on olfactory and visual plant cues and aids in more accurately predicting the environmental safety of M. crucifer.