By Boone Holladay
County Extension Agent-Horticulture
As of early July, we are seeing the hatch of the second generation of Walnut Caterpillars feeding on pecan foliage. It is critical for tree health that we address these leaf feeders the best way we can. With the spring floods and now hot and dry conditions, trees are very prone to stress. Leaf loss right now would be devastating to the ongoing health of the trees. For quality control of these pests, you may have to turn to an insecticide product, though many specialized and safe options are on the table.
For commercial orchards, products such as Intrepid and Confirm have minimal to no impact on non-target insects such as beneficials or bees. These are insect growth regulators and would need to be sprayed into the canopy for them to work effectively. What we have recently observed is that orchards that were treated for Pecan Nut Casebearer in spring are not showing significant populations of Walnut Caterpillar now. That’s a good thing!
The safest products for homeowner situations is 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. of 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 in-sects. With the large populations of these, ingestion of treated insects by birds and other beneficial organisms may prove harmful.
We ask that if you scout any young populations of the Walnut Caterpillar in your area, that you contact our office at https://fortbend.agrilife.org. We will then visit the area to confirm the outbreak and if confirmed, will add it to our mapping. Thanks in advance for your help.