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Insects and Diseases

Check your evergreens and pick off the bagworms, disposing of them to help protect your trees and shrubs from damage next season.

Bagworms: After hatching the larvae will begin to construct their "bags" around their bodies exposing only their head and front legs to feed and move. As the caterpillar continues to grow it will increase the size of the bag using silk and pieces of the plants they feed on. Although they will feed on numerous trees and shrubs they will most likely be seen on arborvitae and juniper. These caterpillars in their bags look very natural and are often mistaken  for pine cones. If left untreated severe defoliation and possible death of the host plant may occur. If there are not too many they can be hand picked and disposed of. (could be a fun day for the kids) If the population has gotten out of control, spraying may be required. This can be done using Bacillus thuringiensis (B.t.) which is a biological control and should be done in the early stages in mid to late June. There are other pesticides which can be used to control bagworms. As with all pesticides be sure to follow all label instructions and safety recommendations.

Oak Gall: There are a variety of gall-forming species of small wasps that commonly infest oak, Quercus spp., trees in New Jersey. Most leaf galls on oak cause little or no harm to the health of a tree. However, twig or branch galls may cause injury or even death to a heavily infested tree. Two common species of twig gall-producing insects are the horned oak gall wasp and the gouty oak gall wasp. These species are in the insect family Cynipidae. Both the horned oak gall wasp and the gouty oak gall wasp are known to occur from southern Canada to Georgia.

Galls are abnormal plant growth or swellings comprised of plant tissue. Galls are usually found on foliage or twigs. These unusual woody deformities are caused by plant growth-regulating chemicals or stimuli produced by an insect or other arthropod pest species. The chemicals produced by these causal organisms interfere with normal plant cell growth.

The horned oak gall wasp, attacks the twigs of pin, scrub, black, blackjack, and water oaks. The gouty oak gall wasp, develops in the twigs of pin, scarlet, red, and black oaks. Both of these woody twigs galls on oak look similar, but the horned oak gall has small horns that protrude from around the circumference of the gall. One adult gall wasp emerges from each of these horns.

In general, most leaf galls on oak in New Jersey do not affect the health of the host tree. A few can cause leaves to drop prematurely, or distort them so that photosynthesis (the plant’s food-making process) is interrupted. Galls generally are aesthetically objectionable to homeowners who find them unattractive and fear that galls will cause damage to the health of their oak trees.

Chemical control is seldom suggested for management of leaf galls on oak. Cultural methods of control may be effective in reducing the impact of these insects. Some fallen leaves may harbor various life stages of gall-producing pests. Therefore, it may be useful to collect and destroy all infested leaves. Some of these pests overwinter in twigs and branches of oak. Where such woody galls are detected, prune and destroy the infested plant material when the galls are small and have just started to develop.

Once a gall begins to develop, it is almost impossible to stop or reverse its development. Unless registered insecticides can be applied when gall wasps are flying, they offer little or no effective measure of control. Lack of serious plant damage from leaf galls and the difficulty in proper timing of insecticide applications pose a strong argument against the use of insecticides to reduce galls on oak.

Bacterial Leaf Scorch or BLS as it has become known is evident again throughout the county. This bacterial disease is most evident on red oak and pin oak. The symptoms can be seen on the leaves which show an irregular shaped scorch, brown in color often with a red or yellow margin appearing between the scorch and the green section of the leaf. The bacterium clogs the xylem of the tree which are the cells that transport water between the roots and the leaves. At this time there is no cure for BLS. Removal of the infected branches along with proper watering during times of dry weather will help maintain tree vigor and may slow down the spread of the disease.

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