The relative importance of resources and natural enemies in determining herbivore abundance: thistles, tephritids and parasitoids

Abstract

Summary 1.The relative importance of host-plant resources and natural enemies in influencing the abundance of insect herbivores was investigated in potted plant and natural population experiments, using tephritid (Diptera: Tephritidae) flies, their host plant, creeping thistle Cirsium arvense , and their Hymenoptera parasitoids. 2. Experimental manipulation of host-plant quality (i.e. levels of host-plant nutrients) and resource availability (i.e. the number of buds) increased tephritid abundance. There was no evidence that the seed-feeding tephritid fly Xyphosia miliaria preferentially oviposited on fertilized C. arvense . 3. At low thistle densities, X. miliaria showed a constant rate of resource exploitation. At higher thistle densities, a threshold was detected, above which additional buds were not attacked. 4. Parasitism attack was variable across host (tephritid) densities but levels of parasitism were consistently higher on the fertilized thistles. 5. Experimental manipulation of host-plant quality and resource availability (quantity) not only directly affects the tephritid population but also, indirectly, leads to high rates of parasitism. Both chemical and physical characteristics of host plants affect the performance of natural enemies. 6. Both top-down and bottom-up forces act to influence tephritid abundance, with bottom-up influences appearing to be the most important.

The Roles of Trophic Interactions, Competition and Landscape in Determining Metacommunity Structure of a Seed-Feeding Weevil and Its Parasitoids

“…The same argument can be made for the parasitoids, in which case we would expect a high rate of parasitism where the plant and host weevil are abundant. On the other hand, parasitoids can cause host population size to decrease, which would diminish the association of weevil population size with plant abundance (Jones et al 1994, Walker et al 2008, and could increase the association of patch connectivity with host and parasitoid population sizes (Hassell 2000). We would also expect competing parasitoids to be negatively associated (Hassell & Waage 1984), and to have contrasting distributions in the landscape if, for example, there is a trade-off between dispersal ability and local competitive ability (Calcagno et al 2006).…”

Mycorrhiza-induced trophic cascade enhances fitness and population growth of an acarine predator

“…Upward-directed ''bottom-up'' cascades are resource induced (e.g., plant traits influence traits of natural enemies of herbivores), while downward-directed ''top-down'' cascades are characterized by higher level natural enemies controlling other food web components (Hunter and Price 1992). Both upward and downward forces can be linked via feedback loops and influence intermediate trophic levels (Forkner and Hunter 2000;Walker et al 2008).…”

Plant diversity has contrasting effects on herbivore and parasitoid abundance in Centaurea jacea flower heads

“…However, these effects were variable and did not occur for all measures of the herbivore community or in all years. Herbivore populations are highly temporally variable (Solbreck & Sillén‐Tullberg, 1986; Walker, Hartley, & Jones, 2008) and plant diversity effects may only be detected in particular years. Plant diversity mostly tended to have negative (or in one case neutral) effects, so although there is variation in the strength and significance of effects, the direction is largely consistent.…”

“…This is in accordance with results from a study by Walker et al. (2008), which implies that host density was either not a main driver of parasitism rates or that parasitoids cannot easily determine host density. The latter notion is supported by a reduction in parasitism rates when the number of flower heads was high in potted plants.…”