Climate change is the long-term statistically significant change in climate parameters due to increases in greenhouse gas (GHG) emissions in the earth’s atmosphere, including carbon dioxide (CO2). Increased temperatures, increased intensity and frequency of extreme events such as drought and flood as well as increased precipitation variability are examples of climate change. These parameters can have significant impacts on agricultural production and pest management, among other broader implications. Since temperature is most strongly related to insect pests, we will discuss the effect of increased temperature on insect pests and then present a navel orangeworm case study in a later part of this article. But first, we’ll discuss two other parameters in general terms: elevated CO2 and unpredictable precipitation.
CO2 Levels and Pests
Elevated CO2 concentration can have an impact on both crop plants and pests. In general, a high CO2 level to a certain degree favors plant growth by increasing photosynthetic activity, which can change both quantity and quality (i.e., chemical composition) of the leaves. However, we should not take this as a standalone factor as plant growth is the result of multiple environmental and other factors such as crop type/variety, temperature, humidity, soil moisture, soil fertility and more. Several studies have shown that increased CO2 levels can influence the insect fecundity, consumption rates, insect density and abundance, all of which can directly or indirectly increase the risk of individual insects causing damage to host crops as insects are more attractive/responsive to these plants and can feed more.
Precipitation and Pests
Water is the most critical resource for commercial crop production; however, studies reported that natural water sources have become scarce (i.e., drought) or unpredictable (i.e., high-intensity flash floods) across major crop-producing regions globally. In California, a prolonged period of drought has been reported from 2011 through 2019 (source: US Drought Monitor). A recent report from May 2021 suggested that ~74% of California land is under the “extreme drought” condition (drought.gov/states/california). Droughts affect plant-feeding arthropods (i.e., herbivores) in the following ways: 1) dry conditions may favor the development and growth of herbivores. For example, spider mites do well in hot and dry environments; 2) drought-stressed plants can release stress signals that attract certain insect species. For example, plants lose moisture through transpiration, and as a result, there is a gap or break created in xylem water columns. That condition produces an ultrasonic acoustic emission detected by certain beetle pests that attack those trees; and 3) drought-stressed plants are more susceptible to herbivore (plant feeding insects and other organisms) attack as the plants cannot produce enough secondary metabolites to defend against the pest attack.
In California, we observed an increased problem with flatheaded borer, a wood-boring beetle pest in walnuts in the last four to five years. Several growers and PCAs were caught off guard by this pest as this was never a big issue in the past. Although multiple factors might be contributing to this problem, including sunburn, increased walnut acreage, sub-standard land quality for walnut orchards inducing stress in the plant, etc., the most plausible explanation is the prolonged drought (circa 2011-19).
Temperature and Pests
Temperature is the major driving factor of global warming and climate change. In California studies (e.g., Pierce et al. 2018), it has been reported that future statewide temperature is projected to increase by 2 to 7 °C by the end of this century. More importantly, the rate of increase in the minimum temperatures has been substantially higher than the maximum temperatures. Temperature changes directly affect the biology, physiology and behavior of insect pests, impacting their survival, development and reproduction. Also, temperature influences insect mobility and migration, leading to shifts in insect pest dynamics, abundance and distribution.
Elevated temperature level impacts the pest population and the performance of the natural enemies by altering the synchrony of pest-natural enemy interactions. Although insect growth and development are highly dependent on temperature, plants are also affected by the temperature change, which has several implications in pest management. For example, some existing host plants of insects can be more or less favorable, and in some cases, the plants become a new host (attractive) to the insect. This is due to the physiological and biochemical changes within the plant systems mediated by increased temperature and other associated factors.
Since insect and plant growth tend to respond to temperature differently, elevated temperature can alter their synchrony, positively or negatively affecting the insect-host plant relationship. The general assumption is that increased warming can change or expand the geographic range of the pests and predators, increase the risk of invasive pests, increase insect winter survival and begin in-season pest activities earlier as well as increase the number of generations, desynchronization of pest and natural enemy dynamics or pest and host plant interactions. Not all insects and plants respond to environmental cues the same way. Therefore, individual pests and their interactions with natural enemies and host plants under increased temperature conditions must be studied separately.
Navel Orangeworm and Climate Change: A Case Study
Navel orangeworm (NOW), a primary pest of almonds, walnuts and pistachios, directly feeds on nutmeat and causes economic damage. In addition, the infested nuts become an easy target of Aspergillus mold and increases the risk of aflatoxin contamination in otherwise healthy nuts. NOW is an opportunistic, highly mobile and multi-host pest with up to four generations in the Central Valley. With more than two million acres of the major host crops in the valley and increased drought and dry conditions in the last decade, this key pest has become a significant risk to nut crop growers.
Recently, we published an article (doi.org/10.1016/j.scitotenv.2020.142657) looking at the potential impacts of climate change (primarily focused on temperature) on the population dynamics of NOW. For this study, we selected 23 Central Valley counties (Figure 1) and used five temperature data points to represent each county. Temperature (max/min) data were derived from 10 climate models (GCMs or “General Circulation Models”). We categorized the climate period into historical (1950-2005) and future (2005-2040; 2040-2070; 2070-2100). We used two emission scenarios based on “Representative Concentration Pathways” (RCP) of 4.5 and 8.5. RCP. 4.5 is a “medium” emissions scenario that models a future where societies attempt to reduce GHG emissions, while RCP 8.5 is more of a “business as usual” (i.e., lacking the stringent climate mitigation) scenario.
All results presented here are from the RCP 4.5 scenario. We used one calculation of the model for walnut and almond together and a separate calculation for pistachio because of the shorter generation period of NOW in pistachio. We assumed April 20 (i.e., 148 degrees C degree days from January 1) as the spring egg-laying biofix for this analysis. Biofix is when 50% or more of the egg traps in an almond orchard have NOW eggs on them. Biofix date is used to begin accumulating heat units (i.e., degree days). The duration to complete the first generation used for the calculation was 565 degrees C degree days. In contrast, the duration used for the subsequent generations was 444 degrees C degree days for the almond/walnut model and 402 degrees C degree days for the pistachio model. Degree days measure heat units over time and are calculated based on daily maximum and minimum temperatures.
In this study, we looked at two important aspects of NOW population dynamics: 1) the timing of the occurrence for individual generations; and 2) the duration of each generation for the historical and future periods mentioned above. The results showed four complete generations (in contrast to the current three to four generations) of NOW could occur consistently throughout the Central Valley within the next 20 years in almond and walnut. By 2040, the study predicted that an extra generation (i.e., fifth generation) is likely to occur in the three southernmost counties evaluated (Kern, Kings and Fresno) in the almond/walnut model (Figure 2) or seven counties in the pistachio model (Figure 3). By the end of this century, the occurrence of the fifth generation will extend to 11 (almond/walnut model) or 17 (pistachio model) of the 23 counties included in the study.
Additionally, NOW spring activity will begin earlier than in the past, and it will take fewer days to complete each generation due to projected future temperature increases. The study reported that the duration of the first generation of NOW was shortened by almost six weeks in some cases. For example, in Sacramento county, NOW was expected to complete its first generation in almonds and walnuts around the 193rd day of the year historically, whereas it ranged between the 154th (by 2100) and 180th (by 2040) day under future climate scenarios. Similarly, in pistachio in Tulare county, model results showed NOW was expected to complete the fifth generation around the 292nd day (approximately October 19 of the calendar year) historically, while according to future climate scenarios, this could occur as early as the 262nd day of the year (approximately September 19).
To summarize the results of this case study, climate change models predicted an increase in NOW pest pressure for three major, high-value tree nut crops, and the fifth generation of NOW is likely to occur under future climate conditions. The study presented here is based on models that estimate and predict the effects of climate change in different areas. Of course, there is underlying uncertainty with these scenarios because we don’t know what accelerates different model parameters and to what degree. Therefore, the information provided here should be taken as general guidance about a potential future scenario. Our study aimed to inform the industry and allied stakeholders about possible future threats so that we all can think about the potential mitigating measures needed to address impacts of climate change on the integrated management of NOW. These may include creating awareness, designing experiments around this topic and potentially proactively engaging the industry to address its future needs.
Implications for Pest Management
Climate change directly impacts the pest and natural enemies population dynamics in agricultural production systems. Indirect impacts include changes in insect pest composition, expansion of geographic range for pests and host crops and plants, increased occurrence of invasive species and the reemergence of the minor pests with different pest status.
Adapting pest management practices to a changing climate is an ongoing process. These adaptations can be achieved by formulating modified IPM practices, developing regional networks for monitoring certain high-risk pests, redesigning sampling plans, rethinking insect pest thresholds and using pest population prediction models and tools.
The solution is not just adjusting the pest management strategy by growers; it should be bigger. We all should encourage and help formulate crop production and pest management strategies and policies around these matters. These may include provisional incentives to farmers, enabling robust investment in research to address these issues in the long term while maintaining the viability of production and pest management systems.