Biological Controls for Rhododendron Pests

Although spraying chemical insecticides and fungicides to control pests on rhododendrons has been the recommended method for many years, interest in safer, more natural controls has created a new market for companies specializing in biological controls. These recently developed biological controls fall into a few categories; predators, pathogens, and microbe colonization.


Predatory insects prey on other insects. A predator that works well to control two rhododendron pests, thrips and weevils, is the predator soil mite Hypoaspis miles. When applied to the ground, this mite will feed on the larval stage of both thrips and weevils in the soil. Complete eradication of pests in not usually the outcome, but populations of thrips and weevils will be seriously impacted. The really great thing though is that this soil mite will establish populations that stay in the ground for some time. Available and shipped in a tube filled with vermiculite and peat, this predatory insect can be applied by shaking the carrying medium onto the ground below your plant. These tubes have around 25,000 mites each, and can treat 40-50 plants. The retail price is around $40 for a tube or around $1.00 per plant.

Another excellent predator for thrips is Amblyseius cucumeris: the thrip predator. This predator lives on the leaves and feeds on the young stages of thrips. They will eat about 5 larvae per day. Release rates are about 100-500 per plant. They are available in little bags of 300 predators. These small bags should be hung onto the branches of the plant. The predators crawl out through a small hole in the bag and onto the plant, feeding on the thrip larvae. One bag per plant is recommended. Suggested retail is $3.00 per bag that contains 300 predators.


Another biological control that works well is nematodes, which are classified as pathogens. These little wormlike critters move through the moist soil and burrow into the larvae of the Strawberry Root Weevil and the Black Vine Weevil, the two most destructive insects to rhododendrons. If your leaves have little ‘notches’ around the edges, you have weevils in your garden. Weevils are nocturnal feeders, so you can go out on ‘Weevil Patrol’ at night with a flashlight and see these little brown beetles feeding on your prized plants.

There are two species of Nematodes (Heterorhabditis marilatus and Heterorhabditis bacteriophora) that control these pests. They are generally sold together in a packet that will treat approximately 300 sq. ft. of ground. Fall application is recommended when the soil temperatures are above 50 F. and when there is moisture in the soil. September into October is the best time. Mix with water and saturate soil around damaged plants. A pump sprayer is the easiest way to apply the nematodes. Keep soil somewhat moist during this time.

One packet will treat 3000 sq. ft. Suggested retail is $20.00 per packet.

Microbe Colonization

Recent studies around a new patented product are quite promising for prevention of soil borne diseases including Phytophthora, and foliar diseases such as Powdery Mildew. Both of these diseases are a scourge for us rhodie growers, and a natural control like this is of interest. The product is sold under the label Actinovate SP and is a high concentration of a patented beneficial bacterium on a 100% water soluble powder. When used as a preventative, researchers have seen Actinovate® SP work as well or better than most chemicals. The bacterium is called Streptomyces lydicus and when introduced into the soil this microbe colonizes and grows around the root system of plants.

While settling in the root’s rhizosphere the microbe forms a synergetic relationship, feeding off of the plant’s waste materials while secreting beneficial and protective by-products. This combination of the colonization and the protective secretions forms a defensive barrier around the root system of the plant which in turn suppresses and controls soil pathogens. S. lydicus also 3 has been shown to prey on certain pathogens, disrupting their cell walls and disabling them in the process.

Finally, the by-products of the Actinovate® microorganism also aid plants by assisting in the complex conversion of some of the minerals and micronutrients found in the soil, allowing easier uptake and, thus, creating a stronger, more robust plant. When sprayed onto the leaves of plants as a foliar spray, the microbe forms a layer on the leaves and stems, suppressing diseases such as Powdery Mildew.

The only unfortunate aspect is that, at this time, Actinovate is only sold in packs to treat 50 gallons of water, so is not really a ‘home owner’ product yet.

By: Don S. Wallace owner of Singing Tree Gardens Nursery. Anyone interested in trying these products can contact Don S. Wallace at Singing Tree Gardens Nursery 707-839-8777.