Aphids are common plant pests

By Melanie Mathieson
Gardening Guru

Aphids are common pests to house plants, vegetables, annual and perennial flowers, ornamental trees and shrubs. They can often be found on cabbage, peas, potato, tomato, beans, annual flowers (e.g., snapdragons) perennials (e.g., lupines) flowering shrubs (e.g., roses, Viburnum, snowball bush) and trees including apple, pear, plum, crab apple, birch, elm, ash, maple, oak and pine.
Aphids are small, less than 1/10 of an inch, pear-shaped insects with long antennae extending from the head. Most aphids have soft bodies, which may be green, black, brown, pink or almost colourless. Aphids have antennae with as many as six segments. Aphids feed themselves through sucking mouthparts called stylets, enclosed in a sheath called a rostrum, which is formed from modifications of the mandible and maxilla of the insect mouthparts. They have long, thin legs and two-jointed, two-clawed tarsi. Most aphids are wingless. However, when the temperature is not suitable or over-crowding takes place, some species can form wings and move to new areas.
The aphid life cycle has three stages: egg, nymph and adult. Eggs overwinter on the host plant and a generation of females hatch in the spring. Within ten days of being born, the female aphid can give birth asexually (without mating) to a second generation of females. This process can continue throughout the summer allowing aphid populations to reach high numbers very quickly. In the fall, winged males are produced to mate with remaining females. The eggs which are produced from this union overwinter. The following spring, the cycle continues.
Many aphid species are monophagous which means they feed on only one plant species. Although there are some species that feed on hundreds of plant species.
Aphids passively feed on the sap of phloem vessels in plants. In vascular plants, the phloem is the living tissue that carries organic nutrients to all parts of the plant. In woody plants, the phloem is the innermost layer of the bark. In non-woody plants the phloem is the first inner layers of the stem. Once a phloem vessel is punctured, the sap, which is under high pressure, is forced into the aphid’s food canal.
During feeding, many aphids secrete a clear, sticky, sweet substance, called honeydew. It is often found on or below the host plant and may act as an attractant for other insect pests, such as ants, who use it as a food source. Large numbers of ants, marching up and down the trunks of trees are usually a good indication that aphids are present. Honeydew also acts as a host for a harmless black fungus called sooty mould which often grows in the secretion. Sooty mould is a secondary symptom of an aphid infection. In addition to sooty mould, aphids often transmit plant viruses to the plants they are feeding on. These viruses can sometimes kill the plants.
Healthy plants can tolerate a small number of aphids, however, damage results when large numbers are present. When a plant is attacked by a large population of aphids, damage such as reduced growth, wilted leaves, and stunted needles occurs. Many species of aphids prefer the soft plant growth that results when plants are over fertilized with nitrogen. In addition, some aphids prefer plants that are drought stressed. Ensuring that garden plants have adequate but not excessive fertilizer and water may help reduce aphid problems.
Look for aphids on susceptible plants in the late spring. If they are present they can often be found clustered around leaf or flower buds, or on the under side of leaves. Puckering, cupping or yellowing of leaves can also be a sign that aphids are present. Catch aphid infestations early as one aphid can produce up to 80 offspring in as little as a week and is capable of doing this several times over.  Slowly walk through your garden several times a week paying close attention to the underside of leaves.  Also check for aphids at the nursery before you purchase plants.
There are several natural enemies which feed on aphids like lady beetle larvae, lacewings, assassin bugs, syrphid fly larvae, various wasps and spiders. Lady beetle larvae consume the largest number of aphids but parasitic wasps are also very effective in aphid control. As these naturally bugs are only effective when they are present in the garden, avoiding the use of pesticides in the garden will help conserve the populations of these natural enemies.
Sometimes there are not enough of the aphid’s natural enemies around to keep the infestation under control. If this is the case or you have an indoor plant infested with aphids, there are several physical control methods that can be used to control aphids.
One such method is to spray a steady stream of water at the host plant to knock aphids off. For outside plants the hose will work, but inside houseplants will need a shower in the sink or tub. This treatment may have to be repeated several times. I always use this method as the first step and then catch the aphids missed by the shower with any of the following methods.
Rubbing or hand picking aphids from affected plants will also reduce populations. If only a few small colonies of aphids are present, prune affected areas of trees and shrubs to remove aphids and overwintering aphid eggs and dispose of in a tightly sealed plastic bag or container. In the vegetable garden, place floating row covers over the plants when first planted, to act as a barrier to many pests including aphids.
Aphids are attracted to the colour yellow. Try controlling aphids by using yellow sticky traps which can be bought or made. To make a sticky trap, spread petroleum jelly over a yellow index card and place in the area where aphids have been observed.
Pyrethrum insecticides or insecticidal soap can be sprayed on infested areas of plants to control aphid populations. Pesticides only work on contact so it is important to apply it directly to the aphids. As some sensitive plants may be harmed by pesticides, always carefully follow the label directions when using it.
You can also use my homemade insecticide, using dish soap, preferably Sunlight, mixed to a ratio of 1 tablespoon per gallon of water. This can be put in a spray bottle and applied directly to the aphids. The soap washes off the aphid’s protective waxy coating and causes dehydration. Another solution is to mix three parts lukewarm water to one part vegetable or horticultural oil and a couple drops of dish soap. When this mixture is sprayed on the aphids, it clogs their breathing holes. Either remedy should be applied once a week. If using these solutions on food producing plants, be sure to wash the produce well before eating. If using the oil solution, don’t spray it on the plants on very hot and sunny days as the oil can magnify the sun and possibly harm the plant.
With some vigilance and the knowledge contained in this column you are now ready to battle aphids. It is amazing how these wingless insects can show up out of nowhere and start inflicting damage. A careful eye and these tips can help you stay in control.