Mites601

Mites, Aphids, and Spruce Galls, Oh My!

by Amy Gray

Very few things are scarier to a gardener than the site of a beloved tree or bush being slowly devoured by insects. That oak tree your kids used to swing from is now looking forlorn, in need of some help from you to protect it from invasion. Here's a look at some of the common culprits and what you can do to ward them off.

Spider Mites

Part of the arachnid class (Family: Tetranychidae), spider mites are an unseen menace to both indoor and outdoor plants. Mites are nearly invisible to the naked eye and vary in color, depending upon species and time of year. However, spider mites often produce protective webbing on leaves of plants, a tell-tale sign associated with infestation. Another clue pointing to spider mites is the appearance of small, light flecks on the damaged leaf surfaces. These flecks are caused by the feeding mites, which injure the plant cells with their whip-like mouth parts and then ingest the sap.

Spider mites often take residence in drought-stressed plants and trees, including garden vegetables, fruits, and flowers. Well-intentioned doses of insecticides such as Sevin can and do kill off predators of the spider mite and may also lead to an infestation.

If spider mite infestation is suspected to be causing damage to plants, several things may be done.

First, consider the insecticide you are using to ensure you are not killing off the spider mite's natural enemies. Among the more valuable enemies are the "spider mite destroyers," a group of lady beetles (Stethorus species), as well as mites in the family Phtyoseiidae. Many of these natural predators can be commercially ordered, but have limited effectiveness outdoors.
Second, increase watering frequency and occasionally soak leaves with a forceful jet of water to knock mites and eggs loose. Spider mites do not prosper under humid conditions, while their predators do.
Third, you may try a miticide such as Kelthane, Orthene, or a sulfur-based product. Once again, these have limited effectiveness and you must check the label before using them on food crops.

Aphids

Many species of aphids cause problems in shade trees and woody ornamental plants for Coloradoans. Aphids are quite small, ranging from less than 1/8 inch to 1/4 inch. They may have wings and typically display shades of orange and red or gray. Aphids will lays eggs in the summertime, which hatch within one to two weeks. This new generation of aphids will begin to produce aphids as well, causing large population surges in the warmer months. Mating occurs at the end of the summer.

Due to their small size, it is difficult to identify aphid infestations by the appearance of the aphids themselves. Instead, it is helpful to look for drops of "dew" and the presence of many ants. Aphids feed off of the plant sap and then secrete a honeydew that is harvested by the ants. Another symptom is the curling off young leaf edges, caused by feeding aphids.

Once the problem is identified, boosting the numbers of natural control can sometimes solve the problem. Decreasing the ant population removes much of the aphids' defense to natural predators, such as the ladybug, green lacewings, and small parasitic wasps. Many insecticides are available for use against aphids, too. DiSyston, Malathion, insecticidal soaps, and dormant oils can decrease aphid populations. Use EXTRA CAUTION when using Diazinon; it will kill birds as well as aphids. Finally, forcefully spraying plants reduces numbers, just as with the spider mites.

Cooley Spruce Galls

Cooley spruce galls are the result of Cooley spruce gall adelgids (woolly aphids). While the galls themselves are not a serious concern, rarely causing great harm, they may indicate an underlying problem, as healthy trees rarely experience infestations.

Galls form on the edge of new growth on spruce trees. They appear initially as light green, seed cone shaped growths in the spring, drying out to a bright brown by mid-July. Galls are caused by the saliva of the adelgid, which then inhabits the gall chambers, continuing to develop until mid-summer. At this time, the galls dry out and winged forms of the adelgid leave the spruce to continue their life cycle on Douglas-fir trees. Eggs are laid on these new hosts, causing yellow spots and bent needles when the young emerge to feed, but no galls are formed. Late summer, some adelgids will return to the spruce to overwinter and the cycle begins again.

To control Cooley spruce galls, it is imperative to apply treatments in the fall, before galls are formed. A spring application may be effective if made prior to April, when the insects begin to lay eggs. Sevin, permethrin, and horticultural oils will all decrease adelgid populations if this is desired, as it is not considered necessary for the health of the trees. One other option to consider is not planting spruce and Douglas in close proximity, as this hinders the completion of the life cycle.

For more information about spider mites, aphids, and Cooley spruce galls, please call 539-1631 or stop by the fairgrounds and ask for a fact sheet.