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Hedgerows and Herbaceous Plant Strips
They Promote and Conserve Predators, But Do They Promote Pest Control? | 9
by Gail Langellotto, Post-doctoral Associate, UC Davis

It has been repeatedly shown that conserving edge habitat such as hedgerows and herbaceous strips in the vicinity of a cropping system promotes predator aggregations. However, how often predators venture into a cropping system to forage (versus foraging in the edge habitat) has yet to be determined.

Experimental Approach
Using a relatively novel ecological technique called stable isotope analysis, I am attempting to resolve the effectiveness of edge habitats for pest control. Isotopes are different forms of an element with the same number atomic number (e.g. they share the same number of protons), but with different atomic weights (e.g. they differ in their number of protons). Most elements of biological interest such as C, H, O, N, and S, have two or more isotopic forms that are stable (i.e., they do not decay). These stable isotopes are persistent in the ecosystem and are present in different ratios with lighter versions of atoms present in much greater abundance than the heavier isotope(s). Stable isotope-containing compounds are naturally present in the atmosphere, on earth, and in all living organisms.

How it works
In ecology, stable isotope analysis utilizes the carbon and the nitrogen ‘signatures’ of organisms to determine where and on what predators are feeding. Carbon signatures of C3 plants (such as cotton) v. C4 plants (such as corn and other grasses) are fairly distinct. Furthermore, the carbon signature of the plant remains relatively unchanged up the trophic chain (plant to herbivore to predator). Thus, by getting the carbon signal of a predator, we can determine where they were feeding, especially if the edge habitat has a distinct, C4 signature (cropping system versus edge habitat). Nitrogen signatures increase on average 3.4 parts per thousand up the trophic chain.

This is due to the retention of ‘heavy’ nitrogen 15 isotopes and loss of ‘light’ nitrogen 13 during excretion. Thus, by getting the nitrogen signature of a predator, we can determine whether they are feeding on an herbivorous pest (relatively low nitrogen signature) or whether they are feeding on other predators (relatively high nitrogen signature). Together, these two pieces of information can be combined to tell: (1) where the predators are feeding (hedgerow vs cropping system), (2) what the predators are eating (herbivorous pests v. predators), and (3) what proportion of a predators diet comes from the hedgerow insects versus insects in the cropping system.

The First Results
Last summer, with grower cooperation, I collected preliminary samples from 3 farms associated with the Sustainable Cotton Project. Cotton vegetation, edge vegetation, and arthropods (both predators and herbivores) were collected from each farm on a single occasion. Samples were brought back to the UC Davis lab for sorting and processing. While the data is still coming in (it takes time to make sure a sample is ‘pure’ for stable isotope analysis), initial data suggests that predators in general, and big-eyed bugs in particular, move between edge habitats and the cotton crop to forage. Data for herbivores is forthcoming. What remains to be seen, and what will be a major focus of my research in 2003, is the directionality and frequency of predator movement (e.g. do predators start out foraging in the edge habitat and move to the cotton crop, or vice versa). Additional work for the summer of 2003 will focus on verifying the strength and nature of the pattern of predator movement that was suggested by the preliminary 2002 data.

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