RESEARCH RESULTS

Managing Natural Habitat Can Aid in Organic Pest Control

Determing habitat requirements for natural enemies
of farm pests

OFRF funding was awarded for this project in Spring 2007.

Investigator: Rebecca Chaplin-Kramer, University of California, Berkeley
Project location: Salinas Valley, California

Summary
Maintaining a healthy population of beneficial insects that serve as natural enemies to crop pests is an important component of pest management for organic farmers. To do so, we must understand the habitat requirements of these insects.

In this project we worked with growers in the Salinas Valley and surrounding areas to understand how landscape factors around the farm contribute to natural pest control. For the study, we selected 18 organic broccoli farms representing a gradient of landscapes ranging from less than 5% to more than 80% natural habitat within a 3 km radius of the farm.

Cabbage aphids are a major pest of broccoli and can be controlled by natural enemies if not disrupted by pesticides. There are many important natural enemies of aphids. In this study we focused on syrphid flies because larvae of flies in the family Syrphidae are by far the most abundant aphid predator in broccoli.

 Adult syrphid flies are extremely mobile and search in many different habitats for aphid colonies on which to lay their eggs. This makes the surrounding habitat an important variable in their distribution on farms. Syrphid flies are especially vulnerable to pesticides and cannot be acquired commercially; they must migrate into the fields from surrounding areas. Syrphid flies are therefore an excellent study species to use to explore the effects of landscape on pest control.

Our study was composed of two main components: habitat mapping using ArcGIS software and weekly insect surveys at each of the farms over the growing season in three separate years (2006, 2007, and 2008).

Two additional components emerged out of questions raised by the insect surveys: a field experiment to measure pest control function at each end of the landscape gradient and laboratory experiments focusing on the physiological impacts of a species of wild mustard which serves as an alternate host plant for aphids. We investigated whether cabbage aphids feeding on these mustards become more toxic to predators as a result of sequestering chemicals that essentially constitute a “mustard oil bomb.”

GIS habitat mapping
From aerial photographs we used Geographic Information System (GIS) methods to create maps that were classified into the following land-use categories: annual agriculture, perennial agriculture, fallow agriculture, industrial, residential, road, bare, water, and natural habitat. We defined a “landscape complexity” gradient in terms of the amount of natural habitat around the farm. Areas with lower proportions of natural habitat (usually corresponding to higher proportions of agricultural habitat) were considered “simple” while areas with higher proportions of natural habitat (or lower proportions of agricultural habitat) were considered “complex.”

Results of insect surveys
Syrphids showed a strong response to landscape complexity in all three years of the study with populations increasing significantly as natural habitat increased in the area surrounding the farm.

Similarly, cabbage aphid population growth was reduced on farms with higher syrphid densities, resulting in a weak but significant decline of aphid densities in more complex landscapes. Pest control services delivered by natural habitat may be masked by inter-annual variation in environmental factors and by competing direct and indirect effects (i.e., landscape effects on the pests themselves versus on the natural enemies’ control of pests). Therefore, longer term datasets are necessary in order to detect the true magnitude of pest control services provided by natural habitat. In the short term, experiments can reveal the relationship between pest and predator more clearly.

Results of field experiment in pest control function
The weekly insect surveys allow tracking of the distributions of aphids and their natural enemies over time. However, it is not possible to establish cause and effect from this method of observation. In order to determine whether changes in aphid populations are due to natural enemies as opposed to other exogenous factors (weather, dispersal patterns, sources of aphids), it is necessary to control for these other possible factors.

In a cage study using potted broccoli plants, pest control provided by natural enemies in complex landscapes (>50% natural habitat) was nearly double that found in simple landscapes (<15% natural habitat), with pest suppression approaching (and at one site even exceeding) 100% in complex landscapes.

While the effect of landscape complexity on pest control is significant, the variability was not consistent. The error or statistical noise in the data for simple landscapes was more than twice that for complex landscapes. A closer inspection of the data reveals that two of the simple landscape sites exhibited much greater pest reduction than the other two. Interestingly, this difference corresponds to a difference in local farm diversity. The sites within each landscape category break down neatly into two local categories: those with large monocultures lacking any floral resources for beneficial insects (locally simple) and those with diverse plantings and hedgerows adjacent to the field (locally complex). Reanalyzing the data using these categories shows that there is a significant effect at both landscape scales and a significant interaction between the landscape and local scales of complexity.  Specifically, local complexity only appears to enhance pest control in simple landscapes where locally complex sites have nearly four times the pest reduction as locally simple sites. 

Results of laboratory experiments: physiological impacts of mustard
A major factor that may affect the potential for natural habitat to provide pest control services is the presence of alternate host plants for pests within the weed community surrounding the farm. Physiological experiments show that one weedy relative of broccoli, black mustard (Brassica nigra), could serve as more than merely an alternate host. Brassica species contain chemicals that allow cabbage aphids to build a “mustard bomb” for self-defense. Black mustard has more of these chemicals than broccoli, and therefore may provide aphids a refuge from predators.

The first question was whether aphids feeding on mustard contain more glucosinolates, the building blocks for the mustard bomb, than aphids feeding on broccoli. The answer to this is a definitive yes. Chemical assays showed that mustard-fed aphids contained more than ten times the glucosinolates of broccoli-fed aphids.

Daniel Kramer Counting insects under the dissecting microscope after bringing the field plants back to the lab.

Rebecca Chaplin-Kramer A syrphid fly larvae attacks an aphid on the underside of a broccoli leaf. Syrphid larvae are voracious predators that can eat up to 100 aphids per day.

The next question was whether syrphids are less effective predators of mustard-fed than broccoli-fed aphids. Initial results indicate that the answer to this is also yes. Syrphids ate almost five times more broccoli-fed aphids than mustard-fed aphids.

But the most impressive result to come out of this study was the impact on syrphid larval development.  Syrphid mortality in the broccoli treatment was around 40% while mortality in the mustard treatment was 95%.  This high mortality, combined with our field observations of not finding any syrphids on mustard, suggest that adult syrphid flies may have learned to simply avoid mustard altogether in nature. Weedy mustard may provide predator-free space for aphids around farms, and thus could serve as a source of aphid pests to crops.

Conclusions
The results of this project demonstrate that natural habitat does indeed provide a pest control service to farms. This research has established that farms having a substantial amount (>50%) of natural habitat in the surrounding landscape have essentially double the biological pest control found on farms with less than 15% nearby natural habitat. Pests are constrained to some extent by ecosystem services but the degree to which this constraint can contribute to overall pest control is dependent on pest colonization and population growth early in the season. The potential for habitat around the farm to benefit pests as well as natural enemies is an important consideration in understanding the impact of habitat complexity on pest control services.

A final report on this project was submitted in December 2009.