June 20, 2013

By Trish Hanson

Cerceris fumipennis wasps, known to some as “smoky-winged beetle bandits,” remain one of our most promising tools for detecting the presence of emerald ash borer.   These solitary wasps dig nests in the ground, fly up into trees to search for, sting and capture buprestid beetles (including the emerald ash borer, if present) then fly back to their nests with the stunned, but still living, prey. 

When safely inside the underground nest, the wasp caches its prey in a separate cell, lays an egg in the beetle, and seals the cell with a plug of soil. The process may be repeated until a number of cells are living nurseries. When the eggs hatch, the young larvae eat the paralyzed prey and develop into adults that emerge the following year.  Cerceris wasps are in flight for only about two months, during the months of July and August.

Emerald ash borers have been recovered from wasps in several areas with known populations of EAB (Ontario, NY and IL) but July 16, 2012 marked the first time that initial detection was made through monitoring Cerceris wasps.  This occurred in the town of Prospect, CT, and gratified regional “wasp watchers.” 

Thanks to many volunteers, including a number of First Detectors, we have located and monitored Cerceris nest sites in Vermont for several years.  Last year, through the efforts of volunteers, Cerceris nests were found in Bennington and Grand Isle Counties for the first time.  No emerald ash borer have been found in Vermont to date, but in 2012, 996 other buprestid beetles were collected at 18 Cerceris sites in eight Vermont counties.

Seasoned wasp watchers can now easily recognize the characteristic profile of wasps carrying large beetles, and some of the more experienced are becoming familiar with the sight of a wasp carrying a smaller buprestid species.  Stunned beetles are held upside down, head forward, under the wasp’s thorax and a “loaded wasp” is apt to make a very direct and quick return trip with a beetle in tow.  Wasps are often chased by satellite flies which  are eager to lay their own eggs on the buprestid beetle prey, making Cerceris nest sites drama-rich locations.

When will the smoky-winged beetle bandits emerge and start scouting for buprestid beetles?  Claire Rutledge at the Connecticut Agricultural Experiment Station calculated that wasp activity begins at 880 DD () at base 50. Growing degree days (DD) are a measure of how warm the season has been. At base 50, the number is a sum of the heat from any day the mercury’s gone above 50°.

Claire provides this advice: “If you want to check the status of a colony site near you, here’s how!  Go to this link, http://uspest.org/NE/VT/index.html, where you can view a list of weather stations in Vermont.  At the top of the page is a form asking what model of DD you want.  In the spot where it says 41 (the default), type in 50. Then pick a weather station that is near to your colony and click in the little circle for 2013. It will bring up a page showing DD accumulation thus far (the far right column) and then projected DD based on historical data.  Look for 880 DD and that is your predicted emergence date. Keep in mind that this is an average, and you should start looking a couple of days before that date.” 

If the DD model holds, the Vermont wasps are likely to begin emerging at the beginning of July.  First emergence at some sites could be 1-14 days later.

For more information, or to volunteer to watch wasps, contact me at trish.hanson@state.vt.us.