Investigating the Effect of Hedgerows to Enhance Natural Biological Control

Key insect natural enemies and pests attracted to hedgerow plants:
Numerous natural enemy taxa important in the natural biological control of crop pests were present in Central Coast hedgerows. Lacewings, minute pirate bugs, ladybird beetles, syrphid flies and parasitic wasps were commonly found on hedgerow plants. The minute pirate bug (Orius spp.) was the most abundant predator sampled on hedgerow vegetation, followed by Coccinellidae, Geocoris spp., Chrysopidae and Hemerobiidae, respectively. Overall, wasp parasitoids were consistently more abundant on hedgerow plants than generalist predators or herbivore pests. Indeed, Chalcidoidea, a super family of minute parasitic wasps, was the most abundant insect group overall, comprising 54% of the entire indicator insects collected in 2006. Chalcidoidea is a large and diverse super family that has been most frequently and successfully used in classical biological control programs.

Key pests of vegetable and fruit crops were also present on hedgerow plants and cumulatively represented 10% and 14% of the indicator insects sampled in 2005 and 2006. The most abundant key pests found were the western spotted cucumber beetle (Diabrotica undecimpunctata undecimpuntacta), followed by the western striped cucumber beetle (Acalymma trivittatum), the western black flea beetle (Phylotretta pusilla) and the western tarnished plant bug or lygus (Lygus hesperus). The western spotted cucumber beetle represented 8.5% and 10% of the indicator insects collected across the sampling periods in 2005 and 2006 respectively. However, the majority of cucumber beetles collected originated from Site C where its highest abundances were found at H. arbutifolia and B. pilularis. Other key pests of vegetables such as the silverleaf whitefly (Bemisia argentifolli), the cabbage looper (Trichoplusia ni) and the imported cabbageworm (Pieris rapae) were either not collected or extremely rare.

Association between Floral Resource Availability and Insect Abundance
In 2005 and 2006, the abundance of Orius spp. was positively correlated with the floral resource availability of all hedgerow plants, except for C. griseus. The strongest associations were with A. millefolium (R2 = 0.52) and B. pilularis (R2 = 0.56) in 2005 and with B. pilularis (R2 = 0.53) in 2006. The mean abundance curves of Orius spp. overlap at hedgerow plants tracking the plants bloom periods, such that it’s abundance is highest on A. millefolium in the spring, on E. giganteum in the summer and B. pilularis in the late summer.

The minute pirate bug is a generalist predator of eggs and soft-bodied insects such as thrips, mites, aphids, whiteflies and small caterpillars, many of which are key pests in vegetable systems (UCANR 2008). Orius spp. have been shown to be effective biological control agents in augmentative releases in greenhouses to control thrips (Chow et al. 2008) and in natural field conditions to control aphids (Costamanga and Landis 2006, Harwood et al. 2007). Given that minute pirate bugs are the most abundant predator found in hedgerows and they appear to be utilizing hedgerow resources (pollen, nectar or alternate prey), hedgerow planting may be a particularly important practice for the management of aphids or thrips.

The abundance of Chalcidoidea was also positively associated with the floral resource availability of A. millefolium (R2 = 0.21) and only weakly associated with the floral resource availability of the rest of the plant species, except for B. pilularis with which there was no association.

Pest species also responded to floral resource availability. The abundance of D. undecimpunctata undecimpunctata was positively correlated with floral resource availability of H. arbutifolia in 2005 (R2 = 0.21) and of B. pilularis (R2 = 0.21) and E. giganteum (R2 = 0.32) in 2006. The abundances of A. trivittatum and L. hesperus were also positively correlated with the floral resource availability of B. pilularis in 2006.

Comparison of Hedgerow Plants for Harboring Indicator Insects.
In order to compare the habitat quality of different hedgerow plants for supporting indicator insect groups, I used Cohen’s d estimates, which measure the magnitude of the difference between two means, known as the “effect size.” In other words, d estimates reveal not whether the means are significantly different from each other, but by how much they are different. Cohen’s d estimates range from 0 to greater than 1, where values equal to 0.2 are considered a small difference between two means and values equal to or greater than 0.8 signifies a large effect (Cohen 1988).

I found that B. pilularis supported a sizably higher number of parasitic wasps than other plants and this pattern was evident across years and sites. For the hymenopteran families, Chalcidoidea, Braconidae and Ichneumonidae, comparisons between B. pilularis and other plants resulted in Cohen’s d estimates between 0.5 and 1.72 indicating medium to very large effect sizes. B. pilularis generally supported a higher abundance of Orius spp. in comparison to other hedgerow plants, at all sites except for site B, where B. pilularis was late in flowering. B. pilularis was also favorable habitat to Chrysopidae, Hemerobiidae, and Coccinellidae in several plant comparisons at multiple sites. At site C, B. pilularis harbored a higher abundance of cucumber beetles than other plants (d estimates = 0.41 – 0.55). In general, B. pilularis is an important insectary plant for hedgerow design.

A. millefolium was another plant that tended to host a higher abundance of indicator insects in comparison to other plants, evident by d values greater than 0.4. Geocoris spp. was almost exclusively found at A. millefolium with d values greater than 0.7 for most plant comparisons. A. millefolium also supported a higher number of Orius spp. in comparison to C. griseus at site D (d = 0.85), C. ‘Ray Hartman’ at sites A, C and D (d = 0.26, 0.53, and 0.79 respectively), H. arbutifolia at site A (d = 0.70) and R. californica at all sites (d = 0.67, 0.41, 0.52, 0.77 respectively).

Another relevant finding was that the mean abundances of Syrphidae tended to be higher on Ceanothus ‘Ray Hartman’ at the majority of sites (d estimates > 0.2). While Ceanothus griseus and ‘Ray Hartman’ tended to host fewer of the other indicator insects in comparison to the other plants. Syrphid flies selective use of C. ‘Ray Hartman’ (Rhamnaceae), a tree with blue flowers, is a new finding. Syrphid flies have primarily been shown to be attracted to yellow and white flowers (Cowgill et al. 1993, Lunau and Wacht 1994) and prefer plants from the Apiaceae family (Colley and Luna 2000). As a nitrogen fixing tree that produces pollen rich flowers, Ceanothus ‘Ray Hartman’ offers many advantages to hedgerow design – architectural complexity, microclimate mediation, windbreak, nutrient recycling, and accessible pollen for syrphid flies and other obligate pollen feeders, such as bees. Furthermore, C. ‘Ray Hartman’ has the advantage over the prostrate species, C. griseus, of having two bloom periods, one in the spring and one in the late summer, which may explain the overall higher mean abundances of syrphid flies.

There is consensus among CBC researchers that adequate pest suppression via CBC depends upon the presence of a suite of generalist natural enemies in the farmscape (Ehler and Miller 1978). Since the insects monitored in this study differentially partitioned themselves across hedgerow plants and showed some indication of plant selectivity, the current design of species rich hedgerows with A. millefolium, B. pilularis, Ceanothus spp., E. giganteum, H. arbutifolia, and R. californica as foundational plants, is effectively conserving a diversity of natural enemies in Central Coast vegetable farms. In conclusion, the establishment of perennial hedgerows in the borders of vegetable systems greatly increases the potential for habitat stability and the development of resident natural enemy communities. Based on the findings of this research, hedgerow plantings aimed at conserving insect natural enemies should have a diverse mix of plant species that include B. pilularis, which is particularly important habitat for wasp parasitoids.

The dispersal of indicator insects from hedgerows into adjacent crop fields
The fluorescent mark was found on all insect taxa at 25 m and 100 m, except for lady beetles (Coccinellidae) and flea beetles (Phylotretta pusilla), which were only found marked at 25 m (Figure 8). The mean proportion of marked individuals, however, was significantly higher at 25 m and dispersal rates varied across taxa. Less than 10% of the minute pirate bugs collected were marked at 25 m and 100 m showing low dispersal from hedgerows into adjacent crop fields. However, predators as a functional group had a higher overall mean rate of movement (17.3 ± 2.9 SE) than both parasitoids (5.4 ± 1.5 SE) and pests (6.5 ± 1.3) . In particular, the generalist predators, Syrphidae and Chrysopidae, had the highest mean percentage marked for the combined distances, followed by Hemerobiidae. The parasitic wasp family, Ichneumonidae had a combined mean percentage marked of 13.6 ± 4.4 SE, but due to the variance was not significantly different from Chalcidoidea (1.7 ± 0.4 SE) and Braconidae (2.3 ± 0.9 SE). Excluding L. hesperus, pest taxa generally had lower rates of movement into crop fields than natural enemies. Percentage values are probably inflated for L. hesperus because of their low average abundance. Because of this probable inflation, this taxa was excluded from ANOVA analyses.

Results from a two-way ANOVA with an interaction term showed that taxa, distance, and distance * taxa were significant effects for the mean percentage of marked individuals (P < 0.01 for all effects). These results support the hypothesis that natural enemies and pests move from hedgerows to adjacent crop fields, but the rate of movement varies across taxa. The Effect of Hedgerows on Parasitism Rates of a Sentinel Pest
In 2006, parasitism was detected mostly in the June trial and at only two sites, a field with a hedgerow and a field without a hedgerow. These fields were not in the same location nor did they have the same crop matrix, thus they are not comparable. However, results from this trial suggest that the crop matrix influences parasitism rates. In the San Juan Bautista field, percentage parasitism ranged from 70% in the brassica stand to 0% where there was bare ground. Whereas in the field with an unmanaged field margin, where the entire field was planted in brassica, parasitism rates ranged from 20% at 100m to over 80% at 10m. Parasitism rates also declined with distance from the field margin in the comparison field, indicating that parasitoids may be more abundant on the edges of fields.

In 2007, parasitism was detected in all three trials, however, there was a high variation in rates within and between fields and across dates. Results from a mixed model ANOVA showed that distance nested within field margin type was a significant effect (F=4.61, P=0.0165) while field pair was weakly significant (F=2.729, P=0.0582) and distance by itself was not a significant effect (F=3.0470, P=0.09). This result is best seen in Figure 10 where the average percentage parasitism is greatest at 100m in comparison fields, although there is no difference in percentage parasitism between field margins at 25m.

Results from the 2007 trials however are not conclusive because the crop matrix was not adequately controlled within and across fields. The crop matrix affects the background population of T. ni and thus the diversity and abundance of parasitoids present in the system. Furthermore, the stage of development of the crop stand affects the apparency of the sentinel pots, which in turn affects wasp foraging behavior. Being able to place sentinel larvae in a standardized crop matrix is essential for detecting the effects of added floral resources. In complex agri-landscapes such as the Central Coast of California, on-farm CBC research may be more successful if growers were compensated for planting the same crop matrix at the same time.