By Boone Holladay, County Extension Agent-Horticulture,
and Bill Ree, Pecan IPM Extension Specialist
Walnut Caterpillars currently active in Fort Bend County have eaten all the pecan tree foliage they are likely to consume by now, but pecan growers would do well to check their groves for egg masses before a new generation hatches later this summer.
The population that has been quite active during the past few weeks is most likely the second of the summer. Most are in their final growth stage and through eating.
Based on the caterpillar’s developmental time line, the period since the region’s last frost makes likely the prospect of a third generation hatching in mid-July, one whose larvae – and consumption of leaves – would be more widespread. Though the trees will rebound, this kind of defoliation can be devastating to the crop of young pecans on the trees.
The second generation was very bad, causing complete defoliation in the hot spots around pecan groves. The critical potential is that the third generation, if successful, will infest a much larger area and may move into local commercial orchards, which have a good crop of pecans on them.
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.
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. Eggs will take approximately 10 to 12 days to mature and larval feeding will be approximately 23 days before larvae leave the tree to seek pupation sites.
This would be the best stage at which to treat these pests. With-out blindly spraying for them, we ask individuals to scout for new populations. If you see the new egg masses on mature foliage, it should be about a week before the larvae hatch out.
The safest products for homeowner situations would be Bt, or Bacillus thuringiensis (Dipel, Thuricide, Caterpillar Killer) and products with spinosad (such as Green Light Lawn and Garden with Spinosad), or the use of wetable Sevin (Carbaryl) plus soap.
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 usu-ally 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). 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, inges-tion of treated insects by birds and other beneficial organ-isms may prove deadly.
Commercial growers will want to be on the lookout early and often. For commercial producers, products listed for walnut caterpillar will include Intrepid, Confirm, Belt, Delegate and Altacor. For organic producers, products will include Entrust, certified Bt insecticides, Azera and Neemix.
Caterpillars may be observed moving to other tree and shrub species, such as oaks and crapemyrtle, but they will not feed on them. They have simply used up their food stock and are desperately searching for more (pecan, hickory, or walnut) and may die on their own without enough energy to pupate into adult stage.
For more information, contact Boone Holladay, horticulturist with the Fort Bend County Texas A&M AgriLife Extension Service in Rosenberg at 281-342-3034.
Additional information and regular updates on pecan-related issues throughout the state may be found at the Texas Pecan Integrated Pest Management website, http://pecan.ipmPIPE.org.