Thursday, June 21, 2012

Walnut caterpillar hatching

    Just like fall webworm, walnut caterpillars have arrived right on schedule--late June. Today, we found a couple of newly hatched walnut caterpillar colonies (photo at right). Newly hatched larvae are quite small and olive green in color.
    I looked around the foliage near the colony of feeding caterpillars and found the egg mass from which they hatched (photo at left). Looking closely at the egg mass, I learned two things. First, the egg mass is much larger than normal. And second, not all the eggs hatched indicating possible parasitition by the wasp, Trichogramma minutum. The laying of large egg masses is often a precursor for an outbreak of walnut caterpillars. The fact that less than 10% of the eggs were parasitized means that wasp parasites were not numerous enough to keep walnut caterpillar in check.  Everything I saw today, points to a walnut caterpillar outbreak, at least in and around the Pecan Field.

     To scout for newly hatched walnut caterpillar colonies look for brown, skeletonized leaves in the canopy of your pecan trees (note brown leaf in center of photo at right). The mouth parts of newly hatched larvae are so small they can not eat the entire leaf. Instead, the young insects scrape both upper and lower surfaces of the leaf, turning the leaf brown. The photo below shows a close up of the brown, skeletonized leaves and a feeding colony of first instar walnut caterpillars.