Dealing with pests on herbs

  Raymond CloydThe availability of pest control materials (insecticides and miticides) for herbs is limited. This is primarily due to the fact that many herbs are used for culinary purposes. Despite this, herb growers must still  control the same pests (e.g., aphids, fungus gnats, mites, thrips and whiteflies) that attack other ornamental plants.

Other than reading the label of specific control materials, there is no simple, comprehensive listing of materials registered for use on greenhouse-grown herbs.

Pest control materials
Nearly all of the control materials labeled for herbs are contact products with short residual activity, which means that supplemental applications may be required. A number of these materials have broad-spectrum activity against different insects and/or mites and some are only active against one or two insect groups.

 Table 1. Pest control materials labeled specifically for use on geenhouse-grown herbs. Refer to product label to make sure that the use on any specific crop is included on the product label.

Soaps and oils
Insecticidal soaps (potassium salts of fatty acids) and oil-based material (paraffinic oil) are broad-spectrum with activity against many different soft-bodied insects and mites. Thorough coverage of all infested plant parts is essential since activity ceases once residues dry.

Soil-bacteria
Several control materials are derived from soil-bacteria (Bacillus thuringiensis subsp. israelensis and B. thuringiensis subsp. kurstaki) and the target insects (caterpillars and fungus gnats) must ingest the active ingredient to be killed.

Pyrethrin
The active ingredient in two of the control materials is pyrethrin, which is derived from the flower of chrysanthemum species Dendranthema cinerariaefolium. This is a fast-acting, short-residual, contact material with broad-spectrum activity against many soft-bodied insects. A synergist such as piperonyl butoxide (PBO) is commonly incorporated into the formulation to increase insect mortality by preventing insects from detoxifying the active ingredient.

Azadirachtin
Azadirachtin is an insect growth regulator that is active on the larval stages of insects. It disrupts the molting process via inhibiting biosynthesis or metabolism of the molting hormone ecdysone.

Sugar components
Two products are based on sugar components: SucraShield and SorbiShield. The active ingredient in both products is an extract derived from the leaf hairs of wild tobacco. The restricted entry interval for SucraShield is 48 hours, although it is permitted for use in organic production systems. The active ingredients kill insects and mites by disrupting cuticle membrane integrity and suffocation.

Biologically-based materials
There are several biologically-based materials containing fungi (Beauveria bassiana) or beneficial nematodes (Steinernema feltiae) as the active ingredient. The beneficial nematodes have been shown to be very effective in controlling fungus gnat larva populations.
 
There is a formulation of spinosad (sold as Conserve for ornamentals) commercially available for use on herbs called Entrust. This is the product of choice when dealing with thrips. It is important to avoid relying on this product (and mode of action) to avoid resistance developing in thrips populations.

Scouting is critical
Since there are a limited number of pest-control materials available for use on greenhouse-grown herbs, it is important to scout the crops regularly. Scouting can allow you to effectively manage pest populations early enough, which may reduce the frequency of applying pest-control materials.
 
Always be sure to read product labels prior to using any material and avoid applying any pest-control material when the ambient air temperature is greater than 80ºF. 
 

Raymond A. Cloyd is associate  professor/ext. specialist in ornamental entomology IPM, K-State Research and Extension, (785) 532-4750; rcloyd@ksu.edu.