Tank-mixing insecticides can reduce rates

The loss of certain pesticides from the market, as well as heightened environmental awareness, has led to increased popularity of biorational products such as horticultural oils, insecticidal soaps, microbials and botanicals. Researchers and practitioners are looking for ways to integrate these materials into plant-health-care (PHC) programs, and certain developments are making this easier:

Improved efficacy and application. Several new biorational products show efficacy and ease-of-application comparable to conventional products. Neem, a botanical extract, shows potential for controlling several important pests. Insecticidal soaps can be quite effective for controlling an assortment of sap-feeding and defoliating pests.

New strains. New strains of biological and microbial controls are widening the control spectrum of older products. For example, Bacillus thuringiensis (Bt), a microbial product in use since the 1960s, controls a variety of leaf-eating caterpillars. However, in recent years researchers have developed isolates that are effective against specific pests such as the Colorado potato beetle and certain leaf beetles.

New uses. Applicators are finding new uses for older products. Fruit growers began using horticultural oils in the early 1900s as a dormant treatment for San Jose scale. However, only in the last few years have turf-and-ornamental applicators begun to use oils as a foliar spray.

Combinations with conventional products. Recent studies have found that biorationals tank-mixed with conventional products at reduced rates are as effective as full-rate applications of the same products used alone.

In spite of these developments, applicators have not fully exploited the use of biorationals in urban horticulture and nurseries. An industry survey revealed that 57 percent of arborists do not or never have used horticultural oils. Of those, 1 percent were not even aware that this product existed. Of those that did use horticultural oil, 54 percent routinely applied it as a general-purpose spray with 65 percent of the applications occurring in the spring. However, only 8 percent of the applications occurred in the summer months. A significant reason for the limited foliar use of horticultural oils (and insecticidal soaps) is the fear of phytotoxicity. Many applicators incorrectly assume oils and soaps invariably injure foliage.

A factor barring greater acceptance of biorationals is the poor reputation they have with many operators. Unfortunately, this stereotype has dissuaded many applicators from trying the newer and more effective biorationals now available. Simple lack of awareness also inhibits greater utilization of these materials.

Tank-mixing biorational and conventional products One method of integrating biorational products into a plant-management program while possibly increasing confidence in their efficacy is tank-mixing them with reduced-rate conventional pesticides. This approach enables you to minimize the application rate of conventional active ingredients and still retain their benefits such as long residual activity.

Researchers at several universities have conducted studies that demonstrate the effectiveness of reduced-rate synthetics tank-mixed with biorational insecticides. Although these studies have provided a wealth of information, we still need to learn more about control spectrum and efficacy as well as phytotoxicity relating to these tank mixes. Here is what we have learned from field and laboratory studies on tank-mixed products (see table, at right, for a list of products evaluated). Consult with a state extension agent before using below-label-rate applications. Such treatments do not violate federal regulations, but state regulations may differ on this point.

Aphids. In a series of studies over several growing seasons, researchers evaluated spray oil, insecticidal soap, chlorpyrifos and cyfluthrin for control of spirea, elm-leaf and apple aphids, and aphids on viburnum and oak seedlings. In another study, investigators used 0.25-, 0.5- and 1-percent (by volume) insecticidal soap tank-mixed with one-eighth, one-fourth and one-half the label rate of chlorpyrifos for control of aphids on spirea plants and crabapple seedlings.

Horticultural oil and insecticidal soap were as effective (70- to 100-percent mortality 2 days after treatment) as chlorpyrifos or cyfluthrin at controlling immature and adult aphids. Plus, chlorpyrifos mixed with insecticidal soap (both at reduced rates) was as effective as each of these products alone, even at their full label rates.

Armored scales. Reduced rates of chlorpyrifos and tank mixes of chlorpyrifos with insecticidal soap provided good to excellent control (100-percent mortality) of pine-needle-scale crawlers on mugo pine, euonymus-scale crawlers on euonymus (87- to 100-percent mortality) and oystershell-scale crawler on quaking aspen (90- to 100-percent mortality) 1 week after spraying. Additionally, reduced rates of chlorpyrifos and tank mixes of reduced-rate chlorpyrifos with 1- or 2-percent spray oil provided good to excellent control (70- to 100-percent mortality) of black-pine-leaf-scale crawlers on mugo pine.

Potato leafhopper. Red-maple trees infested with potato leafhoppers were sprayed with one-half and full label rates of permethrin, cyfluthrin or fluvalinate, alone and in combination with 2-percent insecticidal soap. Eight days after application, all combination treatments provided control equal to the full-rate pyrethroids (less than 10-percent tip distortion). The untreated checks experienced 30- to 40-percent tip distortion.

Eastern tent caterpillar. Chlorpyrifos at 0.25, 0.50 and 1.0 pound active ingredient per 100 gallons tank-mixed with 0.25-percent insecticidal soap controlled eastern tent caterpillar as effectively as chlorpyrifos or cyfluthrin alone. After 16 days, average percent-defoliation on untreated trees exceeded 80 percent, while all treated trees had only 10- to 15-percent defoliation.

MVP (micro-encapsulated Bt) was as effective at controlling eastern tent caterpillar as other Bt formulations and cyfluthrin. Treated trees had less than 8-percent defoliation, while untreated trees experienced 60- to 70-percent defoliation. A laboratory study yielded results consistent with this finding: Dry weights of frass from MVP-treated eastern-tent-caterpillar larvae were significantly less than the untreated group.

Leaf-beetle larvae. In a laboratory study, investigators exposed young elm-leaf-beetle larvae to Siberian elm (Ulmus pumila) foliage treated with Bt "San Diego" and label-rate neem extract and then evaluated for mortality or pupation.

Neem provided good control (88- to 96-percent mortality) of both first- and second-instar elm-leaf-beetle larvae and imported-willow-leaf-beetle larvae 1 week after application. Neem also provided excellent control (96-percent mortality) of elm-leaf-beetle larvae 10 days after spraying. However, the field residual for Bt "San Diego" is short as evidenced by poor control of elm-leaf and imported willow-leaf-beetle larvae 7, 9 and 14 days after spraying. This is consistent with the 2- to 3-day residual of other Bt strains.

Imported willow-leaf beetle. In the field, tank mixes of reduced-rate chlorpyrifos and insecticidal soap controlled imported-willow-leaf-beetle larvae as effectively (100-percent mortality) 2 days after spraying as chlorpyrifos or cyfluthrin alone at full label rates.

Phytotoxicity of biorationals As these studies indicate, horticultural oils and insecticidal soaps can be as effective as standard synthetic insecticides for controlling several types of insect pests. None of the treated plants in any of these studies exhibited phytotoxic effects. All plants appeared healthy and exhibited normal growth. Ambient temperatures at the time of application varied between 70 and 75 degrees F with 50- to 60-percent relative humidity. These conditions fall within the recommended range for using horticultural oils and insecticidal soaps without serious risk of phytotoxicity. Be aware that hot weather can reduce the safety of oils and soaps on foliage, so you should adhere strictly to label guidelines.

While not a silver bullet, these studies illustrate that biorational insecticides, alone or at reduced rates in combination with synthetic insecticides, can play an integral role in a progressive plant-management program. As additional products become available and research increases our ability to use biorationals effectively, these materials should play a greater role in landscape pest control.

Dr. Fredric Miller is an urban IPM extension educator with the University of Illinois cooperative extension service (Countryside, Ill.).

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