Milder than average winter temperatures in southwest Ohio are continuing with daily high temperatures forecast to reach the mid-60's over the next seven days; some prognosticators are predicting that records may fall. Our consistent above average temperatures this winter have ignited questions about whether or not we will suffer higher than average numbers of insect pests.
There is no concise sound bite or tweet-worthy answer. It's complicated by the wide ranging strategies used by insects to successfully deal with winter conditions. The answer is made even more complicated because evolutionary selective advantages that reward one strategy over another haven't been consistent across taxonomic groups. So, we can't say that all beetles deal with winter in a certain way, or all flies, or all bees, etc. Questions spawned by warm winters are at the opposite end of those that arise from winters dominated by lower than average temperatures, but the same principles apply.
At one end of the spectrum are insects that clearly benefit from warmer than average winters in Ohio. Goldenrain Tree Bugs (Jadera haematoloma) appeared in large numbers on their namesake host in southern Ohio during the 2012 and 2013 growing seasons. This native insect is common in Florida where it feeds on the seeds of plants in the soapberry family. However, they disappeared from Ohio during the winter of 2013-14. Populations of the Common Bagworm (Thyridopteryx ephemeraeformis) were also decimated. The rise and fall of these insects in relation to warmer or colder than average winter temperatures adds support to the perception that a warm winter means more insect pests. Of course, the winter survival of these insects was based more on luck than evolutionary advantage. This is not the case for most of our Ohio insect pests.
Some southern insects give up the ghost in Ohio during even our mildest winters, but they reappear in our state in the spring or early summer by being blown north from their southern winter enclaves. Potato Leafhopper (Empoasca fabae) is the poster child for this seasonal repopulation strategy. The hoppers spend the winter enjoying the sunny south but mated females are wafted north on storm fronts in the spring to establish damaging populations. Neither colder than average nor warmer than average Ohio temperatures will have an effect on this pest. At least thus far; perhaps climate change will change this.
Other insects handle Ohio winters by seeking over-wintering locations where temperatures remain moderate. However, this may be a risky proposition. All insects are cold blooded (ectothermic) meaning that their body temperature and thus their metabolic rates depend on external heat sources such as sunlight, heated surfaces, or ambient air temperature. Insects that spend the winter hiding from the cold prepare themselves by accumulating fat in the fall then they live off the fat through the winter. A warm winter may mean they can literally starve to death because they are not feeding and high temperatures increase their metabolism causing them to consume their fat reserves. Multicolored Asian Ladybeetles (Harmonia axyridis) and Brown Marmorated Stink Bugs (Halyomorpha halys) commonly seek protected overwintering quarters in the walls or attics of our homes. However, if they venture into our homes, they quickly burn their fat reserves and die. While this is an extreme example, it serves as example of the risks associated with this overwintering strategy and why a warm winter may not benefit all insects.
Rather than depending on finding a protected location in the fall, some insects enter the winter in protected packaging of their own making. Eastern Tent Caterpillar (Malacosoma americanum) moths produce eggs that appear to be surrounded by bubble wrap. No doubt the air in the hollow structures surrounding the eggs provides some insolating protection against cold temperatures. Since they are protected against the cold, warm winter temperatures would just be icing on the cake. On the other hand, common bagworms overwinter as eggs protected by both their bags and the dead bodies of females. However, research has shown the eggs are subject to low temperature mortality with 50% of the eggs killed if exposed to 6.8F.
Some insects are simply unaffected by even our coldest Ohio winters because they can protect themselves with antifreeze; literally! It's not cold temperatures that kill insects, its sharp-pointed ice crystals that form below 32F to pierce and destroy cells that kills insects. Insects may rely on a wide range of chemicals that lower the freezing point of their blood (hemolymph) to prevent the growth of spiky ice crystals. This includes ethylene glycol, the same chemical we use as antifreeze in our cars. Of course, this ability means a warmer than average winter has little impact because even a colder than average winter does not kill these insects.
Finally, some insects dodge the winter bullet by employing several strategies including diapause. This is a physiological state that’s much deeper than hibernation. The insect’s metabolism slows way down – regardless of temperature – and does not return to normal until the insect experiences certain environmental cues that cause it to come out of diapause. If the insect combines diapause with producing antifreeze, finding a protected location, or making their own protection, it will substantially increase the insect’s success rate with handling winters one way the other.
Gypsy Moth (Lymantria dispar) spends the winter as diapausing 1st instar larvae inside eggs beneath a protective covering of scales deposited by the females. Research has revealed that this combination of overwintering strategies has its limits with the larvae being killed if temperatures of -20F last from 48 to 72 hours. However, warmer than average winter temperatures does not mean we will see a higher than average number of gypsy moths; it just means this low temperature threshold (which would be rare for much of Ohio) was not experienced. Indeed, research has shown that pathogens, predators, and parasitoids usually have a greater impact on gypsy moth numbers compared to just cold or warm winter temperatures.
The bottom line is that overwintering strategies of insects are varied and complicated meaning that we can't paint with a broad brush to predict what effect, if any, our balmy winter temperatures will have on insect pests. This early season question is much like the late season call for predicting fall colors. About all we can say with certainty is that we will have insect pests during the upcoming growing season regardless of winter temperatures … and leaves will change color at the end of the season.