The Grape Leaf Skeletonizer (GLS), Harrisina metallica, is a serious challenge for grape growers and home gardeners. This insect causes significant damage to grapevines, leading to defoliation, sunburned fruit, and weakened plants. Unchecked populations reduce the vine’s ability to photosynthesize, impacting both the current harvest and the vine’s long-term health. Successful management requires a multi-pronged approach, starting with accurate identification, followed by cultural, biological, and, if necessary, chemical control strategies.
Recognizing the Grape Leaf Skeletonizer
Identifying the Grape Leaf Skeletonizer involves recognizing both the adult moth and its destructive larval stage. The adult moth is small, about three-quarters of an inch long, and features a metallic dark bluish-black or greenish-black color. These moths are active during the day, often seen flying and laying eggs on the undersides of grape leaves.
The damaging stage is the larva, or caterpillar, which can grow up to three-quarters of an inch long. Young larvae are cream-colored, while mature larvae are bright yellow with two wide purple bands and several narrower blackish bands. They possess tufts of hair that can cause skin irritation upon contact. The most telling sign of an infestation is the leaf damage, where young larvae feed side-by-side on the underside of the leaf, leaving only the veins and the upper cuticle intact. This feeding pattern creates the characteristic “skeletonized” appearance, which gives the pest its name.
Physical Removal and Cultural Management
For smaller infestations, direct physical removal offers an immediate, non-toxic solution. Carefully inspecting the undersides of leaves and hand-picking the larvae or egg clusters is effective. Gloves should be worn due to the larvae’s stinging hairs. Collected larvae can be crushed or dropped into soapy water for disposal.
Cultural management focuses on reducing overwintering sites and maintaining vine health. Pupae overwinter in silken cocoons within loose bark crevices or in debris on the ground. Removing and destroying fallen leaves, old fruit clusters, and other debris from around the vine during the dormant season reduces the number of pests emerging in the spring. Maintaining a healthy vine through proper watering and fertilization helps the plant withstand minor defoliation damage.
Using Biological Controls for Targeted Action
Biological control leverages natural enemies and specialized microbial agents to target the larvae. The most widely used agent is Bacillus thuringiensis subspecies kurstaki (Btk), a naturally occurring soil bacterium effective against many caterpillars. Btk is a stomach poison; it must be ingested by the feeding larvae to work. Once consumed, the alkaline environment in the caterpillar’s gut activates a crystalline protein (an endotoxin), which paralyzes the digestive system, causing the larvae to stop feeding and die.
This microbial treatment is most successful when applied against young larvae (first to third instars) that are actively feeding in groups. Targeting these early stages is important because Btk loses effectiveness against larger, mature larvae. Btk is a selective insecticide, posing no threat to beneficial insects or humans. Natural enemies, such as the parasitic wasp Apanteles harrisinae and the tachinid fly Ametadoria misella, also suppress populations by attacking the larvae.
Applying Chemical Treatments Safely
Chemical control should be considered when physical and biological methods are insufficient to manage a large infestation. Newer, reduced-risk insecticides like spinosad are often preferred, as they are derived from a naturally occurring soil bacterium and are acceptable for organic operations. Spinosad works by contact and ingestion, offering effective control against the larvae.
Other chemical options include methoxyfenozide, an insect growth regulator that disrupts the larvae’s molting process, and synthetic broad-spectrum insecticides like carbaryl. When using any chemical treatment, follow the label instructions precisely for mixing ratios and safety gear. Applications should be timed to hit the young larval stages, ensuring thorough coverage, especially on the undersides of the leaves where the larvae feed. Growers must strictly observe the pre-harvest interval (PHI) listed on the product label, which specifies the minimum number of days between the last application and harvesting.