Variation in pesticide use across crops in California agriculture: Economic and ecological drivers.

Pesticide use is a key component of efficient crop production, but is associated with a suite of costs. Understanding the main drivers of pesticide use will help us target research to develop effective alternatives. Although economic models predict, and empirical tests confirm, that the value of the crop being protected is an important determinant of between-crop variation in pesticide use, previous tests of this prediction have examined only modest numbers of crops and have not assessed the relative importance of crop value versus ecological determinants of pesticide use. Here we analyze variation in pesticide use across 93 crops grown in California, USA. We examine the joint roles of crop value and ecological determinants of pesticide use, including (i) the number of pest species associated with each crop; (ii) the distinction between annual vs. perennial crops; and (iii) the distinction between unprocessed vs. processed crops. As predicted, crop value was the dominant driver of the use of pesticides directed at arthropods and at plant pathogens, explaining 52.7% and 54.6% of total deviance, respectively. Ecological determinants of pesticide use were, however, also detected. Pesticide use was greater on crops that hosted a larger number of arthropod pest species (r = 0.32) or plant pathogen species (r = 0.29); for these pest groups, we saw no differences in pesticide use between annual vs. perennial crops, or processed vs. unprocessed crops. Perhaps surprisingly, crop value failed to explain the substantial between-crop variation in use of pesticides targeting weeds (1.7% of deviance explained, n.s.). Instead, an ecological factor, whether the crop was an annual versus a perennial plant, was the most important predictor of pesticide use against weeds, with more frequent applications on perennial crops. We conclude that both economic and ecological drivers influence the magnitude of potential crop losses, thereby shaping farmer pest control practices.

An Ecoinformatics Approach to Field-Scale Evaluation of Insecticide Effects in California Citrus: Are Citrus Thrips and Citrus Red Mite Induced Pests?

Experimental approaches to studying the consequences of pesticide use, including impacts on beneficial insects, are vital; however, they can be limited in scale and realism. We show that an ecoinformatics approach that leverages existing data on pesticides, pests, and beneficials across multiple fields can provide complementary insights. We do this using a multi-year dataset (2002-2013) on pesticide applications and density estimates of two pests, citrus thrips (Scirtothrips citri (Moulton [Thysanoptera: Thripidae])) and citrus red mites (Panonychus citri McGregor [Acari: Tetranychidae]), and a natural enemy (Euseius spp. predatory mites) collected from citrus groves in the San Joaquin Valley of California. Using correlative analyses, we investigated the long-term consequences of pesticide use on S. citri and P. citri population densities to evaluate the hypothesis that the pest status of these species is largely due to the disruption of natural biological control-i.e., these are induced pests. We also evaluated short-term pesticide efficacy (suppression of citrus thrips and citrus red mite populations immediately post-application) and asked if it was correlated with the suppression of Euseius predator populations. Although the short-term efficacy of different pesticides varied significantly, our dataset does not suggest that the use of citrus pesticides suppressed Euseius densities or worsened pest problems. We also find that there is no general trade-off between pesticide efficacy and pesticide risk to Eusieus, such that highly effective and minimally disruptive compounds were available to citrus growers during the studied time period.

Does use of pesticides known to harm natural enemies of spider mites (Acari: Tetranychidae) result in increased number of miticide applications? An examination of California walnut orchards.

Integrated pest management (IPM) offers guidelines to reduce spider mite (Acari: Tetranychidae) outbreaks by avoiding pesticides known to be harmful to the natural enemies of spider mites. However, in practice, these guidelines can be inconsistent in their effectiveness. The project examined whether California walnut (Juglans L.) growers, following IPM guidelines to avoid pesticides harmful to the natural enemies of spider mites, achieved lower miticide use. Significant statistical tests suggested that fields with harmful applications were 40% more likely to have a miticide application than fields without. Although the IPM guidelines achieved the goal of reducing miticide use, further analysis of other potential causal mechanisms behind outbreaks could strengthen the effectiveness of the guidelines, potentially increasing IPM adoption.