A shining star of the midsummer garden, butterfly weed (Asclepias tuberosa) draws crowds of butterflies and hummingbirds with its gaudy clusters of undeniably orange or yellow blooms. Suitable for U.S. Department of Agriculture plant hardiness zones through 9, this 18- to 30-inch Milkweed (Asclepiadaceae) family perennial is a major food source for monarch butterflies and their caterpillars. Position it to highlight its brilliant flowers, and expect a pair of pests to join its parade of admiring insects.
Named for the mealy white wax camouflaging their flat bodies, mealybugs could easily pass for fungus growing on your flowering butterfly weeds’ stems and foliage. Littering the plants like wads of cotton wool, colonies of these pests feed on sap. In large numbers, they drain enough moisture and nutrients to rob butterfly weeds of their vigor. Mealybugs excrete whatever sap they don’t metabolize as sweet, sticky waste. Ants flocking to harvest this honeydew protect their supply by killing the parasitic wasps that prey on the mealybugs. The honeydew also feeds a variety of sooty mold spores. They cover the plants’ leaves in layers of sun-blocking, photosynthesis-reducing black fungal mats.
Long, narrow tissue-penetrating mouthparts and a pair of tiny tubes, or cornicles, protruding from their hind ends distinguish aphids from other insects. Like mealybugs, tiny, pear-shaped aphids consume butterfly-weed sap and excrete honeydew. Look for colonies of the green, yellow, red, black or brown pests clinging to stems or leaf backs. Some species give birth to up to a dozen live offspring in a single day. Others — in Mediterranean climates — lay eggs on alternative host plants for overwintering and hatching the following spring.
Eradicating the flood of ants streaming to your honeydew-laden butterfly weeds may give aphid and mealybug predators an opportunity to eliminate the pests. To make a low-toxicity ant bait, smear a small amount of apple jelly on a few butterfly weeds and an equal amount of peanut butter on a few more. Watch to see which leaves attract the most ants, and mix 1/2-cup of that food with 1 1/4-teaspoons of boric acid powder. Place the bait in tape-sealed jar with tape and puncture several holes in the lid. Conceal it near the butterfly weeds where kids or pets can’t reach it. You’ll see a noticeable decline in the ants collecting honeydew on your plants about a week after setting out the slowly acting bait.
While waiting for the aphids and mealybugs predators to return to your ant-free butterfly weeds, try dislodging the pests from the plants with a jet of water. Unlike chemical treatments, a water spray won’t leave residue that might harm any monarch butterfly caterpillars as they feed. The water also cleans the plants of honeydew. Without fresh honeydew to consume, sooty molds eventually weather away.