The effects of drought on the Solidago altissima-Eurosta solidaginis-natural enemy complex: population dynamics, local extirpations, and measures of selection intensity on gall size

Abstract

Environmental catastrophes, such as severe drought, can reduce host-plant quality and/or abundance, which in turn decrease levels of herbivore populations. Such changes in herbivore populations affect populations of their natural enemies. As part of a long-term field experiment (1983-1991), galls of Eurosta solidaginis from 16 fields in central Pennsylvania were systematically collected from goldenrod ramets. Galls were dissected to compare the occurrence of E. solidaginis mortality caused by its natural enemies in 2 drought years (1988, 1991) with 5 pre-drought years (1983-1987) and 2 post-drought years (1989-1990). Gall diameters were significantly smaller in both drought years and early larval death significantly decreased E. solidaginis survivorship in the first drought year. Of the natural enemies, the parasitoid wasp Eurytoma gigantea caused significant selection for larger gall size in all pre-drought years, the 1991 drought, and both post-drought years, due to its differential attack of smaller galls. In spite of drought-induced small gall size in 1988, there was negligible selection on gall size by natural enemies. However, populations of E. solidaginis did suffer local extirpations at nine of the 16 fields during the first drought year and population recoveries of the gall inducer and natural enemies varied among fields in the post-drought years. As a consequence of reduced herbivore abundance in drought and post-drought years, some natural-enemy populations were absent. Drought therefore drastically reduced the abundance of E. solidaginis and natural enemies resulting in slow recoveries to pre-drought numbers.

Can dispersal mode predict corridor effects on plant parasites?

“…More frequent gall-inducing insect oviposition along patch edges may have been caused by higher levels of water and shade available to plants, corroborating other findings. Psyllid leaf-gall density was 55-65% higher on shade leaves than sun leaves in Redbay (Persea borbonia) (Legee 2006), while tephritid gallinducing flies were less common in water-stressed environments (Sumerford et al 2000). In connected patches, higher insect movement to connected patches could increase parasite densities and oviposition at less preferable locations away from patch edges.…”

Gall wasp biocontrol of invasive Acacia longifolia: implications of strong bottom‐up effects

“…In insect gall makers, between‐ and within‐plant variation in gall size positively influences gall maker performance through increased resource quality and quantity, buffering against water loss and protection against parasitoid attack (Price and Clancy , Price , Sumerford et al. , Ito and Hijii , Albarracin and Stiling , László and Tóthmérész ). Gall development (growth rate and final size) has been shown to be limited by the resource status of host plant tissue with galls on high‐quality host plants, and host plant modules, growing faster and to a larger final size than galls on low‐quality host tissue in accordance with the plant vigor hypothesis (Price and Clancy , Price , Albarracin and Stiling ).…”

Bottom‐up and top‐down effects in a tritrophic system: the population dynamics of Plutella xylostella (L.)–Cotesia plutellae (Kurdjumov) on different host plants

“…For instance, variation in host‐plant quality may affect the body size of herbivorous insects, which, in turn, can determine life‐history parameters such as fecundity, longevity, and survival, all directly relevant to population dynamics (Bezemer & Jones, 1998; Bjorkman, 2000; de Bruyn et al ., 2002). Furthermore, the effects of resource variation, which impacts on the behaviour or the physiology of herbivores, may cascade up to the population dynamics of higher trophic levels due to significant influences on the fitness and performance (such as survival, development, size, longevity, fecundity, and sex ratio) of natural enemies (Moon et al ., 2000; Sumerford et al ., 2000; Teder & Tammaru, 2002).…”