Chemical defense: incorporation of diet-derived pyrrolizidine alkaloid into the integumental scales of a moth ( Utetheisa ornatrix )

Abstract

Evidence is presented that pyrrolizidine alkaloid acquired by Utetheisa ornatrix (Lepidoptera, Arctiidae) as a larva from Crotalaria foodplants is incorporated in part into the scales of the adult. A single forewing of a male or female moth may contain in the order of 6 to 13 µg monocrotaline in its scale cover or about 1 to 2% of the moth's systemic monocrotaline content. Based on estimates of the number of scales per forewing, the monocrotaline content of individual scales is calculated to be in the order of 0.1 and 0.2 ng monocrotaline per male and female scale, respectively. This amounts to concentrations of about 1 and 3%, values roughly at a par with the average systemic concentration (0.5-0.6%) previously determined for monocrotaline in Utetheisa. It is argued that the presence of pyrrolizidine alkaloid in the moth's scale coating could account for the promptness with which adult Utetheisa are rejected by spiders. It is suggested further that chemical impregnation of scales with substances deterrent to predators may be more widespread among insects than generally assumed.

The costs and effectiveness of chemical defenses in herbivorous insects: a meta‐analysis

“…larvae of the tobacco hornworm, Manduca sexta, exhale nicotine through the spiracles (kumar et al 2014), while tortoise beetles incorporate plant allelochemicals in fecal shields (vencl and Morton 1998). Insects that store their defensive compounds in the body can also demonstrate externalization, such as reflex (easy) bleeding, when hemolymph containing defensive compounds is released in response to disturbance (Boevé et al 2013), or the deposition of defensive compounds in the integument (Nishida 2002, Trigo 2011 or wing scales (Rossini et al 2003) occurs. Insects that store their defensive compounds in the body can also demonstrate externalization, such as reflex (easy) bleeding, when hemolymph containing defensive compounds is released in response to disturbance (Boevé et al 2013), or the deposition of defensive compounds in the integument (Nishida 2002, Trigo 2011 or wing scales (Rossini et al 2003) occurs.…”

Tiger moth responses to a simulated bat attack: timing and duty cycle

“…Arctiids are often dropped after capture with no discernable damage (Acharya and Fenton, 1992;Hristov and Conner, 2005a;Ratcliffe and Fullard, 2005) likely due to defensive odors [i.e. pyrazine (Scoble, 1995), reflex bleeding (Scoble, 1995) and bad tasting scales (Rossini et al, 2004;Hristov and Conner, 2005b)]. Clicks stimulated by contact with the wing and tail membranes would only speed prey discrimination by the bat.…”

Effectiveness of tiger moth (Lepidoptera, Arctiidae) chemical defenses against an insectivorous bat (Eptesicus fuscus)

“…Sequestered defense compounds may be altered to special insect forms (Hartmann et al 2003;Hartmann et al 2004). Once procured the defensive chemicals are stored in the eggs (Hare & Eisner 1993;Eisner et al 2000;Eisner 2002;Bezzarides et al 2004), in larval setae (Rothschild 1985) in larval integument and hemolymph (Nickisch-Rosenegk et al 1990;NickischRosenegk & Wink 1993), in the cocoon , or in adult integument, scales, hemolymph, and reproductive structures (Dussourd et al 1988;Rossini et al 2003). In many of these locations they are likely to be encountered by an attacking predator.…”