The cactus moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), spends most of its larval life feeding within the cladodes of Opuntia cactuses, but the gregarious caterpillars begin their life outside the plant, and in the later instars make intermittent excursions over plant surfaces to access new cladodes and to thermoregulate. The study reported here showed that when the caterpillars move en masse, they mark and follow trails that serve to keep the cohort together. Artificial trails prepared from hexane extracts of the caterpillar's paired mandibular glands were readily followed by the caterpillars. The glands are remarkably large, and their fluid contents, which constitute approximately 1% of the total wet mass of a caterpillar, are secreted onto the substrate as they move. Although the caterpillars also lay down copious quantities of silk, the material in itself neither elicits trail following nor is it a requisite component of pathways that elicit trail following. Previous analyses of the mandibular glands of other species of pyralid caterpillars showed that they contain a series of structurally distinct 2-acyl-1,3 cyclohexane diones. Chemical analysis indicates that the glands of C. cactorum contain structurally similar compounds, and bioassays indicate that trail following occurs in response to these chemicals. While the mandibular glands' fluids have been shown to act as semiochemicals, effecting both interspecific and intraspecific behavior in other species of pyralids, the present study is the first to report their use as a trail pheromone.
The larvae of Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) are internal feeders on cactuses, most notably those occurring in the prickly pear genus Opuntia. The insect is best known for bringing under control the runaway growth of prickly pear in Australia (Raghu and Walton 2007). In 1989, the insect was discovered in Florida feeding on native species of Opuntia (Habeck and Bennett 1990; Dickel 1991; Stiling and Moon 2001). Since then, the range of the caterpillar has expanded westward to Louisiana (Rose et al. 2011), and there is concern that it may eventually reenter Mexico, where a small population on Isla Mujeres, Quintana Roo, was eradicated in 2007 (NAPPO 2008). In Mexico, Opuntia is not only an ecologically significant member of the native flora but also an agriculturally important species.
Although C. cactorum has been the subject of numerous studies due to its success as a control agent for invasive species of Opuntia cactuses, and its more recent status as an invasive species itself, little is known of the fine structure of the foraging behavior of the caterpillars. Indeed, the larvae are found mostly within cladodes, where their behavior is largely unobservable. Pettey (1947) and Zimmerman et al. (2004), however, confirmed Dodd's (1940) observation that, following the depletion of the contents of a cladode, colonies may move to a new cladode by traveling over the surface of the host plant and that colonies divided and wandered over the plant surface when they were from one-half to twothirds grown. Colonies Monro (1967) observed required on average four cladodes to complete their development. The newly eclosed caterpillars may also fail to enter a cladode at the oviposition site, due to either cuticular toughness or sticky exudates, forcing the small caterpillar to move en masse over the surface of the plant to a new site (Pettey 1947; Zimmerman et al. 2004). In addition, the caterpillars move from the interior of a cladode to the outside to seek shade on hot days and may bask outside the cladode on cold days (Dodd 1940; Pettey 1947; Zimmerman et al. 2004).
The possibility that C. cactorum caterpillars mark and follow chemical trails during the time they spend on the surface of the plant was brought to the attention of the senior author by an illustration of a cohort of the caterpillars moving together over a plant, behavior suggestive of trail following, that accompanied a paper by Raghu and Walton (2007). To our knowledge, Dodd (1940) is the only investigator to have previously recorded trailing behavior. He noted that the caterpillars may travel over the surface of a p