Trophic cascades in a cranberry marsh: Can detritus-removal improve biological control?

Background/Question/Methods Biological control is predicated on the concept that crop plants are protected when predators suppress herbivore populations. However, many factors, including detritus-based food-chains, may modify the effectiveness of a predator in a given agroecosystem. The addition of detrital subsidies, and the corresponding increase in detritivore density, has been shown to change the community dynamics between generalist predators, herbivores, and detritivores. In some cases increased detritivore numbers can fuel large predator populations, increasing biological control, although more frequently this abundance of alternative prey reduces predation on pest species. We re-examined this question by removing detritus, instead of subsidizing it, through a spring flood in commercial cranberry production, which washes away detritus as well as enacting pest suppression. We hypothesized that the removal of detritus will reduce detritivore populations, a major food source for generalist predators. This should force the predator community following the flood to prey-switch to feed increasingly on herbivores, thereby engendering more effective biological control. To test this hypothesis, cranberry beds were assigned to either a spring flood or an insecticide treatment, and the diversity and density of the most common species of predators, detritivores, and pest herbivores were assessed for the following six weeks.

Results/Conclusions Detritivore populations rapidly declined following both flood and spray treatments, and did not recover. Spider populations in the flooded beds rose markedly by week-2 and stayed significantly higher than the populations in sprayed beds. Spiders commonly prey on detritivores, and low observed detritivore densities throughout the summer suggest that spiders may have continued feeding on them. Further, the differences in detritivore numbers between the flood and spray treatments may have been dampened by spider predation. Parasitoids targeting moth pests were also higher in the flooded beds. Populations of the three main moth cranberry pests were the same in flooded as in sprayed beds. Flooding, therefore, supports spider and parasitoid populations that, together, reduce both detritivore and herbivore populations. These results provide evidence that a cultural control method, such as removal of detritus, has early-season effects on the trophic dynamics of the arthropod community, and that these changes in turn affect the ability of natural enemies to suppress pest populations. Alongside the theoretical implications in food web ecology, these findings provide evidence for the importance of biological control in Wisconsin cranberry production, suggesting that managing the marsh to support predator populations may be a valuable tool in cranberry pest control.