Getting Rid of Plant Pests in Nurseries

Dale K. Pollet  |  5/17/2005 2:18:59 AM

Dale Pollet

The LSU AgCenter offers help to nursery owners in their continual battle against pests, especially insect pests.

Efforts to control mosquitoes because of the threat of West Nile virus have inadvertently increased some pest problems for nurseries and homeowners. That’s because the insecticides (pyrethroids) used for control of these mosquitoes also reduce the numbers of predators and parasites that feed on such plant pests as scales, aphids and whiteflies.
 
Additional pests in nurseries include the Asian ambrosia beetle and the red imported fire ant.

Scales

Scales are the largest group of insects, representing some 31 families, 16 of which are found in Louisiana. This diverse group can infest all parts of the plant or turf system, from the roots to the foliage. These piercing, sucking insects feed on plant tissues and, in many cases, as with aphids and whiteflies, excrete vast amounts of honeydew. This sticky substance acts as a medium for the development of sooty mold, which reduces photosynthesis. Also, bees, wasps and ants feed on the honeydew.

Many scale populations infesting the plant and turf industry go undetected until some irregularity or injury on the plant becomes visible. Population density and the protective covers and waxes secreted by the scales make them blend in with some plant systems. In addition, scales like the white peach scale, San Jose scale, ground pearls and the cottony cushion scale inject toxins into the system as they feed, killing the plant.
 
Pit scales can clog plant tissues and kill the plants by restricting nutrient and water movement. Identification is critical to effective control.

Aphids

Aphids are a large complex of insects that damage either a broad spectrum of plant material or a specific host. Aphids have piercing, sucking mouthparts and, like scales, feed on all parts of the host. They also excrete honeydew, which can lead to sooty mold buildup. They are potential vectors of plant diseases. Management of aphid populations can be difficult because of the varied and complex life cycles of some species.

Whiteflies

Whiteflies are a management nightmare for nurseries. Populations can easily hide in the planting system and can be dense and damaging before detection. Like scales and aphids, whiteflies feed through piercing, sucking mouthparts and excrete large amounts of honeydew. They also transmit diseases. The immature forms look very much like scales feeding on the underside of the plant foliage. There are 19 recorded species from Louisiana, although not all are ornamental pests.

The newest introduction is the giant whitefly, which is about twice the size of other whiteflies. The giant whitefly has been found on canna lilies and ginger. It produces large amounts of wax that coat the foliage and another wax that hangs down from the leaves like long shiny wax threads. Some are 6 inches or longer.

Asian Ambrosia Beetle

The Asian ambrosia beetle was a serious pest in nurseries in 2004. This beetle attacks live trees rather than stressed trees. It initially bores into the trees and extrudes an ash-like mass of chewed wood. The beetle, though, does not kill the trees. It’s the fungus carried by the beetle that eats the wood, and the adults and larvae feed on the fungus. This is typical of many of the bark beetles. Because it does not attack all types of trees, it is imperative that nursery owners check trees. And when the first signs of an infestation are noted, remove the infested trees and destroy them as a preventive, while treating surrounding trees.

Fire Ants

Fire ants affect nurseries by damaging the roots of plants and causing a nuisance with their stings. Management is critical for shipping. Fire ants are part of the Louisiana Department of Agriculture and Forestry’s quarantine program. New management systems using insect growth regulators (IGR) or a combination of IGRs and insecticide baits should help to reduce cost of fire ant management. Programs are being tested to evaluate the use of these materials similar to the communitywide programs for subdivisions.

Pest Management

Management of pest populations is based on three components: identification of the pest, an effective management material to apply and proper timing of application. In many cases the volume of water used to apply insecticides is critical. This affects coverage on the plant, so proper contact with the pest is achieved. The larger the plant, the more water required. Plants that have a waxy or hairy leaf surface or where the pest is wax-covered require the addition of a spreader sticker to increase the pests’ contact with insecticide. 

Water pH is another potential problem for growers. Insecticides are acid-forming materials, and many water systems are basic (alkaline). The state average is 8.3-8.4 (range of 4.3 to 12). When mixed with insecticides for spraying, this basic water causes a process known as alkaline hydrolysis, which breaks down the insecticide, reducing its effectiveness. Repeated applications for control become necessary, and this causes the potential for the development of tolerance or resistance to the insecticide.
 
The use of buffers to reduce the water pH to the optimum range of 5.5 to 6.5 will allow the insecticide to work more efficiently. The use of ultra fine oils, in many instances, will enhance the effectiveness of the insecticide. In some cases, especially against mites, these oils are an effective management tool alone.

Timing is the most critical factor for optimum control. By knowing the identification of the pest and its life cycle, you can time control measures at the most susceptible stages. This will help to make the management programs environmentally safer, economical and effective.

Dale Pollet, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2005 issue of Louisiana Agriculture.)

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