Carabid Beetles, Filter Strips and Biological Control of Annual Weeds
Vegetative barriers such as filter strips, cross wind trap strips and riparian buffers have proven to be an effective means of reducing soil erosion in the Great Lakes Basin. These habitats may also affect biological control in agricultural croplands by providing shelter from adverse conditions and alternate food sources for beneficial arthropods. Adults of many carabid beetle species are predaceous on insect pests, while others feed on weed seeds. During 1997 and 1998 we examined the abundance and activity of carabid beetles in a large crop field and two adjacent filter strips; one composed of switchgrass, the other a legume-grass mixture. The 10 hectare field was located in Midland Co., Michigan with soybean planted in 1997 and corn in 1998.
In 1997, a total of 5953 ground beetles representing 46 species were collected in the soybean field and the adjacent strips. Of these, 966 individuals and 25 species were collected in the crop field, while 1504 individuals and 32 species were collected in the legume-grass mixture strip. The greatest number of individuals (1581) and species (38) were observed in the switchgrass strip. Six of the trapped species (Harpalus pensylvanicus, H. herbivagus, H. compar, H. affinis, Anisodactilus sanctaecrusis, and Amara aenea) are known to feed on weed seeds. Due to their potential as weed control agents, we were particularly interested in assessing the abundance and activity of these six species. The number of seed predators differed among the three habitats. Only 73 seed predators were captured in the soybean field, 572 in the legume-grass strip, and 1581 in the switchgrass strip. To evaluate the impact of carabid beetle abundance on weed biological control giant foxtail (Setaria faberii) seed removal by invertebrates was compared between the center of the crop field and the adjacent filter strips. Seed removal was estimated twice during the 1997 growing season by placing 50 seeds on water proofpads and recovering them after 7 days. Seed removal by invertebrates followed a trend similar to the numbers of seed predators captured in each habitat. On average, 32% of the foxtail seeds placed into switchgrass strips were removed in one week vs. 14% in the legume-grass mixture strip and 4.8% in soybeans.
From May to August 1998, we captured a total of 4836 individuals and only 528 seed predators. While 1635 and 1139 individuals were trapped in the legume-grass and the switchgrass strips respectively, 2062 individuals were sampled in the corn field. However, the corn field showed the lowest number of seed predators: 112 vs. 202 in the legume-grass, and 214 in the switchgrass strip. In accordance with the low number of seed predators captured, our preliminary observations indicated substantially less seed removal than in 1997. These experiments demonstrate that habitats such as filter strips make a difference in the abundance of ground beetles. However, ground beetle species composition and seasonal abundance has significant year to year variability which influences their effectiveness as biological control agents.
- Fabian Menalled and Doug Landis, Michigan State University
Beetle Introduced in Canada for Bladder Campion Control
Bladder campion, Silene vulgaris (=S. cucubalus), is an introduced noxious weed with low fodder value that outcompetes alfalfa on some soils, and is a serious contaminant in certain seed crops. The European leaf beetle Cassida azurea--which feeds on shoots of the young plants as adults and on apical leaves, buds and flowers as larvae--was released in three Canadian provinces from 1989 to 1993. It was considered established at 8 of the 18 release sites but has not yet increased to densities sufficient to reduce weed populations.
Peschken, D. P., R. Declerke-Floate, and A. S. McClay. 1997. Cassida azurea: Host specificity and establishment in Canada as a biological control agent against the weed Silene vulgaris. Can. Entomol. 129: 949-958.
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