There are more than 200 insects that feed on the roots, wood, foliage and nuts of a pecan tree. Of these insects, 17 are nut feeders, with one of the most important nut-infesting insect pests being the pecan nut casebearer (Acrobasis nuxvorella Neunzig).
The pecan nut casebearer (PNC) has a wide geographical range, extending from Florida to southeastern New Mexico, and north to southern Illinois. This covers most of the pecan producing regions in the U.S., excluding Arizona, California and parts of New Mexico.
Casebearer larvae tunnel into nutlets shortly after pollination, often destroying all nutlets in a cluster. The most reliable method of control is a properly timed insecticide application to the tree to kill the recently hatched juveniles before they can enter the young nutlets. However, treatment is a judgement call based on moth catch, egg scouting and pecan crop load. So part of this equation depends on properly identifying the adult male moths captured in orchard traps.
First-generation larvae are usually considered to cause the most economic loss. This early in the season, a single larvae can destroy an entire nut cluster due to the small nutlet size. Later generations will typically only have to feed on a single nut to complete development. For this reason, control is directed primarily at the spring generation.
There is a fairly small window of opportunity for insecticide applications to control newly hatched casebearer larvae. Once the larvae has bored into the nutlet, they are protected from insecticide treatments. As mentioned earlier, the necessity of control measures is determined on the severity of the infestation and the size of the nut crop. The alternate bearing cycle of pecan trees often has an impact on your management decisions. In the “on” years, when the crop load is heavy, many growers will opt not to treat and allow PNC to naturally thin excessive nuts from the tree. However, in the “off” years, when the crop load is light or if the infestation levels are severe, treatment will be necessary to maintain a commercial crop load on the trees.
Determination of the infestation severity is based on egg counts on nut clusters. To pinpoint when you should be scouting for egg lay, many growers rely on monitoring PNC pheromone traps (Figure 4).
Figure 4. Pecan nut casebearer pheromone trap with lure attached.
The pheromone trap uses a lure containing PNC female pheromone that attracts males looking to find a mating partner. You will need to monitor the traps often — at least three times per week — and record the first capture of male PNC moths. The optimum time to apply an insecticide to control PNC larvae is about 14 days following the first capture. However, this is also the time that misidentification can have the greatest impact on your spray program.
A common imposter moth that can be caught in the pheromone trap before PNC flight is the pecan bud moth. It is slightly larger than the PNC moth and lacks the raised wing scales on the forewings. Its numbers increase in the orchard earlier than PNC, but it does not feed on the young pecan nutlets. Misidentifying moths in the trap can result in wasted hours scouting for PNC eggs when there aren’t any in the orchard yet. Some managers will actually spray 14 days after first moth capture without scouting for eggs. If this is the case, then the insecticide will be applied before any PNC are in the orchard. This is not a recommended protocol.
Research suggests that trap catches cannot be used to predict the threat of casebearer larvae damage or the need to apply an insecticide. Instead, you should begin scouting for eggs on the nutlets approximately seven to 10 days after the first PNC moth is captured in the pheromone trap. Nut entry will occur 14 to 20 days after the first capture.
When scouting for egg lay, remember that most casebearer eggs are found on the stigma (distal tip of nutlet) or hidden just under the sepals of the nutlet. Many find that a good hand magnifying lens makes the job easier to find the eggs and also determine what developmental stage they are at (white, pink, red, or hatched). Ideally, insecticide applications should be made when the majority of the eggs are pink in color. A simple rule of thumb to determine if an insecticide should be used is to treat when 5 to 10 percent or more of the nut clusters sampled are infested during the “on” years. During the “off” year (light crop), reduce the threshold to only 3 to 5 percent infestation. Usually, one insecticide application is needed to control the first generation of pecan nut casebearer. However, if infestations are extremely heavy or emergence of adults is prolonged, a second application may be necessary.
Three to five traps are usually adequate for orchards smaller than 50 acres. Larger orchards will utilize more than five traps to provide adequate coverage. You may want to use additional traps if your orchard has considerable changes in topography, such as a river bottom site transitioning to an upland site. Traps are usually attached to a nut-bearing limb at an easily accessible height to allow easy monitoring. While it is true that traps placed higher in the canopy will capture more moths, you must remember that you are monitoring moth activity, not the actual number of moths captured.
Before purchasing and applying any insecticide, always read the label to determine if the product is labeled for use on the target plant or site. Many insecticides are labeled for controlling pecan nut casebearer on pecans. Base your insecticide choice on applicator safety, grazing restrictions if livestock are present, and potential impact of the insecticide on beneficial insects and other pests. Thorough spray coverage, accurate timing to treat hatching larvae, using recommended insecticide rates and proper sprayer calibration are critical for achieving good control of the pecan nut casebearer.
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