Regulation of forest defoliating insects through small mammal predation: reconsidering the mechanisms

Abstract

Population densities of forest defoliating insects may be regulated by small mammal predation on the pupae. When outbreaks do occur, they often coincide with warm, dry weather and at barren forest sites. A proposed reason for this is that weather and habitat affect small mammal population density (numerical response) and hence pupal predation. We propose an alternative explanation: weather and habitat affect small mammal feeding behaviour (functional response) and hence the outbreak risks of forest pest insects. We report results from laboratory and field-enclosure experiments estimating rates of pupal predation by bank voles (Myodes glareolus) on an outbreak insect, the European pine sawfly (Neodiprion sertifer), at different temperatures (15 and 20 °C), in different microhabitats (sheltered and non-sheltered), and with or without access to alternative food (sunflower seeds). We found that the probability of a single pupa being eaten at 20 °C was lower than at 15 °C (0.49 and 0.72, respectively). Pupal predation was higher in the sheltered microhabitat than in the open one, and the behaviour of the voles differed between microhabitats. More pupae were eaten in situ in the sheltered microhabitat whereas in the open area more pupae were removed and eaten elsewhere. Access to alternative food did not affect pupal predation. The results suggest that predation rates on pine sawfly pupae by voles are influenced by temperature- and habitat-induced variation in the physiology and behaviour of the predator, and not necessarily solely through effects on predator densities as previously proposed.

Habitat heterogeneity affects predation of European pine sawfly cocoons

“…Dead wood contributes to habitat heterogeneity (McElhinny, Gibbons, Brack, & Bauhus, ) providing protection for small mammals when feeding and thus have the potential to increase their feeding activity (Kollberg et al., ). Our results show that there is an overall negative effect on the cocoon predation rate of the proportion of pine around the experimental tree, but the negative relation became less steep when dead wood was added (Figure ).…”

“…The presence of dead wood is also to be important for small mammal presence (Manning & Edge, ). Increased habitat heterogeneity, including dead wood, offer protection to small mammals when consuming food, thus facilitating their feeding activity (Kollberg, Bylund, Huitu, & Björkman, ). Further, tree species composition and understory vegetation could provide alternative food resources such as seeds and fruits.…”

“…Dead wood is also considered an important component of habitat heterogeneity and found to be a key element for the conservation of a variety of living species (Lonsdale, Pautasso, & Holdenrieder, 2008), including many red-listed species (Jonsell, Weslien, & Ehnström, 1998). The presence of dead wood is also to be important for small mammal presence (Manning & Edge, 2008 protection to small mammals when consuming food, thus facilitating their feeding activity (Kollberg, Bylund, Huitu, & Björkman, 2014).…”

Influence of soil and topography on defoliation intensity during an extended outbreak of the common pine sawfly (Diprion pini L.)

“…In addition to climate, defoliator populations are regulated by a range of biotic and abiotic factors, including natural enemies (Kollberg et al 2014), topography (Kharuk et al 2007) and soil (Mayfield et al 2007). Their impacts and interactions with each other are, however, only partly known.…”

“…In Scandinavia, the performance of pine sawflies and the frequency, intensity and spatial extent of damage has been related to temperature (Kollberg et al 2015) and host plant quality, especially needle carbohydrate and terpene concentrations (Larsson et al 1986, Lyytikäinen 1994, Kollberg et al 2015. In addition, site and stand characteristics, such as site type and stand age (Larsson & Tenow 1984, De Somviele et al 2007, Nevalainen et al 2015, and natural enemies (Hanski & Parviainen 1985, Kollberg et al 2014) have been observed to modify sawfly performance and affect the level of tree damage. However, the effects of topography and soil properties on sawfly outbreaks have received little attention.…”