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An Abundance of Aphids

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This most common of garden insect pests can be controlled without using pesticides.

By Jessica Dawe Posted May 26, 2016

Aphids

Aphids are the most common insect pests in the garden, and they are certainly the pest I am asked most about. No doubt you have encountered them gathered on the flower buds of your roses or on the leaves of your cabbages.

Aphids arrive in a variety of sizes and colors, they attack new leaf growth, can produce galls in plants and even feed on the roots of crops. Known by a variety of names, aphids may be called plant lice, green flies, black flies or whiteflies and belong to the order Homeoptera or the order of True Bugs. Though there are thousands of species and hosts, a gardener will likely only encounter some of them. The different species are similar in shape, the body resembling a teardrop with the head at the pointed end and 2 cornicles or “tail pipes” on their posterior. This is the key identifying characteristic that almost all aphids have. Determining the size, color and host plant they are on will help determine what species you have and how best to control it.

Aphid Lifecycle

The lifecycle of the aphid is what I find most fascinating as it can be either simple or complex and is specific to each species. For the majority of species, eggs hatch in the spring and a female aphid emerges. The young female is called a nymph and will molt (shed its skin) 4 times before reaching maturity. At this time, depending on species, the female aphid will asexually reproduce and begin giving birth to live young. The larger Lupin aphid, Macrosiphum albifrons, gives the gardener an excellent opportunity to witness birthing aphids in action. This process of reproduction is called ‘parthenogenesis’, and in aphids it is influenced by season, host plant and environment. In either case the offspring are all identical replicas of their mother.

Lupin aphid
Aphids owe their success to this science fiction-like ability as it allows them to produce offspring in very rapid succession. In parthenogenic species, a female aphid contains a daughter within her who also carries a daughter cell within her too. These are ‘telescoping generations’ resulting in a single female to be responsible for thousands of offspring in a season.

In times of food scarcity or over population, females will develop wings to aid in the migration to a new host plant, often an altogether different species of plant. For example the Green Peach aphid feeds initially on the fresh growth of peach trees until females give rise to winged offspring who will then move to more herbaceous crops like asparagus, peppers or spinach. As the season turns to fall, females will begin to produce males that are also identical but lack the Y chromosome. These aphids will reproduce sexually and lay eggs to overwinter on host plants, and the cycle continues.

Female aphid with wings

Aphid Species

The vast number of aphid species and the wide variety of crops they can attack can be intimidating for the home gardener but don’t let it scare you. By educating yourself on which species is an actual threat and what to look for, you can avoid the potential danger of losing a crop or favorite perennial.

Apple aphid (Aphis pomi)

  • Vibrant green/yellow color with dark cornicles and legs.
  • Overwinters as black egg on apple trees then feeds on new growth in early spring.
  • Causes leaf curl and distortion. It can stunt young trees but rarely inflicts severe damage.
  • Completes its lifecycle on single host species.
  • Check the new growth on your apple trees regularly in the spring. Often these aphids are kept in check by natural predators. Avoid excessive nitrogen applications.

Poplar Petiole Gall aphid or Cabbage Root aphid (Pemphigus populitransversus)

  • Pale green or blue green as adults. Nymphs are grey with powdery waxing coating.
  • Found in galls formed on the leaf petioles of Poplar trees in early spring and then on the roots of Brassica crops in the fall.
  • The female inserts eggs in the leaf petiole of the tree that distorts and limits photosynthesis but does not kill the tree. In Brassicas, they do not form galls but colonize the roots. Turnips being the most susceptible, the aphids are easily washed off at harvest.

Cabbage aphid

Cabbage aphid (Brevicoryne brassicae)

  • Small, grey green with a powdery appearance.
  • Found on the underside of cabbage, cauliflower, broccoli, Brussels sprouts and radish.
  • Overwinters as egg in plant debris near soil surface in temperate climates.
  • Arrives in northern climates in late spring but will exist year round in the warmer climes of Florida.
  • Feeding distorts leaves giving a wrinkled, pale appearance. High populations can severely stunt plants.
  • Monitor plants after transplanting and squish early arrivals. Cabbage aphids are highly susceptible to natural predators. Using companion plants draws in beneficial insects and is often enough to manage this pest.

Corn Root aphid (Anuraphis maidi radicis)

  • Blue-green with black head and black or brown eyes.
  • Attacks the roots of the corn plant causing stunting and wilt.
  • Overwintered with the support of ants that bring the aphid eggs into their nests in the winter and return them to corn crops in the spring.
  • They attack other grasses and weeds such as knotweed, mustard, pigweed, plantain, purslane and ragweed.
  • Remove and burn any infested plants and manage weeds around host crops.

Green Peach aphid or Spinach aphid (Myzus persicae)

  • Pale yellow green with 3 dark lines across its back.
  • The most common of aphids found on asparagus, bean, beets, broccoli, Brussels sprouts, cabbage, carrot, cauliflower, cantaloupe, celery, corn, cucumber, fennel, kale, kohlrabi, turnip, eggplant, lettuce, iris, mustard, okra, parsley, parsnip, pea, pepper, potato, radish, spinach, squash, tomato, turnip, watercress, and watermelon, to list a few!
  • Feeding causes leaf distortion and is responsible for the transmission of many mosaic diseases.
  • Monitor crops, encourage natural predators through companion plants and wash aphids off if detected early.

Potato aphid (Macrosiphum euphorbiae)

  • Begins the season as black eggs on roses that hatch into pink and grey juveniles that feed on rosebuds and leaves.
  • In early spring the aphids migrate to potato crops as the summer host.
  • Transmit tomato and potato mosaic virus.
  • Monitor crops, encourage natural predators through companion plants and wash aphids off if detected early.

Natural Aphid Predators

Ladybug
Natural predators are what keep most aphid populations in check. If there is enough plant diversity surrounding the garden it is likely that aphid predators such as aphidoletes, ladybugs and syrphid flies will already be in the area. These ‘beneficials’ are always on the hunt for new sources of prey and will quickly move into an aphid colony. As you monitor your plants throughout the season ensure that you are also looking for the good guys. If the predators are also present within the infestation, keep an eye on them and let them do their job. Most probably you will observe that the good guys overcome the aphids and the plants recover.

Companion Plants

Plant diversity is what sustains natural predators when the main food source of aphids is not available. Pollen and nectar found in many native plants serve as an alternative diet and why you are less likely to encounter aphid problems in areas closer to natural landscapes. If your garden is exposed with limited variety it would be advantageous to plant pollen and nectar rich varieties to provide year round habitat. Creating a garden insectary is an ideal way you can attract natural predators to specific areas of your garden.

Crop Management

It is often observed that aphid colonies will establish themselves on the weakest plants first. Aphids are most likely to become a serious problem in areas of poor plant health or limited plant diversity. These situations are preventable, thus prevention is the first step in any pest management strategy. Those plants that are lacking enough moisture, underfed or struggling for other reasons are an easy target. Ensuring you are planting in healthy soil and not using excessive nitrogen, which promotes aphid development, it is much easier to avoid aphid populations all together.

Monitoring is the next step. As the season moves to spring, check plants you know are susceptible to an aphid attack. Observe the growing tips and shoots of plants, the buds of roses and the underside of fresh leaves for colonies beginning to start. Install sticky cards to catch those first flying aphids of the season.

Should you find an infestation on a few plants in a large crop of cabbages for example, it can be useful. The attacked plants are the indicators that aphids have arrived and they can serve to attract the native beneficials through the release of chemical volatiles. These are the scents or signals that native predators use as cues to find their food source.

Should predators not be moving in naturally, it is possible to purchase them at your local garden center for release into the garden. The Aphidoletes midge and Aphidius wasp are the most excellent of biological control choices. If you are unable to source them locally, there are a few companies who produce them that will ship direct.

Only as a last resort you should reach for pesticides as this will kill any of the beneficial insects that co-habitat the area. Starting with nontoxic insecticidal soaps, spray the underside of leaves directly into infestations. Repeat this process according to directions on the bottle.

Though aphids are a serious pest of crops their presence and damage can be minimized with keen observation, employing preventive measures and encouraging native beneficial insects. Under these practices it is completely possible to avoid a serious aphid infestation altogether.

Related:
Garden ‘Mini’ Insectary

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Jessica Dawe owns a garden center and has been practicing integrated pest management and permaculture since graduating in 1995 with a degree in horticulture.
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Posted in Organic Garden Tags , , , , ,
  • CaptTurbo

    They are hugging the trees to tightly .. as usual here. When aphids find your plants, hammer them with Safer’s soap. Safe and effective aphid control.

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