Aphids

Aphids develop through three life stages : adult, egg, and nymph. Adults and nymphs are oblong to pear-shaped, soft-bodied insects that suck phloem sap. At maturity, most species have a body length of 1/25 to 1/12 inch (1–2 mm). Species that feed openly on foliage are distinguished by the presence of cornicles, a pair of tubelike, rear-pointing appendages on top the abdomen. Root-feeding aphids and those that feed within wax they secrete on foliage and shoots commonly lack obvious cornicles. Eggs hatch within adult females most of the growing season, and generally can be observed only during fall on deciduous perennials where aphids overwinter in the egg stage.

Numerous species of aphids feed on flower and nursery crops. Some are host-specific; these generally feed on only one or a few kinds of plants. Some host-specific aphids migrate seasonally between different hosts or plant parts or both, such as feeding on foliage of their deciduous host during spring through fall and overwintering on the roots of a conifer host. Consult Aphids on the World's Plants: An Online Identification and Information Guide for more information, including complete lists of each species' known host plants. The most common pest species are the cotton, or melon, aphid and green peach aphid, each having over 200 plant hosts.

Identifying the particular aphid species present can be important for reasons including

• Differences in susceptibility to certain pesticides.
• Different host plants, including weeds, that can help maintain a population.
• Differences in biology, which may offer clues on management.
• Their role as virus vectors between susceptible crops and weeds that hosts those viruses. Note that some viruses can be moved in other ways, such as on contaminated propagation and pruning tools.

Green peach and cotton (melon) aphids are generally the most common pest species in floriculture and nurseries. They can be distinguished using a hand lens or other magnification. Green peach aphid antennae attach to bumps or rounded projections (tubercles) on the front of the head, with a distinct indentation between; the tubercles slightly converge (point towards each other) so the head depression is wider at the base than the opening. Green peach aphids are commonly pale green overall, but can be yellowish, red, rose-pink, or whitish. The antennae and cornicles are relatively long, extending beyond the rear of the body. Winged adults have a black to dark green head and blotch on top of the abdomen and thorax.

Melon (cotton) aphids have no antennal tubercles; the front of their head is relatively flat, including where antennae attach. Melon aphids are commonly dark to light green, but can also be gray, orangish, yellow, or whitish. Melon aphids populations commonly consist of a mix of colors. Their cornicles are stout and relatively short and entirely black. Antennae when folded over the back reach only to the middle of the abdomen. Winged adults have a black head and thorax.

Adult (reproducing) aphids may or may not have wings. Winged aphids are produced when plants become heavily infested and the aphids crowded or plant quality declines and when day length shortens during the fall. Green peach aphids produce winged adults at lower population densities than the melon aphid. The optimal temperature for green peach aphid development is 75°F, whereas melon aphid develops fastest above 75°F.

During most of the year adult aphids give birth to nymphs; eggs hatch within females except when laid for overwintering in the fall. Depending upon temperature, aphids commonly mature and begin giving birth when they are 7 to 10 days old. Under optimal conditions, aphids of many species can complete 1 generation in about 1 week.

Aphids have tubular, sucking mouthparts and excrete copious amounts of sticky, sweet honeydew as they feed. The honeydew can foul plant parts and induce growth of blackish sooty mold fungi. The translucent to white cast skins of aphids stick to surfaces and detract from plants' appearance. Feeding by abundant aphids can cause foliage to yellow. Leaves and shoots infested while immature can become twisted as they grow. Melon aphids transmit (vector) at least 44 plant viruses. Green peach aphids transmit more than 100 plant viruses.

Biological control, cultural controls (e.g., excellent sanitation, exclusion, and reflective mulches), and insecticide application are key methods for managing aphids.

Important predators of aphids include brown lacewings, green lacewings, lady beetles and the larvae of various flies, including aphid flies, aphid midges, and flower flies or syrphids. Some of these are commercially available for purchase and release, such as the aphid midge Aphidoletes aphidimyza that is commonly released for aphid control in greenhouses and nurseries and has been well-demonstrated to be effective when properly used. The larval stage of parasitic wasps such as Aphelinus abdominalis, Aphidius spp., and Diaeretiella rapae are important natural enemies of many aphid species. Some of these parasites are also commercially available; banker plants may be the most common method of economically sustaining parasitic wasps for aphid control (PDF).

The most economical and effective method for using biological control in field crops is to conserve naturally occurring parasites and predators. Control ants and dust and rely mostly on types of acaricides (miticides) and insecticides that are not persistent and toxic to natural enemies. These methods are necessary for both naturally occurring and released parasites and predators to be effective. Unless additional, compatible methods are employed for aphids that infest marketed parts of plants, biological control alone generally does not provide satisfactory control. For more information, see Natural Enemy Releases for Biological Control of Crop Pests, Protecting Natural Enemies and Pollinators, and Relative Toxicities of Pesticides Used in Floriculture and Nurseries to Natural Enemies and Honey Bees..

Since common pest aphids feed on a wide variety of plant species. Keep production areas free of weeds that can host aphids. Exclude winged adults by covering greenhouse openings with screens that have a pore width of 0.355 mm (355 microns, about 1/100 inch) or smaller and sufficient surface area to allow adequate ventilation.

Before starting a new crop, carefully inspect the plants to ensure that they are free of aphids and other pests. Treat effectively or rogue any infested plants. Avoid the use of excess nitrogen fertilizer, which can increase aphid abundance.

Reflective mulch
For field-grown crops, applying reflective mulch in the middle of planted rows or entirely covering the soil surface and planting through holes in the mulch can greatly reduce the extent of aphid infestation. Reflective mulch also reduces infection from insect-transmitted viruses when crops are young and most susceptible to damage by these plant pathogens. Reflective mulch can increase crop growth and the yield of cut flowers and reduce the frequency of irrigation by conserving soil moisture. See Reflective Mulches for more information.

Biological and cultural controls are organically acceptable aphid management methods. Certain insecticides are acceptable for organic use, including the botanicals azadirachtin, neem oil, and pyrethrins without piperonyl butoxide (PyGanic), the microbial Beauveria bassiana, and certain oils (Organic JMS Stylet Oil).

Melon aphids tend to occur throughout infested plants, and their presence on the underside of lower leaves can easily go undetected if these areas are not inspected. Green peach aphids tend to occur mostly around growing points, so their presence is more readily observed. Green peach aphids produce winged individuals at lower densities than melon aphids on crops such as chrysanthemums.

Yellow sticky cards can be used to detect  winged aphids. Consider treating if there is an average of 5 to 10 aphids per card per week. However, since aphids produce winged individuals in response to crowding, they may be abundant on plants before they are found on sticky traps. Visually inspect plants for aphids, cast skins, honeydew, and sooty mold. For more information, see Monitoring with Sticky Traps and Sticky Trap Monitoring of Insect Pests.

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