Impact of Climate Change on Insect Pests of Walnut and Almond Crops in California

Dr. Prakash Kumar Jha
Division of Agriculture and Natural Resources, University of California.

Dr. Jhalendra P. Rijal
Division of Agriculture and Natural Resources, University of California.

Dr. Tapan B. Pathak
Division of Agriculture and Natural Resources, University of California.
Department of Civil and Environmental Engineering, University of California, Merced.

Walnut and almond growers in California are expected to face a burgeoning rise of insect pest populations under the influence of climate change. Growers can take proactive actions to minimize future risks associated with these damaging pests.

Our recently published study in the journal Science of the Total Environment¹ has unveiled a looming transformation in the population of three major insect pests affecting walnuts and almonds —the codling moth (Cydia pomonella), the peach twig borer (Anarsia lineatella) and the oriental fruit moth (Grapholita molesta)— as a result of the projected temperature increase in future. Farmers have already been facing challenges dealing with these destructive pests, and any further rise in their population will likely have implications for the supply of these commodities along with impacts on the economy and employment.

Led by a team of interdisciplinary scientists from the University of California Division of Agriculture and Natural Resources, UC Merced, and the USDA California Climate Hub, the study delved deep into the intricate interplay between rising temperatures and insect life cycles and the subsequent impact on insect populations across California’s Central Valley, the “food basket of the world.” Leveraging a predictive model of insect pests’ life cycle based on temperatures and future climate projections from the latest generations of climate models along with comprehensive data analysis, we found that climate change is expected to increase the population of these insects in future.

Central to this study is the impact of temperature on the insect pests’ life cycle and the potential consequences for pest management. Warmer conditions can be more damaging because of the rapid development of insects, resulting in a quick population build-up. Warmer winter can also lead to an earlier-than-normal onset of insect activity in the spring. Our study found that populations of the codling moth, the peach twig borer and the oriental fruit moth are expected to increase in California due to their appearance in spring shifting earlier by up to 28 days, resulting in a reduction of up to 19 days in the time required to complete a generation by the end of the century. In the next 30 years, an additional half-generation of these pests is expected to arise within the growing season. The increase in the pest population is expected to be relatively higher in southern parts of California than in the northern parts. The temperature is projected to continue increasing, resulting in a growing pest population, if the current trends of greenhouse gas emissions are not curbed.

The implications of this projected surge in insect pest populations are serious considering the vulnerability of the almond and walnut orchards to these pests. Almost all walnut plantations in California are susceptible to damage from the codling moth. Similarly, the damage to almond orchards caused by the peach twig borer and the oriental fruit moth can be very serious if not properly managed.

Although further research is needed to quantify the expected increase in damage due to rising populations of these pests, a greater number of generations of these pests in the same growing season generally means more damage to these commodities. It also increases the burden of spraying to control these pests, which would definitely increase production costs. In order to produce quality fruits and nuts, farmers have to manage multiple generations of these pests. Late generations are particularly destructive because they feed directly on the fruits and kernels, making them unmarketable. Moreover, more pesticide spray is not a solution, due to the risk of developing pesticide resistance, a decrease in the effectiveness of pesticide products at higher temperatures and in drier conditions, and a negative impact on beneficial insects and the environment.

We recommend adopting a climate-smart integrated pest management (IPM) approach to mitigate the possible increase in pest pressures in the future. IPM focuses on understanding the best biology, developing and refining monitoring tools, and adopting practices that are preventive and biological-based, keeping insecticide as a last resort. The climate-smart IPM approach may include practices that seriously consider the long-term management of pests or pest complexes, keeping future pest issues in mind. These practices include planting pest-tolerant crop varieties, developing tools to protect and utilize natural enemies, and applying bio-based technologies such as mating disruption. What is most important to deal with this crisis is the development and adoption of pest forecasting systems, including long-term prediction combined with short-term potential outbreak, pest monitoring and early detection systems.

Our study did not look at how these pests would adapt under high temperatures, particularly under various patterns of change in frequency, intensity and duration of high temperatures in the future, how natural enemies of these pests behave, or how these changes overlap with crop growth cycles. More research is needed to provide support and guidance to growers about the latest developments in pest management tools and how to adapt their practices to changing conditions.

Photo: Tapan Pathak.

Reference:

1. Jha, P. K., Zhang, N., Rijal, J. P., Parker, L. E., Ostoja, S., & Pathak, T. B. (2024). Climate change impacts on insect pests for high value specialty crops in California. The Science of the Total Environment, 906, 167605.