Combined use of two defensive traits in pupae of Scymnus posticalis ladybirds

Abstract

Coccinellid pupae have various defensive traits, such as keeping their final‐instar larval exuviae and secreting droplets containing defensive chemicals at the tips of glandular hairs. Although each of these traits has been described separately in different species, it is unclear how each defensive trait functions when more than one trait coexists. We found that pupae of the ladybird Scymnus posticalis have two different types of traits; they were partially covered with their own final‐instar larval exuviae, with thick wax structures, and had many glandular hairs secreting transparent droplets on the exposed parts of the body without waxy exuviae (head and medial parts of thoracic and abdominal dorsum). In this study, the defensive effects of these two traits were evaluated against two ant species, Tetramorium tsushimae and Lasius japonicus, which are potential predators. When encountering a pupa, neither ant species were able to make frequent physical contact with the exposed parts of the pupa because of the presence of the waxy exuviae. When touching the exposed body part that was covered with droplets, both ant species immediately stepped backward and then swept their antennae. The droplets suppressed ant feeding when mixed with sucrose solution, suggesting that the droplets contain repellent chemicals. These results indicate that the two traits of S. posticalis pupae (the larval waxy exuviae and the droplets) effectively play defensive roles against ants, functioning as a physical obstruction and a chemical repellent, respectively.

First report of fatty acid-derived alkaloids produced by species of the ladybird genus Scymnus (Coccinellidae: Coleoptera)

“…Droplets were produced by the pupae of all the species studied and in addition another 3 Scymnus species are reported producing droplets in the literature [18][19][20][21]. Therefore, it is a widespread phenomenon in this genus.…”

“…In addition, all developmental stages reflex bleed when attacked, instantaneously expelling droplets through the femorotibial articulations or dorsal glands that contain high concentrations of toxic or repulsive alkaloids [3]. Furthermore, several species of the tribe Epilachnini [15], as well as Delphastus catalinea (Horn) of the tribe Serangiini [16], Platynaspis luteorubra Goeze of the tribe Platynaspidini [17], and Nephaspis oculatus (Blatchley), Scymnus sinuanodulus Yu and Yao [18,19], S. ningshanensis Yu and Yao [20] and S. posticalis Sicard [21] belonging to the tribe Scymnini, have a more specific chemical defence. In these species, the larval exuviae folds back and total or partially uncovers the pupa.…”

“…The pupal cuticle that is visible bears setae on the tips of which droplets can be seen. Behavioural experiments indicate that these droplets are extremely efficient in deterring attacks by ants [16,21,22]. For species of Epilachnini, the droplets are reported to contain molecules of the azamacrolides family [7] as well as tocopheryl acetates [23,24].…”