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Aphids gathering on plants can deform new leaves and blooms. They seldom kill plants but they can do a lot of damage to young, small transplants and new foliage. As they suck the juices, the leaves crumple and twist.
These pests collect on the growing tips of plants because they move where nutrient levels are highest, especially when plants begin active growth in spring. Spring sap contains many sugars, making a fine aphid meal. This also results in aphids producing "honeydew," a sticky substance that coats leaves and drips from bushes. This honeydew aggravates drivers by splotching cars parked under birch or maple trees in spring. With a heavy aphid infestation, the results can seem like a rain of sugar syrup.
Some bees and other creatures utilize this "honeydew" as food, giving it some benefit. But the stickiness on vehicles and sidewalks can be a problem even when the aphids aren't harming the trees.
And aphids multiply fast. Thanks to a remarkable biological quirk-- or trick -- some aphid species bear live young without mating. The females survive during winter as eggs, hatching in spring. The first hatched females aare called "stem mothers." They bear live young, and the young are all female, and already pregnant with the next generation. This frighteningly prolific multiplication continues until cold weather when one generation hatches male and mating occurs. No wonder it's so hard to stay ahead of growing aphid populations!
So what's a gardener to do? First, realize that many beneficial insects eat aphids. The well-known ladybug (lady beetle), wasps, lacewings, and syrphid flies, and gall midges all relish aphids. A tiny parasitic wasp also reduces populations by laying eggs in the aphids. If you see inert aphids resembling brownish "mummies" looking as if they have been stuffed with brown paper, they harbor beneficial wasps and should be left alone to hatch.
Predators of aphids generally don't arrive in the garden until the populations have increased --why would they come for dinner if no dinner is available? Tolerating some aphids in the garden will help build up the food stocks for the control organisms. Birds also appreciate aphids as food.
While waiting to see aphid populations decrease from predator activity, gardeners can take some simple actions. Put on a glove and mash the aphids between two fingers. The "squish" method works well for small populations of aphids on growing tips of shrubs like roses. Or prune out and destroy severely affected shoots, dropping the trimmings into soapy water. (You can drain the bucket and put the drowned aphids in a compost pile.) Washing the aphids off with a gentle spray of water also works but needs to be done frequently as the aphids multiply.
Watch for both predatory lady beetles and their larvae, the alligator-shaped orange and black stage. Lady beetle larvae eat large quantities of aphids while bulking up for pupating. The surest way to have lady beetles stay in your own landscape is to have them born there, from a resident population of adults. Bags of lady bugs bought will seldom remain where you want them --they respond to release by flying, exactly as they would at the end of a hibernation stage. So look for resident lady bugs in preference to purchasing them.
If you choose to use chemical controls for aphids, choose the least-toxic possible. Insecticidal soaps have low toxicity but should be used only when you are not seeing predators like lady beetles and their larvae. Be sure, if using insecticidal soap, that you avoid applying it when hot sun is striking the plant, and that you get it on both sides of leaves where aphids lurk. The insecticidal soap needs to hit aphids to work. Another chemical control, neem oil (sold as Rose Defense and in other brand formulations) will slow down aphids but will also harm beneficial lady beetle larvae, so do not use it if you spot those. Follow label directions exactly when using any pesticide.
Some gardeners choose to replace plants that are severely and regularly attacked by aphids (such as birch trees, or herbaceous perennial lupine). In fact, gardens will never be entirely aphid-free, which is fortunate, because the food source is necessary to encourage beneficial insects and birds.
Do not spray the garden with broad-spectrum insecticides such as diazinon, or malathion, that kill all types of insects including the beneficials. Regular use of insecticides damages the natural cycle that llows predators to build up to munch the aphids. Also, insecticides including diazinon have been measured as pollutants in surface water in western Washington. They can harm valuable aquatic insects and fish populations.
Hortsense: Managing plant problems with Integrated Pest Management
