ISSUE 2 May 22, 2008
During a visit to Mandan on Tuesday, May 20th, Jackie Buckley and I found our first cankerworms of the year. The spring cankerworm (Paleacrita vernata) and the fall cankerworm (Alsophila pometaria) are defoliators of several deciduous trees including various elms, boxelder, ash and linden. Over the last two to three years, a large outbreak of cankerworms has caused widespread defoliation in the Missouri River Valley. While healthy trees can withstand 1-3 years of defoliation, repeated defoliation causes a lot of stress. Defoliation, combined with the current drought conditions in the western part of the state, will likely result in increased tree mortality over the next few years.
The two individual cankerworms that Jackie and I found are still pretty small, 4-7mm in length. We also found two clusters of fall cankerworm eggs that havenít yet hatched. The eggs could be a late-hatching group or could have been killed over the winter. Iíll observe them over the next two weeks to see if they hatch. The current damage on trees is minimal and not noticeable to the casual observer. But as the individual larvae continue to grow, and as more larvae hatch, damage will become evident and homeowners will likely want to pursue treatment.
Several treatment options are available and should be considered in the next one to two weeks. Several chemical pesticides are labeled for control of "caterpillars," including carbaryl, acephate, malathion, cyfluthrin, esfenvalerate, permethrin and spinosad (Zeleznik et al. 2005). Application timing is critical. Another option is the bacterial pathogen Bt. It is highly specific and has been used for many years.
The Bismarck Forestry Department is planning to spray Bt sometime during the next week as weather conditions permit. Applying a sticky substance such as Tanglefoot ģ at this point would not be helpful. That material is useful to capture the wingless female adult moths as they crawl up the trees to lay eggs. Tanglefoot ģ should be placed on trees in early September to capture fall cankerworm females, and in March to capture spring cankerworm females.