By Caleb P. Goossen, Ph.D., MOFGA’s Crop Specialist
With the changing climate introducing greater variability and weather extremes to our seasons, it is perhaps to be expected that the insect and disease pests that we deal with have also been impacted, and are expected to be further impacted moving forward. Insects and disease pathogens are poikilothermic (e.g., “cold-blooded” ectotherms that rely on their environment for heat), and so their development and activity are greatly impacted by ambient temperature. As conditions warm, so too can their rates of feeding or infection, and maturation and reproduction.
Critical growing degree day (GDD) thresholds are frequently being reached earlier than in years past. Although there are upper limits at which higher maximum temperatures stop increasing plant and pest growth, the greater portion of the increase in average temperatures that we’ve seen in Maine is from higher minimum temperatures, both within individual days and across seasons. This can mean greater lengths of time above GDD thresholds associated with pest activity, and/or shorter or less regular instances of low temperature thresholds like frosts and hard freezes. The combination of greater accumulations of heat, and longer periods of time without low temperatures (which may have arrested growth or other behavior in prior years), have resulted in “new” pest species being found in Maine, as well as new behaviors of “old” pest species that were already familiar to Maine growers.
Climate Impacts on Arrival and Behavior of Pest Species
When a “new to Maine” species shows up, climate change is often mentioned, if not completely blamed. However, the arrival of a new species can also simply be an introduction of an organism that did not occur here previously and may not be met with natural enemies that otherwise limit its population and impact in its biome of origin.
Two examples of relatively new pests to Maine that we cannot blame climate change for are the leek moth and the swede midge. Both of these species were novel introductions to North America in relatively recent history, and while they were new to Maine in the past decade, they entered the Northeast United States from Canada and have actually been working their way south into the country.
An example of a species showing up in Maine that I feel we can essentially blame climate change for entirely is Stemphylium gray leaf spot of tomatoes. Gray leaf spot is not at all a “new” disease of tomatoes for the Southern and Mid-Atlantic states that provide the hot and humid conditions in which it thrives. However, it has gradually been increasing its range northward and creating new problems for Maine growers that, like me, had never previously had reason to know of its existence.
Even if many “new” pests to Maine are simply the result of introductions — and not the sole result of climate change — the continued spread and impact of introduced species are expected to be very much influenced by climate change moving forward. For example, the forest pest hemlock wooly adelgid was introduced into the United States about 100 years ago, but it is still spreading relatively slowly in Maine, as particularly cold winter temperatures can cause high levels of winter mortality. This natural check to the spread of hemlock wooly adelgid is expected to diminish moving forward, depending upon the rate at which Maine’s winter minimum temperatures continue to inch higher.
Other relatively recently introduced species of concern for growers — which may be kept partially at bay by Maine’s current climatic conditions — include the spotted lanternfly and the brown marmorated stink bug (Fig. 1). Both of these species have entered southern New England and have been found in Maine, but, as of now, neither have established a solid enough foothold to cause the level of damage that they may in the future. In contrast, the cabbage white, also known as the imported cabbageworm, familiar to generations of Maine growers since its arrival about 150 years ago, is expected to find Maine’s future climatic conditions to be less hospitable.
For growers and non-growers alike, concerns regarding tick populations and potential consequences for the spread of tick-borne diseases seem near universal. Lone-star ticks, native to states much further south, could easily become commonplace in much of Maine according to some climate modeling (see Fig. 2)
The future spread of species described above are only some of the “known unknowns,” as we cannot say for sure what their eventual distribution will be. I can’t begin to imagine what surprise “unknown unknowns” await our discovery (similar to how gray leaf spot surprised us in the past few years). Regardless, we will still need to deal with more species that are new to Maine, or new to more areas within Maine, than previously. The specific difference between climate-driven spread and simple introduction of new species will not be as important as understanding the biology and impact of each species individually, monitoring their spread, and implementing management strategies as appropriate.
New Behaviors of Old Species
Some pest species that we already know well are expected to shift their behaviors in response to the changing climate, and may have begun to shift already. The example I share most often is the fact that squash vine borers used to only produce one generation of adult moths per growing season but are increasingly emerging in two waves. The second wave is suspected to be a second generation that can take advantage of longer warmer summers, and not simply a difference in how long sub-populations take to pupate. The potential for multiple generations of insect pests that previously had more predictable ebbs and flows may result in the need for more continuous monitoring and management. This may allow pest populations to grow to exponentially higher levels in their final generation of the season than they would have previously — potentially resulting in higher overwintering populations as well.
In addition to temperature impacts, the increasingly prolonged stretches of either wet or dry weather also have an impact on the pests we deal with — though likely in a season-by-season manner instead of a more continuous change as with increasing temperatures. Dry years tend to favor pests like thrips and two-spotted spider mites, while wet years tend to favor moisture-loving slugs and snails, and diseases that can best infect and spread with prolonged periods of leaf wetness, like early and late blights of tomato, or Phytophthora capsici, which has a lifecycle stage that can actually swim through standing water in a field. Some familiar diseases do not need liquid water and can spread rapidly in our increasingly warm humid growing seasons, such as powdery mildews and downy mildews.
The Trade-Off of Controlled Environments
Because many of the most common damaging crop diseases require prolonged leaf wetness to successfully infect, the trend for more controlled-environment growing spaces (i.e., greenhouses and high/medium/low tunnel spaces) has largely removed their impact for many growers. Unfortunately, the honeymoon period of tomatoes with practically no foliar disease issues has ended for many growers, as increasingly humid conditions are making management of diseases that do not require liquid moisture — such as botrytis, leaf mold, and gray leaf spot — much more difficult by limiting the ability to draw in outside air that is actually drier than the air already around the plants when venting protected growing spaces.
Additionally, the protection that controlled environments provide from the coldest winter temperatures, combined with the trend towards higher minimum winter temperatures, can allow these spaces to act as a “green bridge” between growing seasons for some pests that would have otherwise not persisted in a Maine winter, or at least taken much longer to build up to a damaging population in the following year. Examples that I have seen on Maine farms include cabbage aphids overwintering on high tunnel kale and infesting the next spring’s brassica crops, and onion downy mildew jumping from overwintered onions to the much larger main-season onion planting.
Climate Impacts on and Interactions with Organic Pest Management Strategies
Our traditional organic pest management approaches prioritize prevention: selecting disease-resistant varieties; ensuring good air circulation; optimizing soil health to support robust plant defenses; and practicing careful crop rotation and sanitation. This foundation will continue to be important for future efforts — perhaps even more so. New climatic conditions are likely to provide new challenges for management with pesticide products when foundational cultural approaches are insufficient and, surprisingly, new possible opportunities for biological controls.
Issues and Opportunities for Sprayed Materials
Prolonged wet and humid conditions may increase pathogen growth and reproduction rates, increasing the frequency at which preventative fungicides need to be applied. Additionally, greater rainfall intensity may wash away applied materials, requiring reapplications, or other shifts in strategy: commercial growers may need to utilize “sticker” spray adjuvants more frequently, and any grower may want to consider applying protective fungicide as soon after heavy rainfall events as possible, instead of rushing to apply it before a rain event. In addition to escaping the need to reapply the material after it gets washed away, this strategy will hopefully still protect plant surfaces before the wet conditions spur the release of new spores from pathogens already present, and before any spores that arrived with the rain have had time to germinate and infect plant tissue — acting during that limited window of time is critical, however!
Although I may be grasping for good news, a possible upside to increasing precipitation and humidity is the potential for greater success rates of some biological control species and biopesticide products. Entomopathogenic nematodes and entomopathogenic fungi, like Beauveria bassiana, both tend to be more successfully deployed, and survive longer, in conditions with more water and higher humidity.
This article was originally published in the fall 2025 issue of The Maine Organic Farmer & Gardener.