Insect Infestation On Sweet Acacia Trees - Knowledgebase Question

Peoria, AZ
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Question by joedsw
November 29, 1999
My trees are infested with spidermites, white flies, thrips, and we think they have some sort of boring insect as well. The trees are 15 years old and we would like to save them. In addition to the problems with the acacias most of the other trees and shrubs on the site have spidermites. I've had advice that we do a Merit soil drench treatment to posibly remedy the problem. Since Merit is a systemic the label indicates 60 days for full effect. I would like to know if the Merit treatment is the way to go or is there a better solution? Also will the trees survive 60 days?

Answer from NGA
November 29, 1999
First of all, research shows that insects attack plants that are weak or stressed. The best thing you can do to prevent insect problems is to have healthy plants. You need to determine what is lacking for your plants, whether it be inappropriate watering techniques (info to follow), lack of fertilizer, too much or not enough sun, lack of air circulation, etc. No matter what you do to battle the bugs, they'll continue if your plants aren't healthy or have the correct growing conditions. You also need to determine if there is indeed a boring insect.

I always start with the simplest method first, and if that isn't successful, move on from there. A strong blast of water from the hose usually works on whiteflies. Spray underneath leaves where they hang out. Do this daily if you notice insects. Spider mites are also best controlled with a daily blast of water from the hose. (Mites prefer dry, dusty plants.) The humidity from the extra water will also helps reduce water stress. If possible, spray early in the morning, before the sun gets too hot.

Soapy water sprays (as opposed to insecticidal soap) are another alternative. Use 1 teaspoon to 2 tablespoons of liquid detergent soap per gallon of water. Use regular, not concentrated soap, and don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed, and spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Spray early in the morning before the sun heats up. Next on my list would be an insecticidal soap spray.

The insecticidal soaps are made from plant-derived fatty acids and target soft-bodied insects. There's really no way you can target the bad guys without fallout on the good guys. If you can regularly monitor and tolerate some damage to your plants, over time Mother Nature strikes a balance, with the beneficials coming in to control the bad guys.

As a tree grows, its new roots tips, where nutrients are being absorbed, spread out laterally. If you are watering only within a four-foot area at the base of the tree, it's not really being watered effectively. Expand your watering zone out PAST the tree's canopy. As the tree grows, continue expanding that water zone. If you have an irrigation system, you need to move the emitters out. If you use a hose, just drag it out further. In any case, water slowly and deeply to ensure water penetration.

When leaves completely dry out, the problem is often salt burn. This is common in our area with low rainfall, alkaline soil and water high in salts. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply, slowly and infrequently. At least once a month, water deeply enough to "leach" or push salts below the root zone. Frequent, light "sprinklings" allow salts to accumulate in the top layers of soil, where the roots are, which is bad news, as the salty soil can actually draw moisture out of roots.

Similar symptoms occur when too much fertilizer has been applied. (Note that even if you didn't apply fertilizer directly to this plant, it could have absorbed it from elsewhere, such as turf, which is often heavily fertilized with nitrogen.)

When fertilizing, moisten the soil well before and after application. Follow package directions exactly. (A little bit more is never better!) Fertilize just before the growing season begins (late February-early March) and lightly again in the fall (mid- to late September). Fertilizing during the hot weather isn't a good idea because the potential for burn is increased.

I don't think the soil drench treatment is the solution at this point. I suggest you contact a professional arborist or the Master Gardeners at County Cooperative Extension office, 4341 E. Broadway Rd., Phoenix, 602-470-8086, ext. 301.You can bring in a sample of the boring insect or plant parts that show the symptoms. There is a Master Gardener satellite office near you in Sun City West. The main office number can provide you with the address and hours. I hope this info helps!

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