Aphids can be a problem in the home landscape, vegetable garden, or fruit garden. They can vector many viral diseases and can cause significant damage to desired plants if the aphid population is left unchecked. Aphids reproduce quickly and have multiple generations per year. Females of most species can give birth to live young, meaning that within a few days in summer time temperatures aphid populations can grow exponentially. Aphid populations of different species can found at any time from spring to fall in South Carolina. Aphids are most effectively managed when Integrated Pest Management (I.P.M.) is practiced. In short, IPM is the use of multiple control strategies in a comprehensive and preventative approach to reduce pest populations, to maintain plant health, and to minimize the use and impact of pesticides in the environment. These management strategies include mechanical, physical, biological, cultural, and chemical controls.
Scouting & Identification
The first step in solving any pest infestation problem is to determine what exact pest is present. There are many different species of aphids, and the different strategies to manage them can vary with each species. Green peach aphids (Myzus persicae) are the most common aphid species found feeding on many plant species in the garden and the landscape. Another common aphid is the oleander aphid or the milkweed aphid (Aphis nerii). This aphid feeds on oleander, milkweeds, and their relatives.
Aphids can usually be found on young tender growth and on the underside of leaves. Plant symptoms typically are a response to their feeding. Aphids have piercing sucking mouth parts. Some species inject toxins in the plant when they feed; these toxins can cause the tissue to become distorted and deformed. A sign of aphids being present is honeydew, the aphid’s sticky excretion, on plant surfaces. Honeydew looks similar to sugary drink being poured on a plant’s leaves that has dried and has a sticky appearance and feel.
There are many plant species that can be host to aphids. Vegetable garden hosts include peppers, tomatoes, leafy greens, cabbage, kale, and basil are key host plants. Hosts in landscapes include zinnias, roses, coreopsis, and many others. Fruit trees are also not immune and the Woolly Apple Aphid (Eriosoma lanigerum) can be a severe problem. Not only do they feed on trunks, branches, and twigs, but they may move below ground and feed on apple tree roots.
Another common aphid found in vegetable gardens is the Melon Aphid, also known as the Cotton Aphid (Aphis gossypii), which can be a severe problem for watermelons, musk melons, cucumbers, and squash.
There are some basic things home gardeners can do to prevent major outbreaks of aphids. High levels of nitrogen promote succulent, nutritious new growth, which is preferred by aphids and can help boost aphid reproduction. Over fertilizing a plant can enhance aphid population growth and make the problem worse. Using smaller amounts of fertilizer throughout the growing season can help to reduce potential aphid outbreaks.
Another effective preventative method is the use of reflective silver mulch (a Mylar like film placed over the soil surface), especially in vegetable production. A side benefit of reflective mulch is that it can increase crop yields because of the increased amount of solar energy reflected onto the leaves. However, there are some precautions to these types of materials. If purchasing big rolls of the material is not desired, or if the garden space is small, an aluminum pie plate can be cut and placed upside down around the base of the plant.
A good effective method for eliminating aphids is to simply rinse them off the leaves of affected plants. A water hose and nozzle with adequate pressure is enough to knock the aphids from the foliage, but not to damage the plant. CAUTION: A powered pressure washer is much too strong. The jet or shower setting on a dial nozzle is enough to dislodge these pests. Once off the plant, aphids cannot climb back up the plant and will often starve to death. Aphids can also be rubbed off the plants with fingers or a wet cloth. This method effective against small aphid populations and at the very early stages of infestation. Physical removal by rubbing would be ineffective at removing large infestations.
Aphids have several natural enemies that can be attracted or released to help keep populations in check. The most common one that gardeners are familiar with are lady beetles. Lady beetles and their larvae feed on many different types of aphids, and their presence in the garden should be encouraged by reducing the overall use of broad-spectrum insecticides. Lady beetles are available for purchase, but it is not recommended to buy them. They are typically harvested during their hibernation period in the western U.S., and as a result are often confused when they arrive in a gardener’s home. As a result, the lady beetles fly away in search of a new hibernation spot. For home gardeners, it is best use plants in the garden and landscape that will attract lady beetles, such as sunflowers, clovers, liatris, and coreopsis.
Another effective natural enemy, that can be purchased and realesed is green lacewing larvae (Chrysoperia rufilabris). These larvae are extremely aggressive and will eat numerous aphids a day.
They can be released on specific plants that are infested for direct application or released as adults to establish in the garden. These insects can be attracted by adding plant species such as coreopsis, cosmos, and clover in the garden and landscape. Another way to encourage and protect them is to reduce the use of broad-spectrum insecticides. Lacewings are extremely sensitive to insecticides and even drift from an application can be harmful. Many lacewing adults are often killed in bug zappers used for mosquito control. These helpful predators can be purchased from online sources. It is recommended that only lacewing larvae are purchased and released into the garden and landscape.
Another natural enemy are parasitic wasps (Aphidius species) that sting aphids and impregnate them with an egg. The egg then grows inside the aphid, killing and mummifying it, and a new adult wasp hatches out of the mummified aphid.
If these mummified aphids are seen near active aphid populations, it indicates that the Aphidius wasps are nearby and actively parasitizing the current population.
These wasps are too small to sting humans. In fact, different species of Aphidius are very selective and each species attacks a different species of aphid. Therefore, it is important to have the aphid properly identified before making a purchase in online sources.
A biological control that can be applied similar to a traditional is insecticide is any product containing Beauvaria bassiana. This entomopathogenic fungus is usually applied as a foliar spray and is parasitic to many soft body insects. The organism is available in both liquid or powder form, but the powder is more stable and has a longer shelf life. This can be used for control of aphids or other soft body insect. Apply the product as a preventative every 7-14 days to help keep pest populations low. The downsides to the product is that it can only be found online and needs to be kept refrigerated, but not frozen.
When all other control measures have failed to keep the populations under control, a chemical insecticide may be needed. The goal with insecticide use is to choose the one with minimal impact to pollinators and natural enemies, but one that is still effective on the insect causing the problem. It should be noted that although the aphids are killed by insecticides, their dead carcasses can still be on the leaves after the application. Do not panic and make an additional application. Check to see if the aphids are still alive by nudging them with a pencil. Additional applications are only needed if live aphids are still present. The dead aphids can be removed by washing the leaves off with water. Before purchasing and using an insecticide, be sure to read and follow ALL label directions. The label is the law; therefore, the product label is the final authority on what crop or areas the product can be applied and at what rate. When shopping for an insecticide, be sure to look on the package for the active ingredient and choose the product with the proper active ingredient to control the pest. Always spray late in the day for best results and to protect beneficial insects.
The first effective choice to spray would be either insecticidal soap or horticultural oil. These insecticidal products coat the aphid’s exoskeleton (body)and cause it to suffocate. These insecticides also can kill beneficial insects upon contact, but they have no residual activity. So only beneficial insects and pollinators that were directly hit by the application will be affected. Pollinators and natural enemies that arrive after the spray solution has dried, will not be impacted by these soaps or oils. Note that these products may be phytotoxic (damaging to the plant) to drought stressed plants, especially at temperatures 90°F or higher. Applications should be made when temperatures are cooler, such as the mid- to late evening to avoid any potential plant damage. For more information on using insecticidal soaps and oils, please see HGIC 2771 Insecticidal Soaps for Garden Pest Control.
An effective step up from the soaps and oils, are insecticides that contain the active ingredient pyrethrin. This botanically derived compound can be very effective in providing a relatively quick knockdown of aphids. These products only affect natural enemies and pollinators that are directly within the application. Additionally, migrating beneficial insects may be repelled by the residue on plant leaves. However, this effect is not long lasting (only hours), so pyrethrins can be an effective choice to help reduce large populations.
Another effective botanically derived chemical is azadirachtin. This compound is a natural insect growth regulator that modifies the way insects grow by inhibiting the shedding of the exoskeleton. It can be mixed with an entomopathogenic fungi or bacteria to allow more contact time between the insect’s exoskeleton and the pathogenic organism. This ensures that the fungi or bacteria have time to grow, penetrate the exoskeleton, and kill the insect.
Systemic insecticides are available for the control of aphids, primarily on ornamentals, although there are formulations for vegetables and fruits. If applying a systemic insecticide to vegetables or fruits, the label will give specific directions as to when the product can be applied prior to harvest. Systemic insecticides contain active ingredients, such as, imidacloprid, or dinotefuran. These products are applied as a foliar spray or as drench application to the roots. The later provides less impact to potential pollinators and natural enemies, as the chemical is applied directly to the roots and taken up by the plant. Applications to the roots can take several days to enter and distribute throughout the plant. Be sure the systemic insecticide product is labelled for use on the specific vegetable or fruit crop.