Impact of Herbicides on Beneficial Insects of Blueberry & Cranberry (LNE96-064)

Impact of Herbicides on Beneficial Insects of Blueberry & Cranberry (LNE96-064)

Summary

Summary
This two-year project is investigating the effects of herbicides on the diversity and abundance of forage plants and beneficial Hymenoptera (bees and wasps) in blueberry fields in Maine and cranberry bogs in Massachusetts.

Objectives
1. Determine the effects of herbicide use on flowering weeds (diversity and abundance) and Hymenoptera (diversity and abundance).
2. Determine the extent to which field border characteristics reduce the effects of herbicides on the diversity and abundance of Hymenoptera.
3. Determine how the abundance and diversity of beneficial Hymenoptera influences crop productivity (fruit set, berry weight, and seeds per berry).

Background
The ultimate goal of this research is more sustainable production of lowbush blueberry and cranberry through greater reliance on beneficial insects and less reliance on pesticides in these important and extensive agroecosystems.

Due to the characteristics of the cranberry and blueberry agroecosystems, we believe they are particularly susceptible to the detrimental environmental impact that herbicide applications might have on beneficial insects. Broadleaf herbicides kill plants that bees and wasps use for forage. Other herbicides control grasses, rushes, and sedges, which, while not forage plants, may provide important microhabitats for some beneficial bees and wasps. Therefore, the effects of herbicides in these agroecosystems (and the concomitant reduction of floral abundance and diversity) on the Hymenoptera provide excellent model systems for study.

Our 1997 research sampled eighteen lowbush blueberry fields and nine cranberry bogs for diversity and abundance of noncrop plants (weeds) and Hymenoptera.

It is likely that the findings will result in significantly less herbicide use in both agroecosystems. This should lead to increased natural enemy abundance and diversity and decreased insecticide use. Both increased floral diversity and decreased insecticide usage should enhance the habitat of native pollinators, making pollination more sustainable.

Lowbush blueberry is the second largest agroecosystem in Maine. The cranberry agroecosystem is a fledgling industry in Maine with major production areas in New Jersey and Massachusetts. Reduction in pesticide use in these important and extensive agroecosystems will contribute substantially to a cleaner environment and safer, healthier food.

Specific Findings
For lowbush blueberry, the number of flowering plant species present ranged from 20 species to 44 species per study site (includes species present within the adjacent forest). Fields ranged from intensively managed and relatively weed-free (6.8 percent weed cover) to organically managed fields with up to 58.3 percent weed cover.

Weed cover in cranberry ranged from less than 1 percent to 12.8 percent. For cranberry, the number of flowering plant species present ranged from 26 species to 47 species per study site. Based on m2 plot counts during bloom, wild bee (Bombus, Andrena, Halictus, Osmia, etc.) abundance varied from 0.2 per m2 to 1.13 per m2 in lowbush blueberry. In cranberry, wild bee abundance varied from 0.02 to 0.35 per m2. Students are in the process of sorting over 2,000 samples into major categories of beneficial Hymenoptera which will then be correlated with weed cover and yield findings.

In addition, aerial photos were taken of all the blueberry fields and floral distribution maps are being derived from 35 mm slides of fields. A measure of herbicide-intensity is being derived from our periodic visits to the study sites, grower spray records, and interviews with growers.

Our sampling procedure allows us to document changes in diversity and abundance from field boundaries toward the center of the field. To more fully understand the interactions that occur between the Hymenoptera of the blueberry field or cranberry bog and the adjacent forests, we extended our sampling transects into the adjacent forests at study sites. Once all samples are sorted from 1997, we will be able to determine whether smaller fields, with more forest edge, reduce the effects of herbicides on beneficial bees and wasps.

To accurately assess how the abundance and diversity of beneficial Hymenoptera influenced crop productivity more samples will need to be processed.
Reported November 1997.

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