Issue: August 12, 1996
Grapeleaf skeletonizer on Virginia creeperQuestion: There are some small yellow worms eating the leaves off my Virginia creeper. These worms are yellow with black and blue bands. When they eat the leaves, they leave behind what looks like a net.
Answer: You have described the appearance and the damage done by the Western grapeleaf skeletonizer. This is the caterpillar of a dark gray or metallic blue moth with a one inch wing span. The adults lay eggs in clusters on the leaves. As the eggs hatch, the caterpillars begin eating the leaf tissue between the veins on the underside of the leaves. The tougher veins are left behind, creating the net-like skeleton that you described. These colorful caterpillars ultimately reach a length of almost one inch. Though this insect is called the grapeleaf skeletonizer, it can also attack Virginia creeper and Boston ivy, two ornamental vines which are related to the grapes.
Since they are larvae of moths, they may be treated with Bt if you think they are doing sufficient damage to warrant control. Bt is the toxin produced by the bacterium, Bacillus Thuringiensis. It may be purchased as a powder or liquid to apply to your plants. It is not toxic to humans, our pets, birds, and insects other than larvae of butterflies and moths. Other insecticides labeled for use on the affected plants may also be used. A warning: insecticides containing carbaryl can damage Virginia creeper and Boston ivy. Be sure to use any products according to their directions to maximize both safety and effectiveness.
Thank you for providing the samples. Good specimens
of the insect and the plant on which it was feeding makes
diagnosis much easier.
Dodder, a parasitic weedQuestion: There is a tangle of yellow plants with white flowers growing in my backyard. I can't find the beginning where they root into the ground or their leaves, but they are spreading across the yard. What is this? Is it harmful?
Answer: You are describing dodder, a parasitic weed. Its seeds germinate in the soil, the vines grow up, twine around a nearby plant, and the dodder begins to draw nutrition from the neighboring plant. It has no chlorophyll and cannot make its own food, therefore it is yellow instead of green. It may break its contact with the ground as all it needs can be taken from the host plant that it has parasitized.
If it is parasitizing weeds, the only concern is that it not be allowed to produce seeds to germinate and attack ornamentals or garden vegetables in a few years. If it is in the garden, you might have to pick it off the plants or kill the plants it is infesting to control it. Don't let it set seeds as that will allow the problem to continue into the future.