Cold may reduce invasive insect populations

From the talk of old timers in the Frederick County area, you’d think this year’s January lows around zero and highs in the teens were normal a half century ago. It turns out they were, as the mid-1960s had a several years of below average temperatures. Science, though, points to a slow warming trend in Maryland of less than one degree over 50 years, which with some recent warmer winters make this year’s cold snaps seem very cold. At the same time, and more related to global trade, invasive insect populations have been increasing in our area. Many recently-arrived invasive insects come from temperate climates, and do not proliferate in zones of extreme cold. Some of the ones that bother us the most may reduced in number by the cold, but unfortunately not eliminated.

The emerald ash borer (Agrilus planipennis) identified in Michigan about 2002, likely had arrived in the U.S. 10 years earlier. The ash borer spread rapidly to many states and to Maryland. It most likely came to the US in crating or pallet wood from Asia. The brown marmorated stink bug (Halyomorpha halys) was first identified in Pennsylvania in 1999, and has spread rapidly across the U.S. It most likely came to the U.S. in packing materials from Asia. The Asian tiger mosquito (Aedes albopictus) arrived in the continental U.S. in 1985 in water in used tires shipped from Asia. It has spread into Maryland, and in the last five years has become a common pest in our area, seen much more often now that the relatively less aggressive native mosquito. The hemlock woolly adelgid (Adelges tsugae) was first introduced into the U.S. from Asia in the 1920s, migrating to our area in the 1960s.

Credit: - James Gathany, CDC

Since these and other invasive insects come from warmer climates in Asia, they can succumb to extreme cold.

Overwintering in different forms, the insects generally seek sheltered places to assure survival. The Asian tiger mosquito overwinters in egg form, depositing eggs in sheltered wet areas. The emerald ash borer overwinters in larva form in logs. Although wood is a good insulator, U.S.

Forest Service freezer experiments showed 50 percent survival of ash borer larva in logs to – 20F, but little survival below – 30F. The brown marmorated stink bug overwinters in adult form, seeing warm and sheltered places to survive the cold. While zero degree temperatures in our area kill the adult stink bugs, there are many that hide in places such as cracks in house envelopes that will not be affected. The hemlock woolly adelgid is thought to be more susceptible to cold, since it overwinters in egg form, the eggs being deposited on the underside of hemlock tree branches exposed to the freezing temperatures.

Insulating snow cover has come with the zero temperatures to help insulate the ground and insect winter habitats. Native insects are accustomed to the variations in cold in our area and are well adapted to survive.

Although we will see invasive insects return this spring and summer, many will be set back in numbers this year by the cold.

Article by FCFCDB

Nature Note for 2/2/2014