AMES, Iowa – Once vegetable seedlings have been produced, or after they’ve been transplanted into your garden or onto your farm, there may be issues with insect pests or diseases. In this “Gardening while Isolated” edition, specialists with Iowa State University Extension and Outreach offer advice on managing pests.
Your first line of prevention was selecting insect- and disease-resistant varieties to plant. The second line of defense will be biological control, or relying on the natural enemies or beneficial insects that may be in your environment.
The most common beneficial insects encountered in nature are tiny, parasitic wasps that attack insect pests, like aphids and whiteflies, or ladybeetle predators that are voracious consumers of aphids and other small insects. If these beneficial insects are missing from the environment, you can purchase and introduce them, with several caveats.
Augmented beneficial insects work best in an enclosed situation, like a greenhouse or high tunnel. If released outdoors, ladybeetles, for example, are known to fly away, attempting to return to the home where they were collected, like the Sierra foothills in California.
But inside a greenhouse, they can go to work – if sufficient prey, such as green peach aphids, are available. Synthetic pesticides should be avoided, as they can damage or kill beneficial insects. In this latest “Gardening While Isolated” video, Pete Lawlor, agricultural specialist in the Department of Horticulture at Iowa State University, explains some of the pest management treatments used to manage insect pests in the Iowa State greenhouse.
Biological controls
Some of the most important insect pests on vegetable transplants include aphids, thrips, whiteflies and fungus gnats. The first three insects can damage seedlings by feeding on leaf chlorophyll, which can cause a stippling or yellowing of leaves, while fungus gnat larvae can damage transplant roots. There are two species of predatory mites that can help control thrips and fungus gnats. Neoseiulus cucumeris, the cucumeris mite, is a species of predatory mite in the family Phytoseiidae, effective against western flower thrips (Frankliniella occidentalis), whitefly, psyllids, aphids and spider mites. This tiny (between 0.5 and 1 mm) mite is pear-shaped and translucent pink or tan-colored.
Purchased beneficial insects are shipped overnight and should be released immediately upon arrival. These mites can tolerate high temperatures but prefer 60-85 degrees Fahrenheit and 40-90% relative humidity. Mites go through a life cycle that includes egg, larva, protonymph, deutonymph and adult, which can take five to 25 days, depending on temperature. The suggested application rate is one predatory mite for every five pests. The other beneficial mite that can be purchased for release is Stratiolaelaps scimitus, or the fungus gnat predator. This mite is translucent brown to tan and inhabits the top layer of soil where pest insect larvae and pupae dwell. They also consume thrips pupae, springtails, root aphids and spider mites. The predatory mites are shipped as adults in bottles or biodegradable bags in a medium such as corn grit or vermiculite.
Parasitic wasps that are available for purchase and release include Encarsia formosa for control of the greenhouse whitefly. These parasitic wasps do occur in nature so they can be conserved by not using synthetic pesticides and planting flowers with nectar and pollen for these species. Adult female E. formosa are tiny wasps (less than 1 mm in length) with a dark brown to black head and thorax and a bright yellow abdomen that feed on immature whitefly stages by puncturing the body with their ovipositors and laying eggs inside. The hatched wasp larvae feed within the whitefly nymph, which forms a black pupal case for the parasite. E. formosa leaves a circular hole in the whitefly pupa when it emerges and goes on to parasitize more whitefly hosts. These beneficial insects are shipped as parasitized whitefly pupae on cards that can be attached to wire bases to allow emergence in the greenhouse.
Another beneficial parasite is Aphidius spp., a small wasp less than 3 mm long, whose hosts include green peach aphid, melon aphid, pea aphid and other aphid species. The attack of aphid pests is similar to Encarsia, except that Aphidius’ larva feeding within the aphid causes the aphid to become puffy or mummified, turning tan or golden. Similarly, the adult parasite chews its way out of the mummy, leaving a circular hole. Aphidius ervi are shipped in a 125 ml bottle that contains at least 250 mummies and adult aphid parasites.
Another biological control is an entompathogen (insect disease), such as the fungus, Beauvaria bassiana. Fungal spores attack the host insect and the dying insect will change color to pink or brown as its body cavity fills with a white fungal mass. The disease is effective against all stages of insect pests, such as aphids, mealybugs, whiteflies and leafhoppers.
Organic insecticides
The third line of defense includes the use of purchased, organic-compliant insecticides that contain naturally irritating or repelling materials. Integrated Pest Management involves the integration of cultural, physical, biological and chemical practices to grow crops with minimal use of insecticides, even natural ones. In order to keep pests below an economically damaging threshold, growers should monitor insect populations, through the use of yellow sticky cards, for example, to determine when control options are needed. Like all purchased insecticides, users must follow the label instructions on the product for mixing, applying, storing and disposing of the pesticide. If in an organic operation, verify type of product through the USDA-National Organic Program rules, or look for an Organic Materials Review Institute label. One of the more widely used brands is Safer’s Soap, which contains natural potassium salts from fatty acids.
This insecticidal soap kills soft-bodied insects, such as aphids and leafhoppers, by desiccating the insect’s cuticle or skin, while not affecting plants or humans, if applied at label rates. Neem oil is produced from fruit and seed extracts of the neem tree, Azadirachta indica, and acts as an anti-feedant, repellent and egg-laying deterrent for insect pests, including aphids, whiteflies and spider mites. Neem is also effective against some diseases, like powdery mildew.
Another organic-complaint insecticide is PyGanic, which is made from pyrethrins extracted from pyrethrum daisy flowers in the genus Chrysanthemum cinerariifolium and Chrysanthemum coccineum. Pyrethrum is toxic to aphids, beetles, caterpillars, fruit flies, mites and thrips. This natural pyrethrum is very different from the laboratory-synthesized pyrethroid, which is in several chemical sprays. Unlike many synthetic insecticides, these natural insecticides tend to break down rapidly in sunlight, so repeated applications may be needed, based on insect pest populations, and label recommendations.
While gardening, get to know the life cycle and behavior of any pests encountered in order to develop an effective management strategy. Understanding where the weak link may occur in a pest's life cycle (e.g., when first emerging from soil) can help you select the most effective strategy for producing the healthiest plants.
For more information see ISU Extension and Outreach publication “Organic Vegetable Production” or “Biological Control in Greenhouses” from the University of Massachusetts.
The products and suppliers mentioned in this release are for example only. No endorsement of these products or suppliers is intended.
Original photo: Vegetables transplanted into the field. By Kathleen Delate.
