The time for reveling in nature amid our delightful spring-blooming landscapes and gardens is upon us.  We’re formulating watchful strategies for the growing season designed to encourage pollinators, avoid transmission of unwanted plant fungi and viruses, discourage insect “pests” and so on. Although the word “pest” may be eschewed by most — insects represent a vital member of the ecological team in our gardens — there are a handful of insects capable of sabotaging some of our gardening pursuits. Consider the aphid. 

There are about 5,000 species of aphids worldwide and approximately 1,300 hundred in North America.  Aphids are invertebrate animals of the class Insecta, Order Hemiptera, Suborder Sternorrhyncha, and Family Aphididae. Often the species name of an aphid includes reference to the plant it primarily attacks (e.g., Eriosoma lanigerum, the woolly apple aphid).

Most aphids are 1.5 mm long with soft, pear-shaped bodies. All have the classic insect anatomy consisting of three body segments — head, thorax, and abdomen — with three sets of legs and a set of antennae attached to the head. The final segment is the cauda, which extends beyond the underlying anus and can be used to cast off predators.  Another unique predator defense is the small backward pointing tubes called cornicles. Cornicles exude a waxy fluid that researchers suspect emits an alarm pheromone that warns nearby aphids of danger. 

Aphid birthing process

An astonishing feature of the aphid is its ability to give birth in the spring to female aphids that reproduce without the aid of a male or mating. This is called parthenogenic birth.  In fact, over its 30-day lifespan, an adult female aphid can give birth to an adult female that may already be pregnant at the time of birth, which allows a tremendous number of aphids to build up very quickly on host plants. Thus Grandma aphid is the instigator of prolific transgenerational cloning.

But that which amazes can also cause gardeners to agonize. Low to moderate numbers of leaf-feeding aphids are usually not damaging. But in population booms, aphids can cause damage to plants, crops, and trees. They use stylets in their pointed mouthparts to pierce the phloem in leaves, buds, stems, and roots. Stylets excrete a special liquid that hardens the point of entry to allow the stylet to enter undisturbed. 

Plant phloem contains lots of sugars, which the aphid can filter and excrete as honeydew. The honeydew excreted is attractive to ants, which have been known to actually milk the aphid to enhance excretion. In exchange for this tasty food the ant protects the aphid from natural predators such as the ladybug. A drawback of the otherwise desirable honeydew, however, is it can coat the leaves of plants, causing the growth of sooty mold fungi. The fungus interferes with the plant’s photosynthesis, slowing and stunting plant growth.

Large aphid populations can also cause curling, yellowing, and leaf deformation. Some aphid species inject a toxin into plants, which further distorts growth. A few species cause gall formation, which stunts the growth of young plants making them more susceptible to disease and leading to the gradual death of the plant. Attacks can occur on the roots of trees. The woolly apple aphid, for example, infests woody parts of the apple root or settles on pruning wounds causing generalized tree decline after continuous infestation.

What is a gardener to do?

Follow the principles of Integrated Pest Management (IPM). IPM uses a variety of techniques to manage pests: first monitoring, followed by biological, cultural or chemical controls. Pesticides, either organic or inorganic, are considered only if other control methods are not effective.  

Monitoring: Check your plants frequently for aphids during the growing season.  If detected, simply remove with a brisk leaf shaking or strong stream of water. 

Biological control: Encourage natural predators by creating a diverse landscape and by minimizing pesticide use. 

Cultural control: Prune leaves or branches with aphid infestations. Be mindful of applying high nitrogen fertilizer, which encourages aphid reproduction.

Chemical control: Washington state has strict regulations on pesticide use (see state Department of Natural Resources website). 

To learn more about IPM recommendations for garden pests, including which pesticides are approved for use in Washington, go to the Washington State University Extension Hortsense website. You can also attend “Ask a Master Gardener” clinics at the Port Townsend Library on the first Tuesday of each month from noon to 2 p.m. in the Charles Pink House or at the Jefferson County Library on the second Saturday of the month from 1-3 p.m.   

Carolyn Smith, a volunteer for the Master Gardener Program since 2024 and a resident of Port Ludlow, serves as a community educator in gardening and environmental stewardship.