Nitrogen Fertilizer's Effects on Pests and Predators

The use of science and technology for the management of agricultural pests can benefit society as a whole only if the overall effects of management on the environment are considered. Unfortunately, short-term controls, such as broad spectrum insecticides, can affect« more than their target species; studies of these incidental effects may lead to better use of current control measures, and to suggest for alternative practices.

Current crop management practices have created several problems. First, losses due to insect pests have increased nearly twofold (7% to 13%) over the last four decades as our insecticide and fertilizer use over the same period has increased tenfold (Bottrell 1980). Second, insecticides are biotoxins, and as such, are a potential source of public/occupational health problems. For this same reason, the use of insecticides has had harmful effects ` On non-target insects (such as pollinators and natural enemies of pests) and other animals in the environment. Third, synthetic chemical fertilizers, which have been linked to environmental problems such as groundwater contamination, can also influence pest problems by affecting population dynamics of both pest and beneficial species. ;

Conventional crop production includes many attributes (such as monoculture, susceptible varieties, scheduled sprays, and high fertilization rates) that are likely to promote an increase in pests, but not the rapid reproduction of natural enemies. Through the application of ecological principles, some of these problems can be illucidated, studied, and ameliorated.

In a recent project, funded by the US Department of Agriculture Competitive Grants Program, Dr. Laurel Fox and I examined the complex interactions that occur among plants, pests and pests' natural enemies (predators and parasitoids) in response to increasing levels of nitrogen fertilizer added to the soil We used collards (Brassica oleracea var. acephala) as a model system for the production of Brussels sprouts, cabbage, and broccoli on California's central coast Our experimental plots were established on the UCSC campus in association with the Agroecology Program. We tested the hypothesis that both core crop pest incidence and the efficacy of biological control of pests can be affected by the nutritive quality of the plant

Results and Discussion

In contrast to most studies of crop pest interactions, this study emphasized multi-level community responses that reflect the nutritional values of the crop plant. We gathered data on the make-up of the general arthropod community, the numbers of specific pests, and the ram of parasitism of those pests by naturally occurring parasitic wasps. Aphids and caterpillars were our main focus, since they are abundant pests in the system. Species include the cabbage aphid (Brevicoryne brassicae), peach aphid (Myzus persicae), the imported cabbageworm or cabbage butterfly (Artogeia (Pieris) rapae), and the diamondback moth (Plutella xylostella). Fertilizer applications ranged from two-tenths of the recommended nitrogen level to three times the recommended level We used synthetic fertilizer inputs (ammonium sulfate and ammonium nitrate) to precise" measure nitrogen input levels and to better interpret relevant results for local farmers using similar types of fertilizers.

Collards responded to increased N fertilizers levels by producing more and larger leaves. The N content of the leaves differed significantly among treatments in ad experiments. Mustard on levels varied among plants, but except for sinigrin levels, which were inversely correlated with N in the experiment run in 1985, concentrations were not affected by fertilizer treatment. Mustard ode provide the pungent task of cabbage and its relatives (including horseradish and capers). Sinigrin, one of the mustard of compounds, is known to be attractive to some insects and toxic to others.

During the study, the numbers of insects were assessed in weekly censuses. Total numbers of insects were generally low, due in part to prolonged periods of unusually cool temperatures and to the exposed location of our field sites. The imported cabbageworm, the diamondback moth, and thrips were least abundant on plants treated with low levels of nitrogen fertilizers. Cabbage aphids and green peach aphids did not respond consistently to various nitrogen levels from season to season.

We had expected large differences in egg densities of the cabbage butterfly in response to fertilizer treatment, because studies with potted plants have shown that females prefer to lay eggs on high N plants. In our own studio with potted kale, over 100 times more eggs were laid on plants watered with high concentrations of N than on those given very little fertilizer. In contrast, our large-scale field experiments showed only a slight tendency for butterflies to preferentially lay eggs in our heavily fertilized plots. We suspect that typical windy conditions and heterogeneous soils with obscure the effects of N fertilization on cabbage butterfly egg densities. In addition, experimental systems can differ fundamentally from agricultural field conditions and thereby produce specialized results. Our experiments show that these pests are biologically capable of selecting food plants of high quality but also suggest that these effects may not be seen in farmers' fields.

Several interactions closely linked the quality (levels of N and mustard oils) of the collards with the ability of predators and parasitoids to affect populations of herbivorous pests. The abundance of natural enemies generally increased at higher pest densities, and for at least one pair of species, the sex ratios of both the herbivore and its parasitoid varied with the plant quality. Diadegma insulare, a wasp parasitoid which lays its eggs inside caterpillars of the diamondback moth, can determine the sex of the egg it inserts. We found that these wasps are more likely to lay female eggs in caterpillars on high N plants and male eggs in caterpillars on low N plants. There was also a correlation between the level of certain glucosinolates in the plant and the number of female moths produced. These processes, if they occur regularly, would affect both the short-term and long-term dynamics of herbivores over a growing season.

Nitrogen fertilization practices do affect insects pests both directly, by the effect of food quality on pit behavior and growth, and indirectly, by influencing the natural enemies of pests. However, the effects of fertilizer on pests in our system were sometimes not predicable from ecological studies on small-scale model experiments. The complexity of the interactions we found emphasize the need to study these systems in their entirely--we hope to conduct on-farm surveys of pests and beneficials on organic farms in the Santa Cruz area. I have also begun a two-year project with collaborators Drs. Carol Shennan, Ariena van Bruggen, Laurie Drinkwater and Phillip Levine at UC Davis and UC Berkeley, and members of California Certified Organic Farmers, to examine the effects of N fertilization and cover cropping on pests, pathogens, production, soil quality and grower profits. We hope to provide useful information to aid decision-making by growers and to compare conventional and organic practice.

Dr. Deborah Letourneau

Environmental Studies

UC Santa Cruz

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