Integrating Biological Control into Cole Crop Production in the Pacific Northwest

Integrating Biological Control into Cole Crop Production in the Pacific Northwest

Summary

We examined the use of beetle banks to conserve predatory beetles for the control of pest Diptera. Locally, the community of predatory beetles includes several species of small carabid and staphylinid beetles ( 1.5 cm), Pterostichus melanarius, that rarely eats fly eggs but does eat smaller beetles. Predator beetle activity-densities, but not rates of fly egg predation, increased in fields including beetle banks. A series of field experiments was conducted to examine whether two types of polyphagy, intraguild predation and feeding on non-target prey, could be preventing increased egg predation following successful predator conservation. The putative intraguild predator P. melanarius reduced activity-densities of smaller beetles, and thus weakened fly egg predation. The strength of fly suppression increased with increasing densities of small beetles in the absence of P. melanarius but not when aphid alternative prey were readily available. In the presence of abundant aphids, egg predation rates did not increase at higher small beetle densities. Overall, our results suggest that both intraguild predation and the presence of alternative prey could limit conservation biological control that targeted generalist predators. Thus, higher predator densities will not necessarily lead to improved pest control.

Objectives/Performance Targets

Our project had 4 objectives:

1. Evaluate field margins and in-field refuges for predator conservation.

2. Document the seasonal abundance of different types of beneficial arthropods relative to the seasonal peaks in adult Delia spp. populations.

3. Evaluate the impact of the various natural enemies as biocontrol agents of root maggots.

4. Disseminate information to organic and conventional cole crop growers.

Accomplishments/Milestones

Our study consisted of two components, replicated on-farm measurements of beetle bank performance, and a series of manipulative field cage experiments designed to better understand interactions among key species. Our on-farm results demonstrated that densities of predatory ground and rove beetles were higher during the growing season in fields with beetle banks compared to fields without beetle banks, consistent with many earlier studies (Lee et al. 2001). However, despite a dramatic increase in beetle activity-densities, there was no relationship between increasing beetle densities and rates of fly egg predation. Evaluation of conservation programs aimed at promoting natural pest control must include measures of pest suppression, in order to be certain that higher predator densities do indeed lead to improved pest control (Gurr et al. 2000; Snyder et al. 2005).

We found that the ratio of small beetles to P. melanarius varied dramatically over the course of the growing season, particularly in fields including beetle banks. At the beginning of the season the predator guild was composed primarily of small beetles, but by the end of the season numbers of small beetles and P. melanarius were roughly equal. This pattern was consistent with previous work suggesting that P. melanarius acts as an intraguild predator of the smaller beetles (Finch & Elliot 1991; Prasad & Snyder 2004); it is only the smaller beetles that are themselves effective egg predators (Prasad & Snyder 2004).
In our field cage experiments we also observed a decline in both small beetle activity-density, and a concomitant drop in fly egg predation, in the treatment including the highest P. melanarius density. This is consistent with the classic intraguild predation scenario, with higher densities of the large, intraguild predator leading to fewer intermediate predators and lower predation rates on herbivores (Polis et al. 1989; Rosenheim et al. 1993, 1995; Finke & Denno 2004). However, because we measured activity-densities of small predators, rather than absolute densities, we cannot exclude the possibility that small beetles reduced their foraging behavior to avoid P. melanarius, and thus were trapped less frequently rather than actually falling victim to intraguild predation (Moran & Hurd 1994; see Lang 2003 for a similar conclusion regarding impact of carabids on lycosids).

Pterostichus melanarius’ role as an intraguild predator has been documented in several other systems (Dinter 1998; Snyder & Ives 2001). Because P. melanarius often responds positively to agri-environment schemes (Symondson et al. 1996; Shah et al. 2003; Raworth et al. 2004), in systems where P. melanarius acts primarily as an intraguild predator this species may be a particular impediment to the success of conservation biological control programs.

Within the group of smaller beetles, where intraguild predation is unlikely (Prasad & Snyder 2004), one would expect increased egg predation with increasing predator densities. Yet, even during May in beetle bank fields, there was no relationship between small beetle density and sentinel egg predation. While results from one block of Experiment 2 demonstrated that higher small beetle activity-densities resulted in greater egg predation, an unexpected aphid outbreak in the other block suggested an additional disruptive force wherein particularly high aphid densities, combined with plants large enough for foliage to be in contact with the ground further inflating predator-aphid encounter rates, prevented increasing egg predation with increasing small beetle density. In Experiment 3, we directly manipulated aphid densities, and confirmed that fly egg predation rates declined at high aphid densities. A number of studies have reported that predation of a target herbivore is limited if generalist predators prefer or are distracted by alternative prey (Abrams & Matsuda 1996; van Baalen et al. 2001; Harmon & Andow 2004). Aphids, pollen, and fungi have all been shown to disrupt predation of target pests by generalist predators (Dennis et al. 1991; Hazzard & Ferro 1991; Eubanks & Denno 2000; Musser & Shelton 2003). Providing generalist predators with alternate sources of prey, in order to maintain or build up their populations in fields when pests are absent, can be an effective conservation biological control tactic (Settle et al. 1996; Landis et al. 2000). But, when conservation schemes promote higher densities of alternate prey at the same time that pests are present, predators can be distracted by or become satiated on the alternate prey, disrupting biological control (Harmon & Andow 2004). Generalists that feed on multiple prey species complicate conservation biological control. However, this does not render such programmes futile; rather, practitioners need to understand how key natural enemies are responding to multiple-prey communities (Eubanks & Denno 2000; Harmon et al. 2000).

Beetle banks, like some other agri-environmental schemes, target generalist predators. However, the broad diet breadths of generalists makes their community-wide impact difficult to predict (Rosenheim et al. 1995). Previous work in our study system has been characterized by high carabid and staphylinid feeding rates on fly eggs in the laboratory (Finch & Elliot 1994; Finch 1996), followed by disappointing performances in the field (Humphreys & Mowat 1993; Finch & Elliot 1994; Kromp 1999). The manipulative field experiments reported here allowed us to identify two disruptive forces: intraguild predation and predation of aphid alternative prey. These are likely to be common in the field and thus will limit the effectiveness of conservation biological control. The identity of the target prey (pest) species is also likely to be important. For example, in our system P. melanarius is disruptive because this predator is too large to feed on fly eggs (Finch 1996), and instead feeds on the smaller beetles that are the most common egg predators. However, if our target pest were another, larger herbivore, a mollusk for example, P. melanarius would likely make a positive contribution to control (Symondson et al. 1996).

Our results suggest that the indistinct and context-dependent trophic role of these generalists will often limit egg predation under field conditions. However, the polyphagous nature of generalist predators will probably limit the maximum contribution of generalists to pest suppression, rather than eliminate the benefits of including generalists in the community. For example, throughout our study more eggs were eaten in treatments containing predators than was seen in the no-predator controls.

Impacts and Contributions/Outcomes

Recent efforts to incorporate conservation biological control within agriculture have been characterized by a non-mechanistic approach, resulting in limited insight, after the fact, into why particular conservation schemes succeed or fail (Gurr et al. 1998; Kean et al. 2003; Snyder et al. 2005). Perhaps more troubling, the data collected are often insufficient to allow performance to be assessed (Kleijn & Sutherland 2003). We advocate a more mechanistic understanding of how predator conservation impacts agricultural communities, either empirically (this study), using models (Kean et al. 2003), or through a combination of both empiricism and theoretical work (Snyder & Ives 2003). Mechanism-centered approaches facilitate a more directed (Kean et al. 2003) and predictive (Gurr et al. 1998) approach to the management for conservation biological control of species-diverse food webs.

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