Effective Ways to Get Rid of Borers on Alfalfa Plants

Use low impact pesticides

Choose insecticides that are highly selective to a specific type of insect and so have low toxicity for others (signal word of Caution on the label or EPA Reduced Risk product). Other characteristics of low impact pesticides are those that break down rapidly after application and therefore have minimal impact on pollinators and natural enemies. However, using these products requires some knowledge about their relative toxicity to beneficial insects and their potential to cause leaf or flower injury (phytotoxicity). The following types of products have a minimal impact on beneficial insects.

Insecticidal soaps are applied as a foliar application (sprayed on plant leaves) and are effective on a wide range of plant pests when the soap spray comes into contact with the pest. Most commercially available insecticidal soaps are made of potassium salts of fatty acids and kill by disrupting the structure and permeability of insect cell membranes. Insecticidal soaps are most effective on soft-bodied insects such as aphids, adelgids, lace bugs, leafhoppers, mealybugs, thrips, sawfly larvae, spider mites and whiteflies. They are not effective on pests as a residue on the plant surface, and therefore are not toxic to pollinators after the spray dries. They can be safely used at any time to control pests on plants that are not attractive to pollinators. However, on pollinator-attractive plants, spray at dawn or dusk when pollinators are not present.

Generally, concentrations of insecticidal soaps exceeding 3% may cause some leaf or flower injury, and concentrations as low as 1.5% may injure sensitive plants. Read the product label for a list of sensitive plants and avoid spraying those. If uncertain of a plant’s sensitivity, spray a few leaves or flowers first and wait at least three days to watch for symptoms of spray injury, which include yellow, black or brown spots, brown (necrotic) edges on leaf and petal tips, scorch or discoloration. Some landscape plants known to be sensitive to insecticidal soap are horse chestnut, mountain ash, Japanese maple, sweet gum, jade plant, lantana, gardenia, bleeding heart, sweetpeas, crown-of-thorns and some cultivars of azaleas, begonias, chrysanthemum, fuchsias and impatiens.

It is best to purchase a commercial product formulated for use on plants rather than prepare your own spray from dish-washing detergents or other household cleaners because homemade recipes may be more toxic to plants. Most such products are detergents rather than true soaps, which can damage your plants. Only use products that are specifically formulated and labeled for use as insecticide. For more information on using insecticidal soaps, see Miller (1989), Gill and Raupp (1990) and Pundt (2004) in the reference section of this publication. Many insecticidal soap products are listed by the Organic Materials Review Institute (OMRI) at www.omri.org.

Horticultural oil is a term for the various oils used for pest control on plants. Most horticultural oils are lightweight and petroleum-based, but some are made from grains, vegetables or neem tree seeds. Like insecticidal soap, horticultural oils work best when the spray comes in contact with the pest. Once the oil spray dries, it does not have much effect and becomes safe for pollinators and other beneficial insects. Horticultural oil can be safely used at any time to control pests on plants that are not attractive to pollinators. However, on pollinator-attractive plants, spray at dawn or dusk when pollinators are not present.

Horticultural oils give excellent control of armored scales, such as Euonymus scale and oystershell scale, and can also be used for aphids, whiteflies, spider mites, true bugs, caterpillar and sawfly larvae and more. The recommended concentration of horticultural oils for pest control is usually 2%. However, even at 2%, some plants are sensitive to oils, including Japanese maple, red maple, hickory, black walnut, plume and smoketree (Cotinus coggygria). Plants reported as somewhat sensitive are Colorado blue spruce, redbud, juniper, cedar, cryptomeria and Douglas fir. Applying oils during high humidity or high temperatures may have a toxic effect on plant growth. Plant injury symptoms following an application of horticultural oil are discoloration, yellowing, leaf or flower browning (necrosis), black spots and terminal or branch dieback. It is best to spray a few plants first and observe them for three days for these phytotoxicity symptoms. Many horticultural oil products are listed by the OMRI.

Several pesticides sold are derived from naturally occurring pathogens such as bacteria or fungi. These microbial or bio-pesticides vary in their toxicity to bees, butterflies and other beneficial insects. Some bioinsecticides, such as those derived from the fungus Beauvaria bassiana, are toxic to bees and should not be used where pollinators are present. Other bioinsecticides may have low impact on pollinators due to their low toxicity or short residual, which allows them to be applied in the evening or at dawn when bees are inactive.

Considerations for using certain biopesticides

The following active ingredients are found in products that have minimal impact on bees and other beneficial insects.

Products containing B.t. are made from a naturally-occurring soil bacterium. Many different B.t. products are available for landscape professionals and homeowners. Different strains of B.t. target specific pest groups, making them selective pesticides. For example, spores and crystals of Bacillus thuringiensis var. kurstaki (B.t.k.) are highly toxic when ingested by butterfly and moth larvae (caterpillars). The crystals containing the toxin dissolve only at an extremely high pH found in the caterpillar’s gut. B.t.k. is not toxic to bees. However, avoid spraying or allowing spray to drift onto favored food plants of caterpillars such as milkweed, the sole food source for monarch butterfly caterpillars.

Another strain of B.t., B.t. galleriae (B.t.g.), targets several species of beetles in the adult and larval stages including scarab beetles (e.g., Japanese beetle), flat headed beetles (e.g., emerald ash borer), weevils and leaf beetles. B.t.g. is not toxic to bees or butterflies, but applications should be avoided where predatory beetles are active. B.t. galleriae is now available at garden centers and recent testing indicates that it will control Japanese beetle adults for two weeks after it is sprayed. It will not harm pollinators, but it is toxic to monarch caterpillars.

While a B.t. strain works well for its target pest, it also breaks down quickly in sunlight, becoming ineffective after a few days. This makes B.t. very safe for pollinators, predatory insects and mammals. B.t. can be sprayed even when bees or butterflies are present. Many B.t. products are OMRI listed.

The fungus Metarhizium anisopliae is found naturally in soils and infects and kills insects. Commercially available products of M. anisopliae (e.g., Met52) target thrips, weevils, whiteflies and mites on ornamentals, and ticks in turf. Once the product is sprayed on the foliage or drenched in the soil, the spores attach to the surface of the insect, germinate and penetrate the insect, multiply and kill it. M. anisopliae does not detrimentally impact honey bees and is being studied as a bio-insecticide of varroa mites, a pest of honey bees.

This naturally occurring bacterium is used in a fermentation process that produces a product with insecticidal properties (e.g., Grandevo PTO). It is a broad spectrum bio-insecticide/miticide that controls or suppresses insect and mite pests on ornamentals and turf. It has multiple modes of action including oral toxicity (stomach poison), repellency and reduced reproduction. This product is applied as a foliar application and targets numerous caterpillar species in addition to aphids, whiteflies, thrips, psyllids, lace bugs, chinch bugs, mites and certain beetles. It suppresses a broad number of caterpillar species and should not be sprayed or allowed to drift in known habitats for threatened or endangered species of caterpillars and butterflies, such as fields with milkweed where monarch butterfly caterpillars feed. This product may repel bees for up to six days, so time applications to avoid disrupting pollination. Grandevo PTO (active ingredient C. subtsugae) is an OMRI listed product.

Azadirachtin is the active ingredient extracted from seeds of the tropical neem tree. Bio-insecticides with azadirachtin act as an insect growth regulator (IGR) in addition to being an anti-feedant and repellant to insects. It is effective at controlling insect immature stages and is broadly labeled for adelgids; aphids; caterpillars such as budworms, tent caterpillars and webworms; beetles such as Japanese beetles, emerald ash borers, weevils and elm leaf beetles; leafhoppers; leafminers; mealybugs; psyllids; sawflies; scales; thrips; and whiteflies. Azadirachtin must be ingested to be toxic and, when applied as a foliar spray, has short residual activity, making it unlikely bees and other pollinators will be affected (no longer toxic after about two hours for bees). Direct contact has shown no effect on worker honey bees. Azadirachtin products can be safely used at any time to control pests on plants that are not attractive to pollinators. However, on pollinator-attractive plants, spray during late evening, night or early morning when pollinators are not present to minimize contact with adult bees that could potentially bring azadirachtin back to the nest where larvae are present. Many azadirachtin products are OMRI listed.

Spinosad is derived from a soil bacterium and affects the nervous system of insects and mites. It has contact activity, but is even more active when ingested. Several products containing spinosad are labeled for ornamental (e.g., Conserve) and agricultural uses to control a broad spectrum of pests including caterpillars, sawfly larvae, leaf beetle adults and larvae, thrips, leafminer and gall-making flies and emerald ash borer beetles. Spinosad is highly toxic to bees. However, toxicity is greatly reduced once the product has dried on the foliage, within three hours to one day depending on the product. Therefore, avoid use if bees are active, and if applications are needed, apply in the evening when bees are not active and product has time to dry. This product suppresses a broad number of caterpillar species and should not be sprayed or allowed to drift in known habitats for threatened or endangered species of caterpillars and butterflies. Some spinosad products are OMRI-listed and on the EPA Reduced Risk list.

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