Abstract
In North America, notodontid caterpillars feed almost exclusively on hardwood trees. One notable exception, Theroa zethus feeds instead on herbaceous plants in the Euphorbiaceae protected by laticifers. These elongate canals follow leaf veins and contain latex under pressure; rupture causes the immediate release of sticky poisonous exudate. T. zethus larvae deactivate the latex defense of poinsettia and other euphorbs by applying acid from their ventral eversible gland, thereby creating furrows in the veins. The acid secretion softens the veins allowing larvae to compress even large veins with their mandibles and to disrupt laticifers internally often without contacting latex. Acid secretion collected from caterpillars and applied to the vein surface sufficed to create a furrow and to reduce latex exudation distal to the furrow where T. zethus larvae invariably feed. Larvae with their ventral eversible gland blocked were unable to create furrows and suffered reduced growth on poinsettia. The ventral eversible gland in T. zethus and other notodontids ordinarily serves to deter predators; when threatened, larvae spray acid from the gland orifice located between the mouthparts and first pair of legs. To my knowledge, T. zethus is the first caterpillar found to use an antipredator gland for disabling plant defenses. The novel combination of acid application and vein constriction allows T. zethus to exploit its unusual latex-bearing hosts.
Highlights
Lineages of insect herbivores typically specialize on plants that are taxonomically related or that share similar defensive traits such as the presence of latex canals or the production of a particular class of chemicals [1, 2]
This study examines how T. zethus utilizes such atypical hosts that emit copious latex exudates, which in the Euphorbiaceae are often toxic and sticky [7,8,9]
Emerged larvae were reared in the lab on potted Chamaesyce maculata; poinsettia, Euphorbia pulcherrima (Eckespoint Classic Red in all experiments), E. cyathophora, C. hyssopifolia and other Euphorbiaceae
Summary
Lineages of insect herbivores typically specialize on plants that are taxonomically related or that share similar defensive traits such as the presence of latex canals (laticifers) or the production of a particular class of chemicals [1, 2]. Extreme host shifts to unrelated plants bearing drastically different defenses are uncommon, among specialist herbivores. Striking host shifts have occurred within the Notodontidae, a group of over 4,400 species of caterpillars that in North America feed almost exclusively on hardwood trees [3,4,5]. Theroa zethus, feeds not on long-lived trees with tannin-rich leaves, but on herbaceous members of the Euphorbiaceae that emit profuse latex when damaged. A single Theroa species has been described [6]: T. zethus (Druce) occurs in southern Arizona where.
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