Milkweeds and Their Foes!

Q. Christie in Montgomery County PA (near Philadelphia) writes: "I've converted my garden to a chemical free, pollinator & bird friendly area. I now have mostly native plants surrounding my back patio and pathway and have seen an uptick of birds and insects alike.

"However, I'm concerned about my Swamp Milkweed. It grew beautifully this summer, and I was excited to find three monarch caterpillars on it. Then I saw that it was covered with yellow aphids. I also found ladybugs and milkweed bugs. Now I'm seeing tiny reddish beetle-like bugs scurrying around.

"I didn't remove any of these critters out of fear that I'd be upsetting the natural environment, but I'm also wondering if I somehow attracted the wrong type of insects and that they may invade the rest of my garden. Any suggestions or wisdom?"

A. Suggestions I got bags of, but I think I left wisdom in my other cape. At any rate, yours is a perfect question as it appears your plant was invaded by the two most important milkweed pests. Lucky you!

But before we get to those pests, let's briefly discuss the best milkweeds to grow as host plants for monarch caterpillars.

Swamp milkweed Asclepias incarnata, is a North American native that prefers growing in wet soil, making it a perfect plant for soggy spots. The website Monarch Health notes that the sucrose content of the flowers is about 30 percent, which sounds good for pollinators, but stay tuned for the heavyweight champ.

Swamp milkweed is a perennial, spreading via underground rhizomes (as opposed to those extremely rare above-ground rhizomes. Ahem.) As the website notes, this means that a big patch could actually be a single plant with a giant root system. Flower colors range from pink to a light purple with mild fragrance.

Another native, Common Milkweed (Asclepias syriaca) prefers well-drained soils and also spreads via underground rhizomes; again, meaning that a big patch could be a single plant. It is said to be strongly fragrant (the better to attract monarchs flying back from their winter vacation) and its sucrose content is rated at an astonishing 100 percent, making it the most nutritious of the 100 or so native varieties of the species. Flower color is pink to white.

Despite the common name, Butterfly weed (tuberosa) is another native milkweed. It produces showy orange flowers that attract lots of pollinators and butterflies, while its leaves feed the caterpillar form of the monarch. It completes the soil Trifecta by preferring to grow in dry areas. A milkweed for every garden!

Important Note: Because it spreads via those underground rhizomes, milkweed can be aggressively invasive. If you want other plants to prosper, it would be a good idea to surround your milkweed patch with an underground rhizome barrier that will limit lateral growth.

Now, a warning about the non-native Tropical milkweed (curassavica), which the Xerces Society hopes you will not grow in a typical American or Canadian garden despite its showy flowers, as it can confuse and disorient monarchs. Leave it in Central America where it belongs.

Milkweed is toxic; that's why monarch caterpillars (immune to the poison) feed on it, absorbing the toxin to deter predators. Don't grow milkweed if your dog eats everything and never grow it near a farm field, where it can be extremely dangerous to livestock, especially when inadvertently harvested with hay at the end of the season.

Back to Christie (remember Christie?) and her abundant late-season visitors.

Quoting the (excellent) Missouri Botanical Garden: "The Large Milkweed Bug (Oncopeltus fasciatus), is orange-red and black. It has a long proboscis and is a piercing sucking insect. It feeds on the seeds, leaves and stems of milkweed", which many sources note may be a good thing as it slows down the spread of the plant by seed.

This pest is not generally considered to be a big problem, but if they bug you (they are true 'bugs'), you can spray them off the plant with sharp streams of water as soon as they appear. Then clean up underneath the plants after your first hard frost and trash the debris you collect to prevent them from overwintering in the area. Mulch afterwards with an inch of compost to show the plants you love them.

The little red bugs are probably the nymphal stage of the big bug. Spray these with professionally-made insecticidal soap to smother them and prevent their molting into the adult stage, which is how they overwinter.

The yellow aphids are a species known as 'the oleander aphid', a pest of many ornamentals. It's best to keep their numbers low early on, as they multiply rapidly, and a large enough invasion of these sap-suckers could weaken the plant and deter egg laying by adult monarchs. Again, SHARP streams of water will blast them off the plants. Just make sure you don't blast any monarch eggs or caterpillars at the same time. The ladybugs are there to devour the aphids.

Important Note: I found a lot of ridiculous and counter-productive oleander aphid information on the Internet. More than half of what I read was dead wrong, useless, worse than useless and/or could prove deadly to plants and caterpillars. All you need to dispatch aphids is a hose with a nozzle that has a laser-sharp setting and good water pressure.