In Britain, aphids are called plant lice, and for good reason. Considered one of the most destructive pests in the garden, aphids suck valuable nutrients from your plants, leaving behind a “honeydew” (a sugary bug poop) that creates the perfect habitat for fungal disease. And they don’t work alone!
Ants actively protect and “farm” aphids in exchange for honeydew, but there is so much more to this complex relationship. Prepare to be amazed!
One type of butterfly (Niphanda fusca) lays its eggs on plants where ants are tending aphids. When the caterpillar hatches and starts eating the aphids, it emits a pheromone that fools the ants into thinking it is one of their own. The ants take the caterpillar into the nest, where they feed and tend it. In return for their efforts, the caterpillar produces honeydew, which the ants eat. When the caterpillar is grown, it walks to the colony entrance and builds a cocoon. When it emerges as a butterfly, the ants would attack it, but it produces a sticky wooly substance on its wings that temporarily paralyzes the ants’ jaws! Aphids also have arrangement with certain bacteria, as well, but I digress…
Besides sucking the life out of garden and landscape plants, aphids are vectors of disease. As they walk from one place to another, they can carry fungal diseases such as rust and Fusarium wilt.
If you’re not sure what an aphid looks like, go outside and take a close look at plants in the garden and on citrus trees. At this time of year (February), you often see clusters of grey bumps or tiny green crawlers on the underside of leaves and where new growth emerges. These are signs of an aphid infestation.
Aphids are tiny. Most varieties in the Bay area are less than 1/8” long. They have soft, pear-shaped bodies and can be brown, grey, black, green, pink, or nearly colorless. They have segmented antennae and long, thin legs. They feed through sucking mouthparts called styles that emerge from a sheath called a rostrum. Aphids are wingless until conditions change for the worse. Somehow, somewhere in their genetics, the need for wings is communicated and voilà! Wings emerge!
There are over 4,000 species of aphid, worldwide, and 250 that pester U.S. gardens. In our area, there are 17 species of aphid that regular cause trouble, including the European Asparagus aphid, the rose aphid (Macrosiphum rosae), and the apple aphid (Dysaphis plantaginea). The UC Davis Integrated Pest Management (IPM) page on aphids has some amazing photos.
Aphids communicate using pheromones. Pheromones are chemicals that cause others to react. (Perfume and cologne try to replicate the effect of human pheromones.) When aphids are attacked, they release pheromones that tell other aphids to run for it. Lady beetles (ladybugs) have learned to follow those pheromones to find the aphids’ hiding place! Even more amazing, scientists have learned that plants release their own chemical communications when they are attacked, calling in reinforcements of lady beetles and other beneficials! Who knew?!!?
One reason why aphids are so successful is that they can reproduce asexually (parthenogenesis). This means they do not need to mate to reproduce! A single female aphid can give birth to 12 live offspring every day. These offspring are called nymphs. Some aphids lay eggs that overwinter, but you’ll probably never see them.
Aphids grow to adulthood in a week and each adult aphid can produce 80 nymphs a week. According to entomologist Stephen A. Marshall, in his book Insects: Their Natural History and Diversity: With a Photographic Guide to Insects of Eastern North America, a single aphid could produce 600 BILLION descendants in a single season. Try wrapping your brain around those numbers!
There are many beneficial insects that love to chow down on aphids. These natural enemies include:
Applying sticky barriers to plant stems and tree trunks can help eliminate ant support. You can also use Kaolin clay or Diatomaceous earth (DE) to prevent infestation.
Being pretty fragile, aphids can be wiped off plants or squished, but that gets messy. Insecticidal soap can be sprayed on heavily infested plants, but this will only affect the bugs that it contacts directly. The easiest way to reduce aphid populations on specific plants is to simply hit them with water from the hose and to encourage beneficial insects in the garden.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.