Study: Relationship Between Crops, Land, and Pesticide Is Highly Complex

Study: Relationship Between Crops, Land, and Pesticide Is Highly Complex

Earlier this year, University of California Santa Barbara’s Ashley Larsen and University of British Columbia’s Frederik Noack released an eight-year study on how crop diversity and other landscape features drive insecticide use.


In their abstract, Larsen and Noack say they wanted to take a deeper look to see if “concerns that more simplified cropland with lower crop diversity, less noncrop habitat, and larger fields results in increased use of pesticides due to a lack of natural pest control and more homogeneous crop resources.”

The study gathered data on crop production and insecticide use from more than 100,000 field-level observations from Kern County, CA, and ran from 2005 to 2013. It considered how factors like crop diversity, field size, and cropland affect insecticide use in normal agricultural practice.

The results? Here’s what Larsen and Noack say in their abstract:

“Overall, we find that higher crop diversity does reduce insecticide use, but the relationship is strongly influenced by the differences in crop types between diverse and less diverse landscapes. Further, we find insecticide use increases with increasing field size.

“The effect of cropland extent is distance-dependent, with nearby cropland decreasing insecticide use, whereas cropland further away increases insecticide use. This refined spatial perspective provides unique understanding of how different components of landscape simplification influence insecticide use over space and for different crops.

“Our results indicate that neither the traditionally conceived ‘simplified’ nor ‘complex’ agricultural landscape is most beneficial to reducing insecticide inputs; reality is far more complex.”

American Vegetable Grower® reached out to Larsen, Assistant Professor, Agricultural and Landscape Ecology, to learn more about the study.

AVG: How are you defining “more diverse crops”?

Larsen: Here we define crop diversity based on what is known as Simpson’s Diversity Index, which is a measure of the proportional abundance of any given crop. Our main results used this diversity index calculated using the species of surrounding crops, so a field surrounded by cropland composed of different crop types (by species) is more diverse than a field surrounded by cropland of one crop species. We also evaluated diversity calculated based on crop type, agricultural class, and genus and family of surrounding crops.


AVG: Did any of the operations use integrated pest management (IPM) methods, biocontrols, or organic production? If so, how did those practices affect pesticide use levels?

Larsen: Unfortunately, the data don’t include any information on whether an operation uses IPM or organic production. That would be fantastic.


AVG: Does the field size of these particular crops have an impact?

Larsen: Yes, field size is important. Averaged across crop types, we see that field size leads to an increase in insecticide use that is consistent across models. Roughly, a 1 standard deviation increase in field size results in a 0.9-to-1.8-pounds/acre decrease in insecticides (as can be seen in Supplementary Figure 1). Again this varies by crop type. For almonds and pistachios, increasing field size results in a statistically significant 1.8-to-2.7-pounds/acre increase in insecticides, for carrots the relationship is potentially large (5.4 pounds/acre), but we don’t find a statistically significant effect. As the number of fields of a particular crop gets smaller, our statistical power to identify how surrounding landscape features impact insecticides decreases as well.

Figure 1


AVG: Would you share the impact of diversity with other vegetable crops?

Larsen: We are working on those analyses now. As mentioned above, the analyses for individual crops are less statistically robust because they are based on substantially fewer observations than all crop types pooled.


AVG: Are you planning on building on any of these findings? If so, what would you like to learn?

Larsen: Yes, we are working on a couple of exciting directions forward. For example, we are trying to tease apart whether the observation that large fields lead to an increase in insecticides is due to ecological or economic factors. From an ecological perspective, as fields get larger, natural enemies are thought to have more difficulty spilling over into the center of crop fields and providing adequate biocontrol. From an economic perspective, it is thought that farmers with larger fields have more control over the pest population on their field and derive more benefit from using insecticides. Thus, teasing apart the underlying mechanisms will provide potentially actionable information into how to reduce insecticide use on large fields.