Archive for Thursday, August 2, 2007

Aphids, caterpillars leave marks but little damage

August 2, 2007


A recent thorough cleaning of the deck, patio and concrete areas removed a patina of accumulated dirt. A few weeks later, they are again covered with a brown sticky sap. The overhanging walnut tree appears to be the source. As it turns out, the tree is the host and aphids in the foliage are the culprit.

Aphids like many trees. Walnut and oak seem to be their current targets. In small numbers, the aphid does little damage and may not even be noticed. Aphids, however, multiply very fast. These small creatures, less than 1/4-inch long, congregate on the underside of leaves and are almost clear to light brown in color.

Aphids stick their piercing mouthpart into the plant, feeding on the sap. With an abundance of moisture, which we have had, there is an abundance of sap. The aphids respond by overeating only to excrete the excess. This excess is called "honeydew," and this is what deposits itself on any horizontal surface and your car. With the small droplet size, honeydew is easily carried by the wind and may cause some respiratory problems. Honeydew is rich in nutrients and very attractive to ants. Some ants want honeydew so much they raise and tend aphids for this food source much like we would farm animals.

Honeydew also supports the development of sooty mold fungus. This fungus shows itself as a gray covering of the leaves. It is not harmful to the tree except when it covers the leaves and temporarily reduces photosynthesis.

Insecticides containing the chemical malthion are effective for control. The problem comes with the size of the tree and amount needed. Aphids do not harm the tree (maybe in rare instances with really severe infestation), so it is best to let nature run its course. If we must treat, save the chemical for our favorite flower or ornamental. When the aphids are gone, in a very few weeks, the honeydew stops and we are back on our patio.

Tent caterpillars and fall webworms are also prolific this year, and as one unsightly mess goes away another seems to start. These show up as medium to large white silk nests in the crotches or on the end of the branches. Areas inside the nest are defoliated and leaves adjoining are being eaten, soon to be enveloped. Groups of bluish, black, tan or greenish hairy caterpillars with spots or stripes are feeding in or around the nests. The nests will continue to grow until the caterpillars drop to the soil to pupate. There may be up to four generations between June and September. Seldom is an infestation detrimental to the tree. The major issue is the visual unsightliness.

When we first see the silken mass or tent, the larva have all but completed their development, and they can just be ignored. The web mass can be removed easily by hand or other mechanical means, and if this is done when the larva are resting in the tent, they too can be eliminated.

If you wish to use an insecticide as a contact spray to control tent caterpillars, be sure that the end of the sprayer wand penetrates into the web mass. This ensures that the webbing itself will not shield the larvae from the spray. If insecticides are used as a stomach poison, apply them as foliar spray treatments. Bacillus thuringiensis (Bt) products such as Dipel only work when ingested. Orthene, Malathion and Sevin work both as contact or stomach poisons.

The aphids and caterpillars have survived many centuries in Kansas, and so have the trees. Their harm is not to our ecology but our designer furniture, pristine patio and perfect view. Our choice could be to live and learn from what nature provides.

- Stan Ring is the horticulture program assistant at K-State Research and Extension Douglas County. He can be reached at 843-7058 or


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