Sprays can halt hibiscus sawfly

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My children gave me two hardy hibiscus shrubs for my birthday. They have been doing well and blooming gloriously, but now something seems to be eating the leaves. I just noticed the damage, but some of the leaves look like lace. I checked for bugs, but I do not see anything. Do you have any suggestions?

A. It sounds as though the hibiscus sawfly has found your new plants. Your description of the damage fits with the damage created by this small, caterpillar-like insect. It can cause similar damage on hollyhocks and possibly other members of the cotton family. Although it is known as the hibiscus sawfly or the hollyhock sawfly, this particular insect has no generally accepted common name among entomologists. Its Latin name is Atomacera decepta.

The larva, or immature insect, is the damaging stage of this pest. It is spiny and green, about 3/8-inch long, with a black head. It is easy to overlook because it is about the same shade of green as the leaves. It resembles a caterpillar, but that term is reserved for the larvae of moths and butterflies. One way to tell the difference between sawfly larvae and caterpillars is to count the fleshy, abdominal prolegs. Sawfly larvae will have six or more pairs, while caterpillars have five or fewer.

This is one of the most important pests to affect hibiscus plants, but it has not been studied extensively. There are several generations of this pest every year, right up until frost. Adults lay their eggs on the undersides of the leaves, generally toward the tip. The adults are small (less than 1/4-inch), fly-like insects that are black with an orange thorax. The larvae hatch and begin feeding and turn hibiscus and hollyhock leaves into lace. They pupate in debris around the base of the plant, and then hatch out as adults, and the whole life cycle begins anew.

Insecticide sprays are very effective in controlling this pest, but you have to scout your hibiscus plants frequently for the damage and the larvae that cause it.

Rotenone and pyrethrins, Captain Jack's Deadbug Brew (spinosad), Sevin (carbaryl) and Bayer Advanced Garden Multi-Insect Killer (cyfluthrin) should provide adequate control. Begin making applications when you first notice damage and find the larvae feeding on the leaves. You will need to make repeated applications to control subsequent generations.

Q. We have an old poplar tree in our backyard. I have noticed large black ants active in a crevice in the trunk that I believe are carpenter ants. What can I use to get rid of them and keep them from feeding on this tree?

A. The black carpenter ant (Camponotus pennsylvanicus) is the most common carpenter ant species in the northeastern United States. Although they might be nesting in your poplar, they are not "eating" it.

Research indicates that they prefer to nest in moist, decaying wood. They are scavengers and primarily eat other insects (dead or alive), the juice of ripe fruit, sap from various trees, and the sugary honeydew from aphids or other sap-feeding insects they tend. They nest in wood, but they do not eat it.

In their natural habitat, carpenter ants help with the decomposition of decaying trees. They nest in logs, stumps and hollow trees. They can become a problem in houses, particularly in wooded areas, when they come looking for food. If old or unprotected wood in the house has begun to decay, they may build satellite nests there to be closer to their food source. Carpenter ants do not usually invade sound wood.

The presence of a colony of carpenter ants in your tree is a strong indication that the tree has some type of interior rot that was caused by fungi rather than insects. I can suggest some products that can help control the carpenter ants and keep them from invading your house. However, you should contact a certified arborist to examine the structural integrity of the tree because the interior rot should be more cause for concern than the ants. They are but a symptom of a more serious problem.

Dust formulations of diatomaceous earth or boric acid can be blown into the crevice of the tree to kill the ants. These products have no or very low mammalian toxicity and are safe to use around people and pets.


Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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