Pest type: Insects
Aphids are small-bodied insects that use their slender mouthparts to pierce the stems, leaves, and tender parts of plants, sucking the sap out for sustenance. Aphids come in a variety of shapes, colors and sizes depending on the species; they can be green, pink, yellow, black, brown or gray. However, they tend to have pear-shaped bodies, with long legs, antennae, and tube-like structures called cornicles that protrude from the back of their torso. These cornicles distinguish aphids from other insects. Many species can develop wings when overcrowded, allowing them to spread to other plants.
Aphids are common garden pests that reproduce quickly and can require quick action to keep under control. As most growers will tell you, small populations of aphids aren’t usually a problem. In low numbers they actually provide an important food source to attract beneficial predators and parasitoids. However, it is important to prevent widespread infestation as significant plant damage can occur when populations get out of control.
Practice aphid prevention by encouraging and protecting natural predators such as ladybugs, brown and green lacewings, and hoverflies by providing a wide diversity of native plant species. You can find a range of plants that attract beneficial insects online, but some suggestions to fill-in your garden include dill, coriander, fennel, parsley, lemon balm, and marigolds. Aphids are also partial to plants fertilized with too much nitrogen, so it may be helpful to get a soil test to see your soil composition. If necessary, modify nutrient applications in favor of slow release fertilizers with a moderate portion of nitrogen such as organic compost, worm castings, fish emulsions, or liquid seaweed.
Also note that certain ant species will protect aphids from natural predators so that they can eat the honeydew aphids excrete after they feed on plants. In this mutualistic relationship where both animals benefit from the other’s actions, some ants actually “milk” aphids to coax out honeydew, and will even store aphid eggs in their nest over winter, essentially farming the pest. If you do see ants around your aphids, consider using sticky bands like Tanglefoot (available at garden centers) or other barriers to prevent them from guarding aphids.
Aphid problems generally peak in the spring and fall, when nitrogen levels are raised during initial plant growth and at the end of its life or before leaf drop. But it’s good practice to regularly check the undersides of leaves for the presence of aphids, honeydew, or other damage. If you notice a small population, take note of how long it is before natural predators move in, and determine based on damage whether to take action. Low levels of aphid populations will not necessarily damage gardens or trees. However, once plants exhibit wilting, stunting, yellowing or loss of leaves, it may be time to consider least-toxic controls.
Before planting vegetables, check surrounding areas for sources of aphids and remove these sources. Some aphids build up on weeds such as sowthistle and mustards, moving onto related crop seedlings after they emerge. On the other hand, these aphid-infested weeds can sometimes provide an early source of aphid natural enemies. Always check transplants for aphids and remove them before planting.
According to UC IPM:
Where aphid populations are localized on a few curled leaves or new shoots, the best control may be to prune out these areas and dispose of them. In large trees, some aphids thrive in the dense inner canopy; pruning out these areas can make the habitat less suitable.
High levels of nitrogen fertilizer favor aphid reproduction, so never use more nitrogen than necessary. Instead, use a less soluble form of nitrogen and apply it in small portions throughout the season rather than all at once. Slow-release fertilizers such as organic fertilizers or urea-based time-release formulations are best.
Because many vegetables are susceptible to serious aphid damage primarily during the seedling stage, reduce losses by growing seedlings under protective covers in the garden, in a greenhouse, or inside and then transplanting them when the seedlings are older and more tolerant of aphid feeding. Protective covers will also prevent transmission of aphid-borne viruses.
Silver-colored reflective mulches have been successfully used to reduce transmission of aphid-borne viruses in summer squash, melon, and other susceptible vegetables. These mulches repel invading aphid populations, reducing their numbers on seedlings and small plants. Another benefit is that yields of vegetables grown on reflective mulches are usually increased by the greater amount of solar energy reflecting onto leaves.
Another way to reduce aphid populations on sturdy plants is to knock off the insects with a strong spray of water. Most dislodged aphids won't be able to return to the plant, and their honeydew will be washed off as well. Using water sprays early in the day allows plants to dry off rapidly in the sun and be less susceptible to fungal diseases.
According to UC IPM:
Natural enemies can be very important for controlling aphids, especially in gardens not sprayed with broad-spectrum pesticides (e.g., organophosphates, carbamates, and pyrethroids) that kill natural enemy species as well as pests. Usually natural enemy populations don't appear in significant numbers until aphids begin to be numerous.
Among the most important natural enemies are various species of parasitic wasps that lay their eggs inside aphids. The skin of the parasitized aphid turns crusty and golden brown, a form called a mummy. The generation time of most parasites is quite short when the weather is warm, so once you begin to see mummies on your plants, the aphid population is likely to be reduced substantially within a week or two.
Many predators also feed on aphids. The most well-known are lady beetle adults and larvae, lacewing larvae, soldier beetles, and syrphid fly larvae. Naturally occurring predators work best, especially in garden and landscape situations.
Lady Beetle Releases
Applying commercially available lady beetles (the convergent lady beetle, Hippodamia convergens) may give some temporary control when properly handled, although most of them will disperse from your yard within a few days.
If releasing lady beetles, keep them refrigerated until just before letting them go, doing so at dusk, as those released in broad daylight will fly away immediately. Mist the lady beetles with water just before release, and also mist the surface of the plant you are releasing them onto. Place the lady beetles at the base of infested plants or in the crotches of low branches. Lady beetles will crawl higher into the plant in search of aphids. University of California research indicates that high numbers of lady beetles are required to control aphids. One large, heavily infested rose bush required two applications, spaced a week apart, of about 1,500 lady beetles each.
Aphids are very susceptible to fungal diseases when it is humid. These pathogens can kill entire colonies of aphids when conditions are right. Look for dead aphids that have turned reddish or brown; they'll have a fuzzy, shriveled texture unlike the shiny, bloated, tan-colored mummies that form when aphids are parasitized.
Weather can also impact aphids. Summer heat in the Central Valley and desert areas reduces the populations of many species, and aphid activity is also limited during the coldest part of the year. However, some aphids may be active year-round, especially in the milder, central coastal areas of California.
If you are thinking about applying insecticides to control your aphid infestation, remember that larger plants can handle light to moderate levels of aphids with almost no damage. In fact, even larger populations of aphids often decline quickly in size due to biological controls or hot temperatures. Usually a strong spray of water or a soapy water solution, even on very large plants or trees, can provide the necessary control.
If it is determined that insecticides are needed, you can turn to insecticidal soaps or oils, such as horticultural oils, or, plant derived oils like neem and canola oils. These products only kill aphids that are present immediately, so applications may need to be repeated. Unfortunately, they will kill some natural enemies if they are also living on the plant and are coated in the spray, but they do not leave any toxic residue that will harm new beneficial predators that migrate to the plant after the application.
Look at your product labels and try to avoid products containing those chemicals listed below:
(A = acute health effects, C = chronic health effects, SW = surface water contaminant, GW = ground water contaminant, W = wildlife poison, B = bee poison, LT = long-range transport)