Theroa zethus Caterpillars Use Acid Secretion of Anti-Predator Gland to Deactivate Plant Defense

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.

Silkworms suppress the release of green leaf volatiles by mulberry leaves with an enzyme from their spinnerets

“…Secretions deposited by herbivorous arthropods on plant tissues during feeding, in some cases, suppress plant defenses against herbivores 9 . For example, Theroa zethus (Notodontidae) larvae produce acid secretions from the ventral eversible gland that created furrows, thereby physically suppressing the release of poisonous latex from the veins of Euphorbiaceae 10 . Furthermore, certain chemicals in herbivore secretions have been reported to suppress defense responses in plants (effectors) 9 .…”

A notodontid novelty: Theroa zethus caterpillars use behavior and anti-predator weaponry to disarm host plants

“…A notable example, Theroa zethus feeds on ephemeral herbs in the Euphorbiaceae (euphorbs) that germinate and grow rapidly in response to seasonal monsoon rains in the American Southwest. When damaged, their Euphorbia and Chamaesyce host plants emit copious quantities of latex [8], a defensive fluid known to be poisonous in diverse plants [9–12], including in related euphorb species [13–15]. The latex coagulates as it exudes from the plant [16], potentially entrapping or gumming up insect herbivores [10, 17, 18].…”

“…Feeding folivores rupture the canals, which causes the immediate flow of latex from high pressure in the canals to low pressure at the breach [29, 30]. Theroa zethus larvae disarm the laticifers by co-opting their defensive weaponry: they secrete concentrated acid onto the plant surface from a gland located ventrally between the head and prothoracic legs [8]. This gland is variously called the ventral eversible gland [31–33], prothoracic gland [34, 35], adenosma [36], cervical gland [20, 37] or repugnatorial gland [38]; we will use the term ventral eversible gland (VEG).…”

“…This gland is variously called the ventral eversible gland [31–33], prothoracic gland [34, 35], adenosma [36], cervical gland [20, 37] or repugnatorial gland [38]; we will use the term ventral eversible gland (VEG). In most notodontids, the VEG serves to deter predators; when disturbed, larvae spray acid precisely aimed at the attacker [8, 34, 35, 38].…”

“…Theroa zethus application of VEG acid to the plant surface causes visible withering, producing conspicuous furrows in leaf midribs or girdles that encircle stems and petioles (Fig 1A). Both furrows and girdles reduce distal latex emission [8]. Previous work has documented that T .…”