Thursday, July 14, 2011

Summer Caterpillars

     Sometimes they seem to appear overnight, stripping the leaves from you pecan trees to satisfy their voracious appetites. Walnut caterpillar and fall webworm are easy to spot at this time of year. The first summer generation of these insects is nearing completion with a second, and larger generation staring in mid to late Augusts.
    Pictured at right are the tell-tale stripped leaves left by the feeding of a colony of walnut caterpillars. Look closely at one of the lower branch terminals and you will see a clump of caste skins left behind by the colony of caterpillars that chewed off all the leaves.
    In an earlier post, you can see a walnut caterpillar egg mass. Once these eggs hatch the caterpillars that emerge stay together in a group, eating together, molting together, and eventually dropping out of the trees together to pupate in the turf. Walnut caterpillar larvae are green when first emerging from the egg (1st instar). After the caterpillars molt, the larvae become maroon in color with log dark hairs. They keep this maroon color during the 2nd, 3rd, and 4th instars. At the 5th and final instar the large become totally black with long white hairs (zoom in on photo at left). These 5th instar larvae do 80% of the defoliation that will occur from this pest. 
     Over the years, we have noticed that walnut caterpillar populations vary greatly. Some years you can hardly find a colony while other year this insect seems to defoliating every tree in the grove. Walnut caterpillar populations are kept in check my a small parasitic wasp. In our area, the colonies we've found are small indicating that the wasp is active and working to suppress an outbreak of caterpillars. However, nut growers in Miami Co., KS and Barton Co. MO have seen much larger colonies indicating that that natural biological controls have broken down in those areas. If you have seen a large number of walnut caterpillar colonies during the 1st generation, be prepared for an even bigger 2nd generation.

   With its dirty white web, the fall webworm, can be spotted a half mile away (photo at right).  These caterpillars appear to stay within the web they create but that web is expanded every night as the larvae devore an ever increasing number of leaves.
     Fall webworm populations flucuate from year to year depending on the survival of pupae over the winter months. Warm, wet winters seem to favor the fungi that attack fall webworm pupae reducing the size of the 1st summer generation. Dry weather provides prefect conditions for insect survival so the second summer generation should prove more troublesome.