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Nematodes Heterorhabditis bacteriophora look like spaghetti inside a black vine weevil larva.The top larva is not infected.

Steinernema carpocapsae nematodes.

Possibilities for effective biological control

Nematodes are an economical and effective method for controlling black vine weevil and strawberry weevil on potted plants in commercial green-houses.They are very cost effective, readily available, and easy to apply with existing chemical application equipment. Nematodes are as effective as or better than many chemical controls.

To be effective, nematodes must be applied shortly after weevil eggs hatch. Applications made before weevil eggs hatch provide no control. Unfortunately, timing the application is difficult because adults do not begin laying eggs until about a month after they emerge. Egg laying occurs over a 4-6 week period. After that, the eggs take only about 2 weeks to hatch, opening a window of opportunity for biological control.Try applying nematodes 6-8 weeks after your peak weevil catch. Most eggs should have hatched by then.

To apply the nematodes, simply mix the package contents with water, as directed on the package. Occasionally, improper storage or shipment has resulted in non-viable nematodes. After adding the nematodes to water, examine with a magnifying glass to be certain that the nematodes are moving. You can apply nematodes as a soil drench to potted plants. Nematodes can withstand pressures of up to 300 psi, so you can apply them with the same equipment used for chemical pesticides. Application rates of 40,000 nematodes per 6-inch pot and 80,000 nematodes per 8-inch pot caused 100% weevil mortality in experimental trials. Application to the soil surface is suitable for plants with shallow root systems, such as cyclamen. At a rate of 20,000 nematodes per liter (1 quart) of soil, a surface application killed 90% of the beetle larvae in pots. For plants with roots throughout the pot, soil injection would be much more effective, but it is twice as time consuming as surface application.

Nematodes should be applied to moist, but not saturated, soil.This may require watering the area to be treated before application.The soil and water temperature must be above 65°F for the nematodes to be effective. Application in the early evening or morning is recommended to avoid exposing the nematodes to extreme heat and sunlight. It is important to keep the treated pots moist after the application.

It may take 10-30 days for root weevil populations to decline, but control can occur 2-5 days after treatment if the larvae are close to the soil surface. More than one application of nematodes may be necessary if soil conditions are not suitable for nematode persistence—that is, moist and moderately warm.

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