Pest Control and Herbs By Raymond Cloyd and Nina Cycholl

Because herbs are considered consumable food crops, chemical applications to them are heavily regulated. But are pest control materials harmful to greenhouse-grown herbs?

Herbs are used throughout the United States for culinary andmedicinal purposes. Total sales of culinary herbs were $30.9 million in 1998.The annual retail sales of medicinal herbs in the United States in 1994 was$1.6 billion and nearly $4.0 billion in 1998. Popular herbs grown ingreenhouses include basil (Ocimum basilicum), chives (Allium schoenoprasum),dill (Anethum graveolens), lavender (Lavandula angustifolia), mint (Menthasp.), parsley (Petroselinum crispum), rosemary (Rosmarinus officinalis), sage(Salvia officinalis), scented geraniums (Pelargonium sp.) and thyme (Thymusvulgaris). Similar to other greenhouse-grown crops, herbs are susceptible to adiversity of insect and mite pests, including aphids, whiteflies, thrips,fungus gnats and spider mites. The number of pest control materials registeredfor herbs, however, is limited because of liability and the fact that herbs areconsidered a minor or specialty crop.

Pest control materials registered for use on herbs aregenerally considered “biorational” because these materials are lesstoxic to workers, have short residual properties and have minimal environmentalcontamination. These materials also have a narrow spectrum of pest activity andgenerally take longer to kill pests. Although some “biorational”insecticides are registered for use on herbs, these materials may not have beentested on all herb species or on plants at different maturities. Currently,only a minimal amount of information is available on the phytotoxicity of theseinsecticides to herbs.

Greenhouse managers need information on which insecticidesare safe to apply to herbs because any phytotoxic effects may limit the use ofthese registered materials, thus making it difficult to manage insect and mitepests. Furthermore, these materials cannot leave unsightly residues or causetissue discoloration, which would reduce the market value of herbs. Theobjective of this study was to determine if certain pest control materials arephytotoxic to a select group of commonly grown herb species at two differentgrowth stages.

Materials and Methods

A greenhouse study, divided into two experiments andreplicated over two growing seasons, was conducted at H. M. Buckley and Sons,Inc., Springfield, Ill. The first experiment was conducted from October 1999through February 2000. The second experiment was conducted from March 2001through April 2001. Within each experiment, there were two treatment groupswith four treatments and an untreated check for all herb species. In experimentone, both groups were done consecutively because of space restrictions at thegreenhouse facility. As a result, the first experiment was conducted over alonger time period (five months). In experiment two, both treatment groups wereconducted simultaneously. Herbs used for the studies were five- andnine-week-old plants of Spanish lavender (Lavandula stoechas), oregano(Origanum vulgare ‘Santa Cruz’), rosemary (Rosmarinus officinalis),St.-John’s-wort (Hypericum perforatum ‘Topaz’), wolly thyme (Thymus vulgaris’Wolly’) and nutmeg thyme (Thymus vulgaris ‘Nutmeg’). These herbs were selectedfor the study because they are widely grown by greenhouse operations and theyare popular with customers.

For each experiment, five- and nine-week-old plants werestarted from cuttings taken from stock plants. After rooting, cuttings werepotted into 4-inch, plastic containers. The growing medium used for the studyconsisted of 30 percent bark, 30 percent peat, 20 percent medium vermiculiteand 20 percent perlite. Plants were fertilized using a constant liquid feedprogram with 20-8.3-8.8 (N:P:K) at 200 ppm. Plants were placed in a glassgreenhouse (521/2 x 17 feet) on four raised benches (12 x 5 feet) in acompletely randomized design. There were five treatments each with five replicationsfor each herb species and age class, for a total of 600 plants (100 plants foreach species). Plants were grown under natural daylight conditions withtemperatures between 68 and 82° F.

The insecticide treatments and rates for the first groupwere Beauveria bassiana Strain GHA (Botanigard) at 32 fl.oz. per 100 gal.,Pyrenone at 25 oz. per 100 gal., Azatin at 16 fl.oz. per 100 gal., orinsecticidal soap at 2 gal. per 100 gal. The insecticide treatments and ratesfor the second group were Cinnamite at 85 fl.oz. per 100 gal. or 64 fl.oz. per100 gal., Ultrafine spray oil at 8 gal. per 100 gal., or hot pepper wax at 50gal. per 100 gal. Both groups had an untreated check that received notreatment. The insecticides used in this study were selected because they areor were registered for herbs in greenhouse production systems. The rates usedwere based on manufacturer label recommendations for herbs. All insecticideswere sprayed to run-off with a 9.5-L capacity hand pump sprayer.

Three applications of each treatment were made at seven-dayintervals, with temperatures during the applications ranging from 64-82° Ffor the first experiment and 73-77° F for the second experiment. Theoutdoor weather conditions during insecticide applications were generally overcast.Plants were watered regularly to minimize plant stress, as environmentalconditions, such as low air speed or high relative humidity, and culturalstresses may predispose plants to phytotoxicity. In addition, all sprayapplications were performed in the morning so that any phytotoxic effects weredue directly to the insecticides. A numerical phytotoxicity rating scale from0-3 (0 = no visible injury; 1 = light injury, 25 percent foliar injury, noinfluence on marketability; 2 = moderate injury, 50 percent foliar injury,reduced market quality; and 3 = complete foliar injury, more than 75 percentfoliar injury, not marketable) was used to describe the extent of phytotoxicityfrom the insecticide treatments. This numerical rating scale is similar to the oneused for evaluating phytotoxicity on chrysanthemums. Typical phytotoxic plantinjury on herbs was a marginal leaf burn (necrosis). Visual evaluations wereconducted seven days after the final insecticide treatment. Plants wereindividually evaluated for phytotoxicity.

Results and Discussion

In experiment one, Pyrenone, insecticidal soap and bothrates of Cinnamite were significantly more phytotoxic than the other treatmentsand the untreated check (See Table 1, above). The high rate of Cinnamite (85 fl.oz.per 100 gal.) had a significantly higher phytotoxicity rating than the lowerrate (64 fl.oz. per 100 gal.). Botanigard, Azatin, hot pepper wax and UltraFinespray oil were not phytotoxic to any of the herb species tested.

Results from experiment two were similar to experiment one,with the same four treatments causing significantly more phytotoxicity than theother treatments and the control. Although plants treated with hot pepper waxand UltraFine spray oil exhibited minor phytotoxic symptoms, they were notsignificantly different from the control (See Table 2, page 23).

Despite the phytotoxic effects from some of these pestcontrol materials, new growth that emerged following the treatments appeared toovercome the initial injury. As a result, plants at the conclusion of the studywere still saleable, as determined subjectively by the herb production manager.This type of response has occurred with other pesticides such asacaricides/miticides where reductions in stomatal openings caused by the phytotoxiceffects were temporary. No significant effect could be attributed to the age ofthe plants when they were treated.

A possible reason why the tested insecticides did not harmmost of the herb types is because the plants possess a waxy or thick leaf cuticle,which may have protected them. This may be due to the physiological nature ofthe leaf surface. Of all the herb types, St.-John’s-wort demonstrated thehighest sensitivity to the insecticides, especially to both rates of Cinnamite.This may be because St.-John’s-wort has a thinner leaf cuticle compared to theother herb types.

Initial research with Cinnamite has shown that it wasphytotoxic to poinsettia, Euphorbia pulcherrima. However, it was noted thatthis might be due to a formulation problem and not a direct effect of theactive ingredient. In those studies, no phytotoxicity was demonstrated onrosemary and thyme at 0.25 percent Cinnamite; however, 0.50 percent Cinnamitewas phytotoxic to thyme. As a result, the rate of Cinnamite for use on herbshas been lowered.

Although oils generally have a higher probability of causingphytotoxicity, this was not the case in this study. For example, UltraFinespray oil, which was one of the treatments in this study, has demonstratedsafety to flowers and fruit.

This study demonstrated that several registered insecticideswere phytotoxic to certain herbs. However, the overall effects were notsignificant enough to reduce market quality and salability. Furthermore, mosttested insecticides showed no chronic phytotoxic effects. These insecticidesare useful for controlling pests on herbs because they have short residualactivity, which minimizes any potential effects from spray residues. Theinformation produced from this study will help greenhouse managers whenselecting pest control materials to manage insects and mites on herbs.

Raymond Cloyd and Nina Cycholl

Raymond Cloyd is assistant professor, extension specialist in Ornamental Entomology/Integrated Pest Management at the University of Illinois Department of Natural Resources and Environmental Sciences. He may be reached by phone at (217) 244-7218 or E-mail at [email protected] Nina Cycholl is greenhouse grower/researcher at H. M. Buckley and Sons, Inc. She may be reached by phone at (217) 546-9350.

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