Yellownecked caterpillars are in full swing

Have had multiple calls, and my own experience, with significant defoliation in oak trees this past week.  And, almost as distressing for some is the frass, or poop, “raining” down out of these trees onto patios, sidewalks, pools, etc.  Both problems are caused by yellownecked caterpillars, Datana ministra.  These caterpillars, or worms, are mostly black, with white to yellow stripes running down their bodies.  They are sparsely covered in long white hairs and usually have a yellow or orange band just behind their black heads, and reddish-orange fleshy legs, called prolegs.  

 

Eggs are laid in masses in trees and caterpillars remain grouped, feeding on foliage until they reach a length of about 2 inches.  They then crawl or drop from trees and burrow just under the surface of the soil to pupate. According to most sources, there is only one generation a year.  However, we have had reports of two generations here in the RGV, one in September to October and another in the spring, around May.  For more information on their biology please click here.

Mature Yellownecked Caterpillar. Notice the way they  raise both ends of their body when disturbed. 

Generally, yellownecked caterpillars are a nuisance and aesthetic pest, but do not cause damage to otherwise healthy trees.  As with any pest, the earlier they are detected and treated, the better the control.  As they are often way up in the tree canopy, their presence is often not noticed until defoliation is significant and/or worms begin falling or crawling out of trees to pupate in the soil. Additionally, because worms are so high it is challenging to get treatments (contact insecticides) to them safely and effectively.  The use of systemic insecticides(soil drenches) or treating the soil for pupating caterpillars has not been shown to be effective.  Chemical options include Bacillus thuringiensis var. kurstaki (biorational) and most home landscape insecticides (conventional).  Again, successful control depends of good coverage of the caterpillars and application timing (when larvae are small).  Killing mature caterpillars in order to prevent them from reinfesting the area has not been reported to be effective, although I have been told by a few that it makes them feel a little better.