2020 Winter - IPM Update

Biological Control Suggested for Asclepias

As the demand for native plants continues to increase, and cut flower growers incorporate natives like Asclepias species into their crop lists, we thought this story would be interesting to ASCFG members.

We recently worked with a perennial nursery that produces several species and cultivars of Asclepias, including A. tuberosa (butterfly milkweed), A.(swamp milkweed), and A. curassavica (Mexican milkweed). It was having trouble with common summer and late summer pest on its asclepias plants

One of the most common of these is oleander aphid, Aphis nerii. It is bright yellow with black legs and black cornicles (small projections near the rear end). I find it rather attractive but the nursery’s customers did not want this pest on their plants, and it was impeding sales. In low numbers you can easily rub these off with your fingers—if you don’t mind squishing bugs barehanded.

However, squishing by hand is not too practical if you have a lot of plants. The nursery did not want to use pesticides since many of their customers grow Asclepias to help feed and increase the population of monarch butterflies, which feed on milkweed foliage. Aphids can weaken plants as they feed, possibly decreasing the nutritional value of the leaves for the monarch caterpillars. They were interested in finding an effective biological control, and wanted something that would quickly take out an aphid population just before the plants were shipped out.

I knew I had to select a psychotic killer insect for this job and I knew just the species to use. Working closely with this operation, we decided that green lacewings were probably one of the more cost-effective methods of controlling aphids, so we purchased both eggs and larvae. The result was dramatic and very fast. The lacewing larvae are voracious killers of aphids. They cleaned up plants in less than a week. Stems were coated with oleander aphids when we started this adventure, and within a week we could not find living oleander aphids.

Another method is to use the parasitic midge called Aphidoletes. The aphid midge, Aphidoletes aphidimyza, is a cecidomyiid fly whose larvae are effective predators of aphids. The adult midge lays eggs near aphid colonies. The larvae, which look like maggots, are tiny, growing through three instars from minute to 2-3 mm. Depending on their food source, they are bright orange to red, and their bodies narrow toward the head. The slug-like larvae grab aphids by the leg and insert their mouthparts in the body, putting in a fluid that paralyzes the aphid so it cannot escape. They then suck the aphid dry. A. aphidimyza attacks over 60 species of aphids. Each female may live for one to two weeks and deposit, singly or in clusters, about 70 upright, orange eggs on leaves among aphids. The eggs hatch in two to four days.

The other interesting item we received in December was a Phlox paniculata with a soft scale insect coating the plant. We identified this as brown soft scale, Coccus hesperidium. Adult female brown soft scales are pale yellowish green to yellowish brown, often mottled with brown spots. Older females are brown. The body is usually oval in outline, 2.5 to 4 millimeters in length, and slightly convex in profile. The shape can vary slightly according to position on the host plant.

This scale feeds on a wide range of plant material but is really a tropical insect. It should not overwinter outdoors in northern climates. If you are in the southern part of North America it will overwinter outdoors. In northern climates, many nurseries and some cut flower growers will overwinter plants in protective structures covered with poly, where this pest may be able to survive.

The scale is predominantly female with very few males. Females will mate but they can also reproduce without mating, so a population can really take off when the weather warms. 

Exposing the foliage to temperatures in the twenties or below should kill the scale. Pruning and disposing of all infested foliage before it starts producing crawlers that will rapidly spread to other plants may cut down on populations. If you find you have an infestation in the spring or summer on a large number of plants I would suggest trying the new systemic insecticide from Bayer called Altus. In trials I conducted in the summer of 2019 it was very effective in controlling this scale.

Stanton Gill

Extension Specialist

Stanton Gill is an Extension Specialist in IPM and entomology,
University of Maryland, CMREC, and Professor
with Montgomery College, Germantown.Contact her at [email protected]