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The P&PDL Picture of the Week

Mid- to Late-Season Caterpillars

Cliff Sadof, Entomology Department

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Yellownecked Caterpillar Catalpa Caterpillars with Ruler
Yellownecked Caterpillar
(Photo by Cliff Sadof)
Catalpa Caterpillars
(Photo by Cliff Sadof)
Walnut Caterpillars Mass of Walnut Caterpillars on Tree Comparison of Several Instars of the Walnut Caterpillar
Walnut Caterpillars
(Photo by Ronald F. Billings*)
Mass of Walnut Caterpillars
on Tree
(Photo by James Solomon**)
Comparison of Several Instars of the Walnut Caterpillar. (Note: the young instars are smooth, but the older ones are hairy.)
(Photo by Cliff Sadof)

Two closely related late season caterpillars, yellownecked caterpillar and walnut caterpillars are quite active this year. Both can be seen feeding on leaves or in clumps on the tree trunks at this time of year. Yellow-necked caterpillars feed on a wide range of hosts whereas walnut caterpillars feed primarily on walnut, pecans and some kinds of hickory. Walnut caterpillars also have an interesting habit of gathering in clumps on tree trunks when they molt. Otherwise the biology and control of these 2 insects are rather similar. After spending the winter as pupae, adults emerge in June and July and to lay egg masses on leaf undersides. White egg masses, each with up to 100 eggs are easily visible on leaf undersides. Eggs hatch in late July into caterpillars that feed in groups through August. Young caterpillars will first skeletonize leaves, and then strip them of foliage as they mature to 2 inches. Large caterpillars pupate in the soil where they spend the winter.

A third late season caterpillar is the catalpa sphinx. This hornworm caterpillars has 2 generations a year, completing the second generation in late August and early September. Growing up to 3 inches this caterpillar is capable of completely defoliating catalpa trees.

Broad spectrum insecticides are more effective against larger caterpillars than Bacillus thuringiensis 'Kurstaki'. Mid season defoliation can harm tree health since it robs the trees of resources. If defoliation is delayed to early September, it has a minimal impact on tree health because the leaves that are being removed close to the time of normal leaf drop.

Information about photographs above:

* Billings, Ronald F. Texas Forest Service. Image 3225052. ForestryImages.org. http://www.forestryimages.org/. 12 August 2001.

** Solomon, James. USDA Forest Service. Image 3067003. ForestryImages.org. http://www.forestryimages.org/. 12 August 2001.

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Last updated: 16 August 2001/tlm.
The Plant & Pest Diagnostic Laboratory at Purdue University.