Walnut Caterpillar update July 29th, 2014

Boone Holladay, County Extension Agent-Horticulture

Bill Ree, Pecan IPM Extension Specialist


Site visits and numerous calls from around the county have confirmed another generation of walnut caterpillars feeding on pecan foliage. The majority of cases range from Fulshear through Pecan Plantation, down to Richmond, and a few calls from south Rosenberg. We assume populations are active further out as well. One thing we are noticing with this generation is a great range of growth stages, from first stage reddish-brown larvae all the way to the large 1 ½ to 2 inch black and white larvae, all in the same areas. This makes it even more of a challenge to treat effectively. So you ask, “What do I do?” Here are your options.

We have found that many pest control companies and local certified arborists have been working in the affected areas. The cost of professional treatment can range from $50-$150 per mature tree. Though very effective, the cost adds up if you have to treat several times throughout the season. Also, with broad spectrum insecticides please be aware of potential secondary toxicity issues with pets, wildlife, and beneficial insects. This leads you to option 2.

We find that the last three larval stages will move down the tree to molt. Usually about half way down the tree, you may find large clusters of the caterpillars nesting while they are molting their exoskeleton. Once done molting, they will head back up in to the canopy to continue feeding. If you can scout the area each day and catch them while they come down to molt, you can use a hose-end tree sprayer to target soapy water or an approved insecticide right on the clusters. This method will not give you 100 percent control, but will greatly decrease populations, is easier on the local ecology, and is a whole lot cheaper!

Most trees in the county are carrying a good crop of nuts. Defoliation from walnut caterpillar will not kill healthy trees, but will greatly affect harvest quality. Trees without leaves right now in mid-summer may experience a complete crop loss. If you plan to market your pecans, you should be very aware of any walnut caterpillar activity in your orchards and plan to respond accordingly to minimize any potential crop loss.

Below is our update from June 19th for your reference.

The Walnut Caterpillar, Datana integerrima, has been quite active in the past few weeks in Fort Bend County. The walnut caterpillar is a foliage feeder of trees in the family Juglandaceae which includes the pecans, hickories, and walnuts and here in Texas. This insect can have two or three generations a year. Based on developmental times, 245 frost free days looks to be an approximate dividing line between areas that could have two or three generations. What this means for us in Fort Bend, is that this is probably the second generation, and we should expect one more, and possibly more wide spread that the last.

Infestations start with female moths depositing egg masses which may contain 300 – 900+ eggs on the undersides of individual leaflets of mature foliage and it is believed that a single female will deposit only one egg mass in her life. Unlike fall webworm where egg masses can be deposited in layers and will be covered with “fuzz”, walnut caterpillar egg masses will be in one layer and free of any covering. Specialists note that you can spotlight into the trees at night and see the white dime size clusters of eggs on the underside of the leaves. Eggs will take approximately 10 – 12 days to mature and larval feeding will be approximately 23 days before larvae leave the tree to seek pupation sites.

Walnut caterpillars feed in a colony and do not construct a web so initial infestations can be easily over looked. Early indications of an infestation might be frass or dropping on the orchard floor or in town on sidewalks and driveways; branch terminals with missing foliage but retained leaf rachis and masses of cast skins on the tree trunk or main scaffold limbs. These insects develop in “instars”, which are the larval growth stages. When the fourth instar molts, the colony molts as a group on the main trunk or a scaffold limb leaving behind a mass of cast skins. The emerging 5th and last instar larvae now feed as individuals and it is during this last instar when most of the feeding damage occurs. Most of the feeding damage will occur in the last 3 – 4 days and when the last instar finishes feeding larvae will leave the tree in search of pupation sites, usually on the ground, in turf, or within mulch.

So now you may ask “what do I do?”

Based off local observation, most of the caterpillars are in their final growth stage (instar), have eaten all they will eat, and are now looking for a place to pupate. Once the adult moths emerge from the cocoon they will mate and begin to start laying egg masses on trees that still have mature foliage. Once the eggs hatch out, they will begin eating. Judging by previous occurrences of these insects this process takes about a month. So we are estimating the next generation to begin around mid-July. So, without 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. This would be the time to treat. The safest products for homeowner situations would be Bt (Dipel, Thuricide, Caterpillar Killer), and products with spinosad (ex: Green Light Lawn and Garden with Spinosad), or use wetable Sevin (Carbaryl) + 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 usually about 6 to 15 feet up the truck 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). Please 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.

You may see these caterpillars moving to other tree and shrub species. They will not feed on these, such as oaks and crapemyrtle. 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.

Commercial orchards will want to be on the lookout early and often. Though the trees will rebound, this type of defoliation is devastating to the crop of young pecans on the tree. 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.

We suspect that this outbreak represents the second generation, based off early calls this spring. If so, the last generation in July-August would be our last for the year. If your trees were completely defoliated during this past spell, you should be safe for the next generation, as the hatching larvae need large mature foliage to feed on. If your trees have no mature foliage, the moths will fly elsewhere and deposit their eggs. With this said, we would still recommend regular scouting for egg masses as noted earlier.

These insects are cyclical in nature, usually lasting two to three years, and mature native trees have lived through these outbreaks for hundreds of years. The foliage will return, but at a cost to the food storage of the tree and will cause stress. If rainfall is low, you may want to irrigate trees to a total of 1 inch per week. Also an application of nitrogen fertilizer would help the trees put on new leaves faster. For large trees, apply about 10-20 pounds of a nitrogen fertilizer (slow release urea 21-0-0-3 or similar) and water in thoroughly to not burn turfgrass beneath the tree. Apply fertilizer around the dripline of the tree, which is out below the outer branches where most of the fibrous roots are.

I have attached below a list of commonly asked questions and responses. Also, check out the Texas Pecan Integrated Pest Management website for regular updates on pecan related issues throughout the state at http://pecan.ipmPIPE.org.

Q: How many generations can we have in Fort Bend County?

A: Three

Q: What kind of trees will walnut caterpillar feed on?

A: Walnut caterpillar is a foliage feeder of plants in the family Juglandaceae which includes walnut, hickory, and in our case, pecan.

Q: Will defoliation kill my tree(s)?

A: Not directly but defoliation will stress trees and limit nut production or quality of nuts (kernels), depending on time of defoliation

Q: How long is a feeding cycle?

A: There are 5 larval stages and total feeding time will average 23 days. A total generation can last 55 – 60 days which includes development time for eggs, pupation and pre-egg laying time for adults.

Q: What are the signs of an infestation:

A: Early signs of an infestation can include branch terminals that are missing foliage but still have the leaf petiole; Droppings or frass on the sidewalk, driveway, on vehicles; masses of cast skins on the tree trunk or mail scaffold branches.

Q: Are there any natural controls that will help

A: There are numerous parasites and  predators that will feed on all stages of the walnut caterpillar but these natural controls which can also include diseases and mammals, really only help maintain populations over time rather than eliminate an ongoing  damaging population.

Q: If I control the larvae and my neighbor does not, will that put me at risk for future damage.

A: No. Larvae feed on the trees where eggs were deposited.

Q: If my tree was defoliated will it be defoliated again during the summer.

A: Larvae seem to require mature foliage so the new re-growth at occurs after defoliation will not be suitable for the next generation. However, trees defoliated during the early summer could be at risk for an early fall generation.

Q: Are the larvae or dropping harmful to my pets?

A: No, but if chemical spray applications are made, secondary poisoning is possible if pets eat the dead larvae in large numbers.

Q: What can be done in order to prevent damage in future years?

A: The best preventive is to be educated on the biology and development (awareness) of this insect in order to spot early infestations so management can be applied that prevents significant defoliation.

Q: Will this happen again next year?

A: Hard to say. Insect populations cycle from year to year and often times for reasons unknown, we will see severe outbreaks or the pest just seem to disappear.

Comments are closed.