We hear about climate change affecting sea levels and increasing the future incidences of extreme weather events. As cold-blooded animals, insects are affected by climate, specifically carbon dioxide levels, precipitation, and temperature. Insects that feed on agricultural crops could become more problematic pests as the climate changes. The predicted rise in temperatures may cause imbalances in the natural ecosystem, affecting the interaction among crop pests, their food crops, and their natural enemies. Area IPM Advisor Jhalendra Rijal and colleagues worked on a case study showing how future changes to California temperatures could affect navel orangeworm populations. Navel orangeworm causes significant economic loss in commercially grown almonds, pistachios, and walnuts. These three crops have a combined cash value over $9.3 billion annually.

Navel orangeworm is well studied in California and has three or four generations per year. Currently, a degree-days model is used to predict when generations or certain life stages occur for timing pest management actions. Rijal and his colleagues plugged in the navel orangeworm degree-days model into several scenarios for climate change to forecast how navel orangeworm populations may change under changing climate scenarios. “Temperature variations can directly affect the developmental rates, behavior, and overall population dynamics of this pest, and it is critically important to understand these dynamics with respect to climate change,” states Rijal and coauthors Tapan Pathak and Mahesh Maskey in a recent article.

The climate is already changing in the San Joaquin and Sacramento valleys. Higher minimum and maximum daily temperatures have been observed. More dry, warm years have been recorded. If temperatures rise, it is expected that some pests will decline, while others may expand their ranges or, as predicted with navel orangeworm, become more problematic.

Using climate change models under two different emissions scenarios, Rijal discovered that California may see one more navel orangeworm generation (five instead of the three or four currently). The time to complete one generation (time to go from egg to adult) was predicted to be faster too, especially in pistachio, where it is already faster than in walnut and almond.

The models were applied to 23 Central Valley counties where all of California’s 2+ million nut crop acres are grown. The climate is already changing in the San Joaquin and Sacramento valleys. Higher minimum and maximum daily temperatures have been observed. More dry, warm years have been recorded. If temperatures rise, it is expected that some pests will decline, while others may expand their ranges or, as predicted with navel orangeworm, become more problematic.

Rijal’s article: Pathak TB, Rijal JP, Maskey M. 2020. Impact of climate change on navel orangeworm, a major pest of tree nuts in California. Science of the Total Environment https://doi.org/10.1016/j.scitotenv.2020.142657.

Long-Term Impacts

The navel orangeworm study could be applied to other pests in other crops. Understanding climate change effects early could prepare for future pest management needs and minimizing product risks as the climate changes, enabling growers to continue successfully producing agricultural crops in California.