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Earlier in January, our area experienced extremely cold temperatures, resulting in many days below freezing.  With some of our recent warmer days, memories of the cold snap are fading but many folks have asked me questions about how winter temperatures impact insect populations, especially pests like Japanese beetles.

Recently, I visited with Plant Diagnostic Outreach Specialist, Diane Plewa from the U of I Plant Clinic to learn more about winter impacts on insects.  “Impacts may vary greatly based on where a particular insect overwinters, “said Plewa.  She went on to explain, “Some insects overwinter on stems or under bark, whereas others overwinter in the ground, like Japanese beetles.  Insects that overwinter on plants have a much greater chance for impacts”  

Those pesky Japanese beetles (Popillia japonica) we all deal with in summer spend the winter months as immature grubs under the ground.  Although these pests are most noted for the foliage they consume on our favorite landscape plants in their above-ground, adult stage, they spend a significant portion of their annual lifecycle underground feeding on shallow plant roots, primarily of grasses.

Alerted by our first frost in the fall, the Japanese beetle grubs burrow deeper into the soil profile for overwintering.  It takes a significant amount of cold to cause mortality, requiring a prolonged soil temperature below 32⁰ F for 2 months or a single event that results in soil temperatures below 15⁰F.

All insects that overwinter in our area have some adaptation to the cold and thus, some resistance to our local conditions.  However, some insect pests, such as thrips (members of the order Thysanoptera), blow in from the south each spring.  In this case, any impacts to thrips populations will depend on winter conditions in the southern US, which can be entirely independent to the severity of our winter here in Central Illinois. 

“The temperatures we are currently having, probably won’t have a huge impact.  However, predicting the impact also depends on a lot of other factors,” Plewa explained in reference to our early January cold-snap.

In the case of Japanese Beetles and many other insects that overwinter underground, snow cover is a huge factor.  Amazingly, that cold blanket of snow actually insulates the soil profile.  Therefore, years with extensive snow cover, and extremely cold temperatures, will have less impact than years with no snow and similar freezing temperatures.  Without the insulating snow, soil freezes to greater depths resulting in greater insect mortality.  

To assess or predict actual insect populations during the coming growing season, there are many other factors that can come into play after winter weather.  “Winter kill not only impacts the bad bugs, but also the good bugs. So, the end result to us can sometimes be negligible,” said Plewa. 

Many insect populations are heavily influenced by populations of predatory or prey insects.  Once excellent example of this dynamic is the relationship between aphids (sap-sucking insects of the family Aphididae) and the multicolored Asian lady beetle (Harmonia axyridi). 

Lady beetles prey heavily upon aphids and scale insects during the growing season.  Therefore, the annual population of lady beetles has a substantial impact on aphid populations.  If winter conditions impact lady beetle populations, we may see more aphids in summer.  Conversely, if summer conditions are not favorable to aphids, we may see a smaller lady beetle population, resulting in fewer than normal overwintering and a smaller population the subsequent summer. 

In attempting to answer the question of how our annual winter conditions impact insect populations in the growing season, I’ve often had to give the answer, “it’s complicated” or “it depends”.  As with many things in nature, there are just too many factors to predict, but that is what makes biology exciting to me.