CASE STUDY 3: STRIP MANAGEMENT TO AUGMENT PREDATORS

As a way to enhance predator abundance in cereal fields, research-ers in Switzerland introduced vegetation edges as successional strips into the field. One 8 ha winter cereal field was subdivided by five wide-strips leaving cereal spaces of 12, 24, and 36 m between the strips (Lys and Nentwig, 1992). Significantly higher recapture rates, indicating higher predator activity, were found in the strip-managed

FIGURE 7.13. Seasonal patterns of predator catches (numbers per yellow sticky trap) in block A, as influenced by the presence of forest edge and the corridor (P < 0.05; Mann-Whitney U-test) (Hopland, California, 1997) (after Nicholls, Par-rella, and Altieri, 2001).

area than in the control area, especially for carabid beetles such as Poecilus cupreus, Carabus granulatus, and Pterostichus melanarius.

Several observations led to the conclusion that this higher activity was generally due to a prolongation of the reproductive period in the strip-managed area.

Besides the marked increase in activity and density, a large in-crease in the diversity of ground beetle species was observed. The most marked increase in number of species was found in the first year. The vegetation structure of the cereal field was enriched follow-ing use of weed strips. After three years of research the authors con-cluded that weed strips offer not only higher food availability but also more suitable overwintering sites. In addition, these weed strips offer refuges during field disturbance or during unfavorable climatic con-ditions, such as droughts. Weed strips increase the chance of survival of many carabid species in arable ecosystems, thus counteracting the faunal impoverishment trends promoted by monocultures. Nentwig

FIGURE 7.14. Seasonal patterns of predator catches (numbers per yellow sticky trap) in block B without the corridor but with an adjacent forest (P < 0.05; Mann-Whitney U-test) (Hopland, California, 1997) (after Nicholls, Parrella, and Altieri, 2001).

(1998) found similar effects with 3- to 9-m-wide sown weed strips di-viding large fields into small parts so that the distance between strips does not exceed 50 to 100 m. A favorite plant to be used as strips within or around fields is Phacelia tanacetifolia (Holland and Thomas, 1996).

In reviewing these studies Corbett and Plant (1993) argued for the need to develop a mechanistic framework to evaluate and predict the response of natural enemies to vegetational arrangements in agroeco-systems. Using a hypothetical field with 10 m wide strips interplanted at 100 m intervals (Figure 7.16), they assumed that these strips are used solely as an overwintering refuge by three natural enemy spe-cies: (1) a predatory mite having very low mobility (a diffusion co-efficient of 1 m²/day); (2) a predatory coccinellid beetle having moder-ate mobility (10 m²/day); and (3) a highly mobile parasitoid (100 m²/day). Once the crop has germinated, the strips do not provide re-sources in any greater abundance than the crop, nor do they provide a

MeanNo.Predators/tr

8 7 6 5 4 3

2

1

0

FIGURE 7.15. Comparison of abundance of generalist predators (numbers per yellow sticky trap) between block A (with a corridor) and block B (without a corri-dor) (P < 0.05, Wilcoxon’s signed rank test) (Hopland, California, 1996) (after Nicholls, Parrella, and Altieri, 2001).

more favorable physical habitat. The mobility (i.e., the probability of making a move in a given time period) is therefore the same in the strips as it is in the crop. Natural enemies overwinter in the strips at a density of ten individuals per square meter.

The spatial patterns in abundance predicted by the model for these three hypothetical natural enemies are illustrated in Figure 7.17. The model predicts that the natural enemies will spread from the strips, resulting in higher abundance in the crop than would have occurred in a crop monoculture. The distance to which they are enhanced varies substantially, however. For the predatory mite, enhancement is con-fined to the region immediately adjacent to interplanted strips, pro-ducing a steep gradient in density with increasing distance. The highly mobile parasitoid, on the other hand, is enhanced throughout the crop—there is no spatial pattern to suggest that strips influenced abundance. As a result, natural enemies with low mobility exhibit no enhancement beyond 20 m from strips, while more mobile natural enemies are enhanced fourfold.

Corbett and Plant (1993) proposed a second scenario using the same field and natural enemies. In this scenario, however, the inter-planted vegetational zones are not overwintering refuges: natural en-emies must colonize the agroecosystem from external sources. The

FIGURE 7.16. Diagram of hypothetical diversified agroecosystem. Interplanted strips are placed 100 m apart within a crop. The model predicts natural enemy abundance along a transect through the field (after Corbett, 1998).

strips do, however, provide more resources than the crop. Therefore, the probability of making a move in a given time period is lower in the strips than in the crop. The resources in interplantings are assumed to be “substitutable,” to some degree, for resources that occur in the crop. They could be either (1) alternate prey or hosts (“supplemen-tary” resources) or (2) floral resources that are an imperfect substitute for the preferred host but that benefit the natural enemy when avail-able (“complementary” resources).

The model predicts that natural enemy mobility would dramati-cally affect the observed enhancement due to increased diversity (Figure 7.18). Natural enemies that are highly mobile would show lit-tle enhancement when plots are 50 m wide because predators are dis-persing among all plots in the experimental field. The observed en-hancement increases with plot size since as strips are farther apart their effect becomes more detectable. However, even plots 200 m in size do not detect the enhancement that would occur in a diversified, commercial-scale field. For less mobile natural enemies, enhance-ment is observed when plots are small, but the observed enhanceenhance-ment

FIGURE 7.17. Spatial patterns predicted by model for field with interplanted strips that serve solely as an overwintering refuge. Peak abundance occurs at interplanted strips. Patterns are shown for hypothetical natural enemies of three different mobilities: low mobility (1 m²/day), moderate mobility (10 m²/day), high mobility (100 m²/day) (after Corbett, 1998).

decreases with increasing plot size. This is because such predators are enhanced only in the area adjoining strips.

The model also predicts that the abundance of the three natural en-emies is higher in the interplanted strips than in the crop vegetation.

This accumulation of natural enemies in the strips is due to the lower tendency for movement there and results in the strips acting as a sink for the natural enemies. The parasitoid exhibits a spatially uniform density in the crop vegetation and the greatest accumulation in the strips. This sink effect results in abundance on the crop that is 60 per-cent of what it would be in an undiversified field. The other natural enemies exhibit some spatial patterning within the crop and a milder sink effect.

FIGURE 7.18. Effect of mobility on the abundance of natural enemies on crop vegetation in a diversified agroecosystem. “Relative Abundance” is the ratio of natural enemy abundance predicted for the diversified system to that predicted for a crop monoculture. Relative abundance is calculated only for crop vegeta-tion more than 20 m from interplantings. Effect of diversificavegeta-tion is shown for three different situations: where the interplantings act solely as an overwintering refuge; where they provide additional food resources but no overwintering sites;

and where interplantings provide both (after Corbett and Plant, 1993).

Chapter 8

The Dynamics of Insect Pests in Agroforestry Systems

The Dynamics of Insect Pests in Agroforestry Systems

Agroforestry is an intensive land-management system that com-bines trees and/or shrubs with crops and/or livestock (Nair, 1993).

Many of the benefits of agroforestry are derived from the increased diversity of these systems compared to corresponding monocultures of crops or trees. Despite the fact that little research has been con-ducted on pest interactions within agroforestry systems, agroforestry has been recommended to reduce pest outbreaks usually associated with monocultures. Although the effects of various agroforestry de-signs on pest populations can be of a varied nature (microclimatic, nutritional, natural enemies, etc.), regulating factors do not act in iso-lation from one another.

The few reviews on pest management in agroforestry (Schroth et al., 2000; Rao, Singh, and Day, 2000) expect that high plant diver-sity protects agroforestry systems to some extent from pest and dis-ease outbreaks. These authors use the same theories advanced by agroecologists to explain lower pest levels in polycultural agro-ecosystems as discussed in Chapter 3. These authors also caution that the use of high plant diversity as a strategy to reduce pest and disease risks in agroforestry systems involves considerable technical and economic difficulties. Whereas a farmer is free to cultivate crops ei-ther on separate fields or in association, the choice of the crops them-selves (and thus the overall crop diversity of the farm) is strongly in-fluenced by the availability of markets for the respective products and the needs of the household. The selection of timber and fruit trees also has to respect local market conditions, although more freedom of choice may exist for “service” trees, for example, trees grown for bio-mass, shade, or wind protection.

THE EFFECTS OF TREES IN AGROFORESTRY