Size and dispersion of urticating setae in three species of processionary moths

Abstract

Larvae of the processionary moths of the Palaearctic region bear urticating setae that are released against vertebrate predators, especially insectivorous birds. A few species are pests of forest and urban trees and, consequently, may threaten human and animal health during outbreaks, causing dermatitis, conjunctivitis and respiratory distress. Although some studies provide detailed information about the setae, particularly those of the pine processionary moth Thaumetopoea pityocampa, there is little knowledge on the morphological traits of the setae and their release by the larvae. In the present study we identify major traits of the setae of 3 species of processionary moth, T. pityocampa, T. pinivora and T. processionea, which are potentially helpful in the understanding of setae dynamics in the environment: (i) diameter and length of setae and (ii) analysis of dynamical properties of the setae in the airborne state. Setae are highly variable in size, with bimodal distribution in T. pityocampa and T. pinivora; in these 2 species, short and long setae are interspersed within the integument fields where they occur. The difference in the seta size has important consequences in dispersion, as smaller setae can spread 5 times further than their bigger counterparts. This information is relevant for a full understanding of the defensive importance of larval setae against natural enemies of the processionary moths, as well for elucidating the importance of the processionary setae as air pollutants, both close to the infested trees and at longer distances.

The urticating setae of Ochrogaster lunifer, an Australian processionary caterpillar of veterinary importance

“…The lengths of setae ranged from approximately 40 to 800 µm, which represents a somewhat broader range than has been reported for northern hemisphere species of processionary caterpillar (Petrucco‐Toffolo et al , ). Short setae of < 100 µm were noticeably distinct from, yet interspersed with, longer setae within the setal fields as found by Petrucco‐Toffolo et al () for Thaumetopoea pityocampa Denis & Schiffermüller and Thaumetopoea pinivora Treitschke. The majority of longer setae measured between 100 and 400 µm, but a few measured > 400 µm.…”

“…The setae conform to the classification of true setae by Battisti et al () and are similar to true setae from northern hemisphere Thaumetopoea spp. (Lepidoptera: Thaumetopoeidae) caterpillars (Petrucco‐Toffolo et al , ), which cause various conditions such as dermatitis, conjunctivitis, oropharyngeal inflammation and asthma‐like symptoms in humans and animals (Battisti et al , ). The setae are inserted into pits in the cuticle and are pointed and detached at the base (proximal end), and thus are easily shed from the integument and readily enter the environment.…”

“…The majority of longer setae measured between 100 and 400 µm, but a few measured > 400 µm. The size of setae affects their dispersal throughout the environment (Petrucco‐Toffolo et al , ) and may affect their penetration through animal tissue. How small and long setae behave after entering the environment warrants further research as it is likely to affect the level of mare exposure to setae and the subsequent risk for fetal compromise or death.…”

Evolution and losses of spines in slug caterpillars (Lepidoptera: Limacodidae)

“…Spines and setae in caterpillars are one kind of morphological-chemical adaptive response to avoid predation. Spines and setae may injure predators or impose a cost in terms of increased handling time (Murphy et al, 2010;Petrucco Toffolo et al, 2014;Sugiura & Yamazaki, 2014). Spines and setae may injure predators or impose a cost in terms of increased handling time (Murphy et al, 2010;Petrucco Toffolo et al, 2014;Sugiura & Yamazaki, 2014).…”

“…At least 13 families of Lepidoptera, including the Limacodidae, have been recorded in which the caterpillars possess stinging (urticating) properties via spines and setae (Battisti, Holm, Fagrell, & Larsson, 2011;Hossler, 2010;Kano, 1977;Kawamoto & Kumada, 1984;Mullen, 2009). Spines and setae may injure predators or impose a cost in terms of increased handling time (Murphy et al, 2010;Petrucco Toffolo et al, 2014;Sugiura & Yamazaki, 2014).…”