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Jessica Walliser

Let the 'good bugs' control aphids on milkweed

Jessica Walliser
| Thursday, May 17, 2018, 4:30 p.m.
A ladybug larva prowls around a colony of oleander aphids on a swamp milkweed plant and looking for lunch.
Jessica Walliser
A ladybug larva prowls around a colony of oleander aphids on a swamp milkweed plant and looking for lunch.

Question: I would like to know if you can help me with a problem I have with my swamp milkweed. In late summer a lot of the plants become infested with bright orange aphids. The plants often become sickly looking. I tried spraying with dish detergent and also a horticultural oil product but the aphids came back. Do you know of any organic solutions which would solve this problem? I don't want to use anything that will harm the monarch caterpillars that are also on my milkweed plants.

Answer: As you know, milkweed plants are the sole larval host food for the monarch caterpillar. These insects are highly specialized feeders who have adapted to the toxic compounds (called cardenolides) found in about 30 different species of milkweed, including your swamp milkweed (Asclepias incarnata).

But, monarch butterfly caterpillars aren't the only insects who have co-evolved with milkweed and can tolerate the toxic compounds found within its sap. In fact, a small posse of native insects rely exclusively on milkweed as a food source. You may sometimes encounter milkweed longhorn beetles, milkweed leaf beetles, milkweed bugs, and milkweed tussock moth caterpillars feeding on your plants, too. All have evolved to rely on this particular plant for nutrition and none is more important than the other within the ecosystem, even though monarchs tend to get all the positive attention.

The bright orange aphids you find on your milkweed plants are another insect that's adapted to using milkweed as a food source, though not exclusively. The aphids found feeding on swamp milkweed are a species called the oleander aphid. In geographical ranges where this introduced aphid species does not have their preferred host plant available (oleander), the insect turns to other plant species for nutrition. Swamp milkweed is a favorite of the oleander aphid and the plants can sometimes become infested with these small, pear-shaped insects.

Aphids have a needle-like mouth part that pierces plant flesh and sucks out the sap. Heavy infestations can lead to distorted growth and weakened plants. Typically, aphid populations are kept in check via the broad diversity of beneficial insects who consume them or use them to house and feed their developing young, including ladybugs, lacewings, syrphid flies, parasitic wasps and many more.

In the case of oleander aphids on swamp milkweed, careful attention must be paid to the pros and cons of getting rid of the aphids. If you check your plants carefully when aphids are present, you will undoubtedly find a diversity of “good” insects on the plants, eating the aphids. In your case, the population of “good” bugs may never be allowed to get high enough to control the aphids because your misguided attempts to wipe out the aphids are also wiping out the beneficial insects that feed on them. You're throwing the natural balance of the garden off kilter. Instead, you should let the “good” bugs bring the aphids under control naturally.

My swamp milkweed plants are loaded with aphids every year and soon after the aphids arrive, I find ladybug and lacewing larvae patrolling the plants, along with plenty of aphids that have been parasitized by a group of tiny parasitic wasps called aphidius wasps. All of these good bugs use the oleander aphids as food and if you get rid of the aphids, yes, you're benefiting the one or two monarch caterpillars that might be using the plant as food, but you're wiping out dozens, if not hundreds, of other very good bugs who help you control many different pests in your garden. Even if you just squish the aphids with your fingers or spray them off the plant with a sharp stream of water from the hose, you are negatively affecting a tremendously important crew of beneficial insects who do mighty good things for your entire garden, not just your milkweed plants.

So, I'd encourage you to do what I do with my swamp milkweed. I let the aphids do their thing and watch closely for signs of beneficial insects. Never, ever spray milkweed plants with a pesticide, whether it's organic or not. Tiny monarch eggs and early instar caterpillars will die if sprayed with something even as seemingly mild as horticultural oil or dish soap.

The best thing you can do is let nature take care of itself in the garden. The natural balance of “good” and “bad” bugs has been keeping plants alive for a lot longer than we humans have. Yes, your milkweed plants may not look so hot for a week or two, but they'll bounce back the next season without any ill effects. If you're worried about the monarch caterpillars, hedge your bets by planting multiple species of milkweed in your garden, not just aphid-prone swamp milkweed. I recommend butterfly weed (A. tuberosa), showy milkweed (A. speciosa), whorled milkweed (A. verticillata), poke milkweed (A. exaltata), and even common milkweed (A. syriaca — but only if you have plenty of room to grow this active spreader. The more milkweed species you have available, the better.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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