Greenhouse IPM: Sustainable Aphid Control

Aphid Photo Courtesy of Hecules Inc., Handbook of the Insect World 60p. Wilmington, DE

Abstract

The following document focuses on least-toxic methods for dealing with aphids in greenhouses. For general information on greenhouse IPM, see ATTRA's publication Integrated Pest Management for Greenhouse Crops, which covers topics such as screening to eliminate pests, weed management, and disease control.

Table of Contents

• Introduction
• Crop Scouting and Trapping
• Biological Control
• Biorational Pesticides
• Insect Growth Regulators
• Summary and Further Resources
• References
• Biological Control Suppliers
• Appendix 1: Beneficial Organisms
• Appendix 2: Biorational Pesticides

Introduction

There are approximately 4,000 aphid species in the world. Life cycles and preferred hosts vary with each type of aphid. Common aphid pests of greenhouse crops include the green peach aphid (Myzus persicae), the melon/cotton aphid (Aphis gossypii), the chrysanthemum aphid (Macrosiphoniella sanborni), the rose aphid (Macrosiphum rosae), the potato aphid (Macrosiphum euphorbiae) and the foxglove aphid (Aulacorthum solani).(1)

The green peach aphid is probably the most notorious aphid pest of greenhouse crops because of its wide host range, worldwide distribution, number of viral diseases it vectors, and difficulty of control.(2)

Aphid management relies on understanding that the females of many aphid species do not have to mate in order to reproduce, and they typically produce live young, rather than eggs. These characteristics contribute to the tendency of aphid populations to "explode."

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Crop Scouting and Trapping

Plants should be visually inspected for signs of an aphid infestation. Look especially carefully at plants prone to aphid problems, and at plant parts like the undersides of leaves, stems, and new growth. Choose plants randomly throughout the greenhouse and inspect undersides of leaves, buds or tip growth and watch for honeydew and cast skins. Since aphids are difficult to see on plants with fine foliage, hold such plants over a white piece of paper and gently tap to dislodge any aphids. Avoid moving infested plants to new areas where susceptible plants are growing. Locations where aphids are found should be flagged, so that population development and control efforts can be evaluated.

Different aphid species tend to populate different parts of their host plants. Green peach aphids tend to cluster on the succulent young growth, whereas melon aphids are usually evenly distributed along the plant stems. Melon aphid populations also have fewer winged adults than do green peach aphids. Knowing which species is infesting the crop is very important in successful detection and monitoring of aphid populations. The Cooperative Extension Service is a good resource for identification of specific aphid pests.

Yellow sticky cards placed horizontally at the top of the pot or container (if you are growing containerized plants) can be used for monitoring winged aphids. However, since winged aphids caught during the summer months may have blown in from the outdoors, sticky cards are not as reliable as visual inspections. Sticky cards are more useful in the winter months when aphids caught on the cards are not likely to have come in from the outside. It is better to rely primarily on visual inspections for aphid detection, and use sticky cards as a backup method.

Signs of an aphid infestation include honeydew or sooty mold on leaves, yellow spots on upper leaf surfaces, cast skins on leaves, curling of leaves, and distortion of new growth.

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Biological Control

There are several biological control options for greenhouse aphid pests. Some common biological control agents (BCAs) include green lacewings (Chrysoperla carnea, C. rufilabris, Chrysopa spp.), aphid midges (Aphidoletes aphidimyza), parasitic wasps (Aphidius colemani and Aphidius matricariae) and lady beetles (Hippodamia convergens). See Appendix 1: Beneficial Organisms for more BCAs and suppliers.

A 1998 study showed that green lacewing larvae did not disperse as well as the parasitic wasp Aphidius colemani.(3) To achieve equal aphid suppression, more of the slower-moving species need to be introduced and from more points (lacewings have to be released on each bench because they cannot move to adjacent benches, for instance). A study performed at Colorado State University compared the effectiveness of parasitic wasps, aphid midges, lady beetle larvae, and green lacewing larvae.(4) The researchers found that lacewings performed better in hot temperatures, while aphid midges and lady beetles were better in cooler temperatures. At all temperatures, Aphidoletes was the best of the four at controlling aphids.

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Biorational Pesticides

Strains of the fungus Beauveria bassiana provide good control of aphids, including green peach aphids. The fungus works by attaching to the outside of the pest, then penetrating into the body and killing it. The fungus is available commercially for greenhouse ornamentals as Naturalis-O™ and for vegetables as BotaniGard™. (See below for suppliers.)

Another fungus, Verticillium lecanii, can also provide good biological control of aphids. Formulations of this fungus are currently being sold in some European countries under the names of Vertalec™ and Mycotal™, but neither of these products is yet registered for use in the United States.

Verticillium lecanii often occurs naturally in greenhouses, so it may be possible to encourage its growth and distribution in the greenhouse without the benefit of a commercially available product. V. lecanii spores require at least 93% relative humidity at temperatures between 59 and 81°F to germinate and grow. High humidity must be present for at least 10-12 hours/day. Unfortunately, most plant disease-causing fungi also grow best at these same temperature and humidity ranges. Fungicides used to control the plant disease-causing fungi would probably also kill any beneficial fungi present. Insecticides may also be harmful to V. lecanii.

Least-toxic pesticides used against aphids in greenhouses include insecticidal soap (M-Pede™), horticultural oil (UltraFine SunSpray spray oil™), and botanical insecticides such as neem (Azatin™, Neemazad™, and Neemix™), or natural pyrethrums. See Appendix 2: Biorational Pesticides for more information and suppliers.

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Insect Growth Regulators

Insect growth regulators (IGRs) are another least-toxic pesticide control option for pests. IGRs typically kill insects by disrupting their development. They have a complex mode of action that precludes insects from rapidly developing resistance. IGRs can work in one of several ways:

1. they can mimic juvenile hormones, so that insects never enter the reproductive stage of development;
2. they can interfere with the production of chitin, which makes up the shell of most insects; or
3. they can interfere with the molting process.

Aphid Photo Courtesy of Hecules Inc., Handbook of the Insect World 60p. Wilmington, DE

IGRs usually work through ingestion, so good spray coverage is essential. They generally don't affect non-target species, such as humans, birds, fish or other vertebrates. For most IGRs there are minimal re-entry restrictions. IGRs typically take several days to have an effect on pest populations. Because IGRs do not affect mature insects, adult beneficials released into the greenhouse after an IGR application are not likely to be affected. Use of IGRs is generally prohibited by organic certification organizations because the products are synthesized.

IGRs can sometimes be used in conjunction with biological control efforts and may provide growers with a "safety net" should beneficials fail to keep the pests below economically damaging levels. The table below lists some well-known insect growth regulators. (Contact information for suppliers is listed at the end of this document.)

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Summary and Further Resources

Greenhouse aphids are tiny insects, but they demand serious attention on the part of the greenhouse grower. Integrated pest management offers a sustainable approach for dealing with greenhouse aphids, and safer pest control products facilitate the adaptation of least-toxic control measures that dovetail very nicely with the IPM philosophy. In the resources sections below, growers are provided with a list of biological control suppliers; and tables that summarize biocontrol agents and biorational pesticides that control aphids.

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References

1) Lindquist, Richard. 1991. A guide to aphid control. GrowerTalks. October. p. 75.

2) Sunderland, Keith et al. 1992. Integrated pest management of greenhouse crops in Northern Europe: Aphids. p. 23-30. In: Jules Janick (ed.) Horticultural Reviews: Vol. 13. John Wiley and Sons, Inc. New York, NY.

3) Heinz, K.M. 1998. Dispersal and dispersion of aphids and selected natural enemies in spatially subdivided greenhouse environments. Environmental Entomology. Vol. 27, No. 4. p. 1029-1038.

4) Anon. 1999. Efficacy of four biocontrol agents on the green peach aphid, Myzus persicae, in greenhouse peppers. Midwest Biological Control News. January-February. p. 7.

Replying to [kivi]: Dear Kivi Sir, very useful info. Let us get rid of all the bugs.

Regards,

Moolan