Limoniic Acid - Major Component of the Sex Pheromones of the Click Beetles Limonius canus and L. californicus.

Wireworms, the larvae of click beetles (Coleoptera: Elateridae), are soil-dwelling insect pests inflicting major economic damage on many types of agricultural crops worldwide. The objective of this work was to identify the female-produced sex pheromones of the Pacific Coast wireworm, Limonius canus LeConte, and the sugarbeet wireworm, L. californicus (Mannerheim) (Coleoptera: Elateridae). Headspace volatiles from separate groups of female L. canus and L. californicus were collected on Porapak Q and analyzed by gas chromatography with electroantennographic detection (GC-EAD) and GC-mass spectrometry. GC-EAD recordings revealed strong responses from male L. canus and male L. californicus antennae to the same compound, which appeared below GC detection threshold. The structure of this candidate pheromone component was deduced from the results of micro-analytical treatments of extracts, retention index calculations on four GC columns, and by syntheses of more than 25 model compounds which were assessed for their GC retention characteristics and electrophysiological activity. The EAD-active compound was identified as (E)-4-ethyloct-4-enoic acid, which we name limoniic acid. In field experiments in British Columbia and Alberta, Canada, traps baited with synthetic limoniic acid captured large numbers of male Limonius click beetles, whereas unbaited control traps captured few. Compared to traps baited with the analogue, (E)-5-ethyloct-4-enoic acid, traps baited with limoniic acid captured 9-times more male L. californicus, and 6.5-times more male western field wireworms, L. infuscatus Motschulsky, but 2.3-times fewer male L. canus. Limoniic acid can now be developed for detection, monitoring and possibly control of L. californicus, L. infuscatus and L. canus populations.

Nontarget herbivory by a weed biocontrol insect is limited to spillover, reducing the chance of population-level impacts.

Insects approved for classical biocontrol of weeds are often capable of using close relatives of their target weed for feeding, oviposition, or larval development, with reduced preference and performance. When nontarget herbivory occurs and is suspected to reduce survival, growth, or fecundity of individual plants, and insects are capable of reproducing on their nontarget host, characterization of spatial and temporal patterns of the occurrence and intensity of herbivory is valuable for predicting potential population-level effects. Here, we perform a novel post-release manipulative field experiment with a root-feeding biocontrol weevil, Mogulones crucifer, released in Canada to control the rangeland weed Cynoglossum officinale, to test for its ability to establish on the nontarget plant Hackelia micrantha. After Cynoglossum, M. crucifer exhibits its highest preference for and performance on Hackelia spp. We released M. crucifer on Canadian rangeland sites with naturally occurring populations of H. micrantha growing interspersed with the target weed or in the near absence of the target weed. Adult weevil feeding on surrounding plants was monitored for three summers after release (years 0, 1, and 2), and, subsequently, subsets of plants were destructively sampled to determine M. crucifer oviposition levels. Additional oviposition and larval development data were obtained from seven non-experimental sites where weevils were released zero, three, or four years earlier. M. crucifer was not detected on experimental sites without C. officinale after two years, and nontarget herbivory was restricted to rare, low-level spillover. Visible evidence of adult herbivory (i.e., scars on shoots) was associated with oviposition in 90% of targets but only 30% of nontarget plants. We infer, through ecological refuge theory, that nontarget population-level impacts from M. crucifer spillover are unlikely because of temporal, spatial, and probabilistic refuges from herbivory, and make recommendations for monitoring and management of biocontrol systems with similar attributes, such as removing target plants around nontarget populations of interest. Because M. crucifer is among the least host-specific of the modern weed biocontrol agents, and H. micrantha is likely one of its most highly preferred nontargets, these conclusions are, arguably, generally applicable to other nontarget plants and biocontrol systems.

Differential host-finding abilities by a weed biocontrol insect create within-patch spatial refuges for nontarget plants.

Many modern weed biocontrol insects exhibit transient "spillover" nontarget herbivory when and where insects are in high density, such as following biocontrol releases, or around dense target weed infestations. Understanding spatial patterns of herbivory is important for predicting efficacy and safety of biocontrol, as refuges from herbivory can buffer plants from population-level impacts. Here, we demonstrate that differential host-finding and arrestment behaviors by an oligophagous biocontrol insect lead to spatial refuges from nontarget herbivory around insect release points within mixed patches of target and nontarget plants. We created transient insect outbreaks by releasing large numbers of Mogulones crucifer Pallas (Coleoptera: Curculionidae) into naturally occurring rangeland patches of the nontarget plant Hackelia micrantha (Eastwood) J.L. Gentry with varying densities of its target weed Cynoglossum officinale L., and monitored spatial patterns of herbivory around release points after 4-7 wk. In complement, we conducted a mark-release-recapture (MRR) experiment to compare M. crucifer's target and nontarget host-finding and arrestment behaviors. For rangeland releases, 95% of nontarget herbivory occurred within 4.25 m of release points, independent of target plant density. Target herbivory occurred throughout our evaluation radii (up to 14 m), where maximum density of diffusing M. crucifer was 1/10 of that in the nontarget herbivory radius. In the MRR experiment, more weevils were recaptured on C. officinale (but not H. micrantha) than expected by chance. M. crucifer's lack of specialized nontarget host-finding and arrestment behaviors means that spatial refuges from herbivory are created for H. micrantha just meters away from sources of high weevil density.