Comparing functional traits and abundance of invasive versus native woodwasps

Abstract

Functional traits are useful for comparing the resource use of invasive and native species, with goals of identifying resource overlap to predict competitive interactions. The invasion of northeastern North America by the woodwasp Sirex noctilio has resulted in competition with the native congeneric Sirex nigricornis for suppressed and weakened pines. We compared sizes of adults, venom glands, fecundity, tree species use, voltinism and abundance of the invasive woodwasp S. noctilio with the native S. nigricornis in northeastern North American pines. Rearing adults from attacked pines showed that these species used the same tree species but S. noctilio were far more abundant, especially with increasing time since establishment. Adults of the invasive S. noctilio were larger than S. nigricornis, female S. noctilio had larger glands carrying phytotoxic venom in relation to body size, average-sized S. noctilio females carried more eggs, and S. noctilio developed faster than S. nigricornis. Sirex noctilio was the dominant woodwasp infesting suppressed pines in our study areas. We hypothesize that the future abundance of S. nigricornis could depend in part on the availability of wood for oviposition by this native that is not available or acceptable to the earlier-emerging S. noctilio.

Biotic and abiotic effects on density, body size, sex ratio, and survival in immature stages of the European woodwasp, Sirex noctilio

“…For example, small body size is frequently associated with both crypsis and rapid population growth rates (Savage et al., 2004), traits clearly associated with introduction and spread in insects. In established populations, selection may favor larger females which confers higher fecundity and in some cases enhanced flight and therefore dispersal capacity (Bruzzone et al., 2009; Hajek et al., 2017). However, stabilizing selection and/or sexual dimorphism is evident where large size equates to reduced agility (e.g., limiting male success in mating swarms) or increased predation risk (Blanckenhorn, 2000; Neems et al., 1998).…”

Symbionts mediate oviposition behaviour in invasive and native woodwasps

“…It is generally assumed that S. nigricornis prefers to oviposit when trees are very stressed, more so than S. noctilio (Hajek et al ., ). The reason behind less total drilling by S. nigricornis compared with S. noctilio could have been that the age of wood being offered was not optimal.…”

“…Native Sirex species usually develop in stressed trees, comprising an ephemeral resource, whereas S. noctilio is considered to be better able to successfully attack healthier trees than S. nigricornis . Sirex nigricornis adults generally emerge and fly later in the year than S. noctilio (Ryan et al ., ; Haavik et al ., ; Hartshorn et al ., ; Hajek et al ., ) and so S. noctilio emerges and finds sites for oviposition before S. nigricornis . Sirex nigricornis drilled and presumably oviposited close to locations where Aa BD is present in wood which suggests that this species might be willing to take advantage of fungal symbionts already established by S. noctilio .…”

“…Thus, the consequences of invasion by the congeneric S. noctilio to S. nigricornis populations remain unknown. We do know that relatively few S. nigricornis emerge from trees co‐infested by S. noctilio (A. E. Hajek, unpublished data; Ryan et al ., ; Hajek et al ., , ). However, changes in the fitness of S. nigricornis when switching to what is assumed as being the invasive symbiont requires further study.…”

“…Sirex noctilio and S. nigricornis that had emerged from bolts were collected daily from barrels and, after emergence, were maintained individually at 4 °C in lidded clear plastic cups (volume 29 mL). Northeastern Sirex nigricornis emerging from wood that we had collected were not sufficiently abundant in 2011 and 2012 (Hajek et al ., ) and so females for the present study were caught in flight intercept panel traps in Grant Parish, Louisiana, in 2011, and maintained at 4 °C until use.…”