Oak and maple caterpillars

Greenstriped mapleworms, as well as orangestriped and pinkstriped oakworms have heavily defoliated red maples and oaks at several scattered locations in east central Minnesota (see map).  This is the third year of rising numbers of these forest pests.  By July 13th , nearly l0% had reached their full length of  l and l/2 inches and  ?chubby? l/4 inch thick bodies.  The chubby caterpillars are ?prepupae? which will soon drop to the ground and form overwintering black pupae.  Since only l0% had reached their full size by July 13th, defoliation will continue until mid to late August. At two other locations, the orangestriped oakworms is considerably later in  seasonal development.  Only egg masses and l/8 inch long caterpillars were found July 15th.   Branch dieback can occur after three years of heavy leaf  loss.  A late August finding of well-formed buds can confirm that no branch dieback occurred this year.  If extensive dieback is occurring, next June or July, consider using Bt on shade and ornamental trees, aiming at the young caterpillars. Biological controls eventually cause declining numbers, but few parasites and diseases have been observed this year. 

triangles = oakworms 
squares = mapleworms 
circles = walkingsticks 


Great numbers of ½  to l ½ inch long walkingsticks were found feeding on oaks and hazel brush in the Birch Lake State Forest area of northern Stearns County on  July 15th .  These green nymphs will turn brown and grow to 3 ½  inches in August, and females will lay up to l50 eggs until late October.  Interestingly enough, females drop these eggs down  from the tree tops. Most remain unhatched throughout the following summer and winter and will hatch in 2001. Birds, insect parasites, and dry weather during hatching are natural controls of walkingsticks. 

When walkingsticks occur in areas where there has been heavy forest tent caterpillar defoliation in May and June, continued leaf loss to walkingsticks in late July and August may cause branch dieback.  This is just what is happening at Birch Lake State Forest. Fortunately, by mid July, the oaks had formed many new leaves and seemed quite vigorous. 

Whenever a woodlot or forest is subjected to heavy spring and fall defoliations, the impact is worse than the sum of their individual effects.  In this case, heavy FTC defoliation occurred in 1998 and 1999 and  is predicted for 2000 and 2001. We expect heavy defoliation by walkingsticks this year, but will wait to see what transpires.   If extensive dieback occurs this fall, we may intervene with a control treatment next spring.  If chemical control is chosen where these two forest  pests are present in high numbers,  the biological insecticide, Bacillus thuringiensis, would be applied to prevent defoliation by the forest tent caterpillar, thereby allowing trees to store energy and form leaf buds next year.