Egg mortality in the pine processionary moth: habitat diversity, microclimate and predation effects

Abstract

1 Little is known about the relative effect of different mortality factors triggering the population dynamics of outbreaking Lepidoptera (e.g. climate, natural enemies, habitat diversity), especially at the egg stage. 2 The presence of broadleaved hedgerows bordering pine stands has been shown to reduce infestation by Thaumetopoea pityocampa (Denis & Schiffermüller, Lepidoptera). This effect of habitat diversity has been proposed to result from the disruption of visual and chemical cues that the female moth uses to locate its host.In the present study, we further hypothesized that egg mortality was higher behind broadleaved hedgerows because of higher predation or parasitism, or as a result of harsher microclimatic conditions. 3 A total of 120 egg masses obtained from laboratory rearing were exposed under three exclusion treatments (large predators, predators and parasitoids, no exclusion).Overall egg mortality was 35%. Predation was the main cause of mortality (22%). Mortality as a result of abiotic factors (abortion) was only 12%. Almost no parasitoids were observed. 4 Overall mortality did not differ between pines bordered (or not) by deciduous hedgerows, nor did mortality differ as a result of predation or abortion. 5 Despite an important contribution of predation to mortality in T. pityocampa eggs, the effect of deciduous hedgerows on tree infestation by this pest is more likely to result from disruption of female moth behaviour than from differential mortality.

Effects of climate on pine processionary moth fecundity and on its egg parasitoids

“…The percentage of hatching failure during the study period was quite low, with parasitism being the main mortality factor. Indeed, in our study site, unhatched eggs ranged from 6.21% to 34.85%, while in studies from other Mediterranean countries percentages were higher, ranging from 20 to 53% (Tiberi 1990;Tsankov et al 1996;Schmidt et al 1997Schmidt et al , 1999P erez-Contreras and Soler 2004;Castagneyrol et al 2014). Among the causes of hatching failure, parasitism prevailed over aborted eggs; however, it was still lower than in other countries.…”

“…, ; Pérez‐Contreras and Soler ; Castagneyrol et al. ). Among the causes of hatching failure, parasitism prevailed over aborted eggs; however, it was still lower than in other countries.…”

Effect of host tree density and apparency on the probability of attack by the pine processionary moth

“…This region is dominated by 800,000 hectares of single species plantations of maritime pine (Pinus pinaster), of similar age. We used and re-analyzed two datasets described in detail by Samalens (2009) and Castagneyrol et al (2014), an overview of which are provided below. The first dataset was used to study the effects of host density (H1), tree distance to stand edge and host apparency (H2.1 vs. H2.3) on PPM infestation, whereas the second dataset was used to test the effect of temperature on egg survival (H2.2).…”

“…Details of the method used have been reported elsewhere (Castagneyrol et al, 2014). Fifty egg batches were distributed between five pine stands, in which two trees were selected at random at each of five different distances from the stand edges (0, 2, 6, 8 and 16 m).…”

“…50 days later). The egg batches were removed at the end of August and egg mortality was determined, as a percentage, in the laboratory (see Castagneyrol et al, 2014). The data for this experiment were recorded as dataset 2.…”