Calls are coming in on infestations of walnut caterpillars on pecan foliage. Historical data had us assuming that we were on the decline of this localized outbreak, but it looks like favorable environmental conditions will bring us yet another bad year. Now is the time to study up on their behavior and life cycle. If you have pecan trees in this area, be prepared with an action plan prior to the trees losing their leaves.
The walnut caterpillar, 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.
The larval stage (caterpillar) has five active growth stages (instars). During the week of May 14-20, we are estimating many young colonies to be in the second stage already. What this means is that in a week or two, they will be in the final growth stage. This is when they do the majority of damage to foliage, stripping all leaf material from the canopy. Inspect trees now to gauge populations and to start working on an action plan.
The safest products for homeowner situations would be Bt (Dipel, Thuricide, Caterpillar Killer) and products with spinosad (such as Green Light Lawn and Garden with Spinosad). These products need to be applied to the leaves where caterpillars are actively feeding. One of the safest contact insecticides is wetable Sevin (Carbaryl). Apply with soap or a surfactant to get better pest contact.
If you cannot reach the top of the tree where the caterpillars 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 dish soap and water (10 drops to 24 oz. water), insecticidal soaps, horticultural oils, or approved contact insecticides.
Be aware of the potential for acute poisoning to pets and wildlife if you use some chemical pesticides 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 http://fortbend.agrilife.org. 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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The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas cooperating.