Cankerworm larva are a type of inchworm. They are brown, green & grey and about 1.5 cm long. Fully grown they can reach up to 2.5 cm.
Cankerworm-Insect Control Branch, Parks & Recreation, Winnipeg, Manitoba Canada (left), Cankerworm-Kansas State University (right)
Cankerworm-University of California (left), Fall cankerworm moth-Matrix Graphics Corp (right)
There are two types, spring cankerworms and fall cankerworms. They are very similar but their eggs are laid at different times of the year. The females about 1.5 cm long and are a greyish moth. They are found everywhere in the US, Canada and several other countries.
Female laying eggs (left), larva (right)-NC State University Cooperative Extension
The spring cankerworm is a wingless moth that climbs up the trunks of host trees in the very early spring and deposit their eggs. The fall cankerworm does the same but lays eggs in October or November. They both hatch in late April or early May. if you have a cold or relatively cool spring you will have a lot of them die off..
Damage by cankerworms-University of Minnesota
Diet and Damage
Apple trees, elm trees, maple trees, hackberry and a few other trees are targets of this pest. They will attack newly opened buds and chew leaves beginning with a ‘shot-hole’ appearance. They will chew until only the leaf veins remain. Because the worms can hang from a silken thread the wind may blow them to any nearby tree.
Fortunately control is relatively easy as long as you do it early enough
1) Sticky Traps
Apply tanglefoot, Stick’em or Tack Trap to the bands and apply to base of the trees in February. If you use the spray make sure you spray it at least 5-6 feet high or at least about one foot below the first limbs. This will trap the wingless female moths when they begin to ascend your tree.
2) Predatory Insects
Parasitic wasps (trichogramma sp) and parasitic flies are an excellent predator of these worms. Birds are also voracious eaters of these worms.
Bt sprayed on the limbs of trees will help control them if they are still small larvae.
4) Hand Pick
If infestation occurs shake the tree and the worms will dangle down on fine silken thread. Drop them in hot soapy water.
5) Dormant Oil Spray
Apply as directed in the spring and again in the fall….should help overwintering larvae