The Western Grapeleaf Skeletonizer is the most obvious pest for those who have grapevines growing in a Las Vegas or a southwest garden. The older larvae feeding on grape leaves are very distinct with bands of yellow, blue and black.
The Grapeleaf skeletonizer moth, Harrisina sp,, is unlike other moths as it is active during the daylight and looks more like a wasp than a typical moth. These moths have been a resident of the Vegas Valley for at least 40 years and are slowly spreading from southern to northern California. There are other grapeleaf skeletonizer moths that inhabit the rest of the USA.
The moths have large slender bluish-black wings in front with smaller wings behind and a long black body that may have a red or orange ring behind the head.
During the summer months there can be up to three generations of this moth and its larvae youngsters. To see pictures of the different stages of Harrisina brillians from a publication by University of CA Div. of Ag and Natural Resources CLICK HERE.
In spring as the weather starts to warm up you might find the moths in the early morning. They move and fly more slowly when the temperature is cooler. Look for them resting on leaves that are in the sun to warm up. Catch them and remove them with your hands or a butterfly net. Mating moths will be attached tail to tail and is a sure sign to watch for eggs laid on the undersides of the leaves.
Eggs are laid in large clusters of 20 or more. They will hatch out within a week or sooner with warmer temperatures. The tiny newly hatched larvae are pale beige in color and will eat only the under surface of the leaf while feeding very close to each other. As they grow all leaf tissue between the veins will be devoured leaving only the tougher veins. This is where the common name of skeletonizer comes from.
Because their life cycle is so quick, you may be surprised one day to find that your grape vines are almost denuded. Full sized larvae are easy to see with distinct yellow and black bands. Each segment has four tufts of long black stinging spines and can cause welts on sensitive skin.
During early summer trap the adults and pick off any leaves with eggs or tiny larvae. When removing the larvae use disposable gloves or cut the leaf and discard without touching them. A pail of soapy water can be used to drop them in.
If you have large grapevines or arbors you can spray with Bacillus thuringiensis which is an organic biological control for caterpillars.
Bt is a bacterium that produces problems in the gut of the caterpillars which causes them to stop eating and die. Younger larvae are most susceptible to Bt so spray as soon as you see eggs that have hatched.
Bt breaks down quickly and should be applied only when the small larvae are actively eating. If new eggs hatch after 24 to 48 hours after the Bt treatment, it will need to be sprayed again. It takes up to three days for the larvae or caterpillars to die completely but they will stop causing damage once the Bt is ingested. Follow label directions and be sure to cover the leaf surfaces as it has to be eaten to be effective.
The mature larvae will crawl into bark crevices or drop into mulch and spin beige cocoons to pupate. There are several predatory insects that attack the larvae and a virus that is being released in California vineyards but I don’t know if any of them are here in the Vegas Valley.
If too many of the leaves are destroyed on the grape vine the caterpillars will start to feed on the grapes. Damaged grapes can develop bunch rot. Loss of leaves also increases sunburn on the fruit. If defoliation is after the fruit are harvested it can affect the health of the vines as there won’t be any more production of reserves in the roots and trunk until new leaves are grown. Some leaf damage is not too serious but the population of moths will increase each year if not controlled.