The Walnut Caterpillar, Round 3

By Boone Holladay, County Extension Agent-Horticulture & Bill Ree, Pecan IPM Specialist

Bill Ree and I have done much scouting and have collected some great data on the Walnut Caterpillars, which have been defoliating pecans in our area since early June. Here are some of our observations that will help you with an action plan.The first generation has done their damage and they are currently pupating in the soil. In a couple weeks, the adult moths will hatch out, mate, and start the whole cycle all over again. What this means for pecan owners is that new young caterpillars should begin feeding on trees again around mid-July and should be quite noticeable again by the last week of July. This would be the time to look out for clusters of the reddish ones on tree trunks, which are easy to treat.
Datana integerrima, is a foliage feeder of trees in the family Juglandaceae, which includes the pecans, hickories, and walnuts. Infestations start with female moths depositing egg masses of 300 to 900 eggs on the undersides of individual leaflets of mature foliage.
Egg masses are deposited in one layer and free of any covering, so that growers can spotlight into the trees at night and see the white dime-size clusters of eggs on the underside of leaves. They will resemble bicycle reflectors. Eggs will take approxi-mately 10 to 12 days to mature and larval feeding will be approximately 23 days before larvae leave the tree to seek pupation sites. So, if you see the new egg masses on mature foliage, it should be about a week before the larvae hatch out.
The caterpillars grow in stages called ‘Instars’. When they are young they stay up at the top of the canopy and feed. Then, after the 3rd instar (usually about 1 week later), they begin to come down the trunk to molt in clusters. This is probably a protective habit, for as they grow larger, they are easier to be spotted by birds, etc. If you cannot reach the top of the tree where the caterpil-lars are feeding, you may be able to catch them when they move down the tree and cluster to molt. This is usually about 6 to 15 feet up the tree and easy to spot with a little inspection. If you can catch them then, you can spray the cluster with a range of soft products listed below.
The safest product for full canopy spray situations would be Bt (Dipel, Thuricide, Cater-pillar Killer). Bt, or Bacillus thuringiensis, is a bacterium that if ingested by caterpillars will kill them, but is completely safe for all other insect and animal species. This has to be sprayed on the leaves where caterpillars are feeding to be effective.
For control of clusters on the tree trunks there are a range of options including: wetable Sevin (Carbaryl), oil sprays (citrus, dormant, or summer oils), or just a mixture of soapy water (10-15 drops of dishwashing soap to 24 oz. water). Spray directly on and completely saturate live clusters.
Be aware of the potential for acute poisoning to pets and wildlife if you use some chemical pesticide products on these insects. With the large populations of these, ingestion of treated insects by birds and other beneficial organisms may prove deadly.
We ask that if you scout any young populations of the Walnut Caterpillar in your area, that you contact our office at We will then visit the area to confirm the outbreak and if confirmed, will add it to our mapping. Thanks ahead for your help.

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