Does a Mild Winter Mean More Pests?

While certain areas of the province have experienced some significant snowfall over the last couple of months, we really haven’t seen extremely low temperatures that could impact overwintering orchard pests. Unfortunately, at this point, it’s still too early to tell just what we can expect this season following a mild winter.

In general, most insect pests that overwinter in Ontario orchards are adapted to the cold temperatures. Many go into what is known as diapause, which is a type of hibernation, while others seek shelter, utilize warming techniques (ie., produce insect “antifreeze”), or a combination of these. For example, codling moth overwinter in diapause in a pre-pupal stage snug within a cocoon, or hibernacula, under the protection of leaf litter or loose bark at the base of the tree (Fig 1A). Japanese beetle and apple maggot overwinter as larvae and pupae respectively in the soil, insulated from the extreme temperatures by the snow cover (Fig 1B).  European red mite and several species of aphids overwinter as eggs by producing specialized proteins that prevent their cells from freezing (Fig 1C).

Figure 1. Overwintering insects in the orchard. (left) Codling moth hibernacula under loose bark on apple limb. (middle) Japanese beetle larva in soil. (right) European red mite eggs on tree trunk.

In a typical winter, insect populations often experience winter kill to some degree, depending on many factors within the micro-climate of the orchard, such as air drainage, snow cover and extreme temperature. While an extremely cold winter could reduce pest numbers, a mild winter with sufficient snow cover to compensate for fluctuating temperatures could favour winter survival. With a warm, dry spring, we could see a quick emergence of European red mite, rosy apple aphid, tarnished plant bug and mullein bug, whereas a wet, cold and rainy spring could delay emergence of these early season pests.

The concern of an early emergence of overwintering pests is the potential for more time for the population to build or for populations to peak earlier than expected – such as during bloom when insecticides can’t be applied. A longer growing season (and pest development season) could even result in seeing some pests getting in an extra generation per year. For instance, potato leafhopper is predicted to go through an extra generation per season if it arrives 2-3 weeks earlier than normal.

The same can be said of most overwintering disease inoculum, which typically overwinter protected from cold weather in infected branches or cankers (e.g., fireblight), in leaves on the orchard floor (e.g., apple scab), in dead wood or mummified fruit in the tree or on the ground (e.g., black rot, bitter rot), or in alternate hosts to be carried in from other areas on wind currents (e.g., rust).

The exception to this would be powdery mildew, which overwinters as mycelium in dormant fruit and shoot buds that were infected the previous season (Fig 2). Conidia grow out of buds in the spring as infected tissue and spread to other leaves causing secondary infection. Overwintering powdery mildew infected buds and shoots lack winter hardiness and can see 95% winterkill at temperatures of -24°C. Following a mild winter, early season powdery mildew risk is often high, especially in blocks with a history of this disease. 

Figure 2. Healthy apple bud (left) and powdery mildew infected apple buds (right).

Powdery mildew requires living tissue. So, if the winter has killed the infected bud, the mycelium will not survive. However, if the bud survives, there is a good chance the fungus survived, too. Since infected buds typically open later than healthy ones, susceptible, green tissue may be already present when the first conidia are produced. If conditions are ideal, even a small powdery mildew population can quickly explode if not managed properly. 

Kristy Grigg-McGuffin
Kristy Grigg-McGuffin

Horticulture IPM Specialist, OMAFRA