Common Milkweed Pests
AphidsAphids which are often found on milkweed plants are yellow, soft, oval, and huddle together on new shoots, stems, buds, and leaves. They damage the plant by sucking liquid from the plant, eventually stressing the plant and killing it when infestation is high.
Hose them off with water using your fingers or a paint brush if you wish. Repeat process when needed. When aphid population is high, use a spray bottle with 10% liquid dish soap mixed with water. Spray solution directly on the infested areas. Wait a few minutes and rinse the plants with fresh water because the soap will burn the leaves. If there are a lot of plants infested, use neem oil. This product clogs the air passages (spiracles) of the insect. There is no need to use insecticides for aphids unless there are hundreds or thousands of infested plants. It is important to watch for natural controlling predators; such as syrphid flies, ladybird beetles, parasitic wasps, and lacewings, to mention a few. If there are young monarch larvae on an aphid infested milkweed plant, use a small paint brush to move them to a clean plant and then wash the aphids off of the plant with water.
WhitefliesWhiteflies are annoying little winged insects that destroy plants when the nymphs suck plant juices and exude a sticky substance. Whiteflies are on the list of the top 10 pest in every part of the west. They are not considered a great pest for milkweed plants but when they are it’s a problem. Their sticky substance on the leaves hinders a monarch caterpillar’s digestive system and appetite. If this begins to happen, the larvae need to be moved to clean plants.
Hose off infested plants, hitting both sides of the leaves. Or use a solution of insecticidal soap that is less harmful to natural enemies. They are easily controlled if the plants are treated often.
Scale insects are closely related to mealybugs and aphids, however they are different because they have a shell-like covering that camouflages them and protects them from natural enemies. Adult scales suck plant juices through tiny filamentous mouth parts and eventually kill the plant. Scale eggs hatch beneath the shell and seek their feeding sites in the spring and summer.
Their waxy shell-like covering camouflages them and protects them from predators and insecticides. Scales on milkweed plants can easily be removed by picking or scraping them off. Insecticides or soap application are effective only during the juvenile “crawler” stage. Systemic insecticide will kill them but the plant cannot be used to feed monarch larvae for about one month.
Spider MitesOften red, sometimes yellow or green, spider mites are tiny arachnids (eight legs). They damage a leaf by feeding from its liquid. This results with yellow-stippled leaves. You will see fine webbing across the leaves, especially on the underside and around the stems. Spider mites thrive in dry dusty areas. They rank among the top ten garden pests throughout the west.
Wash the plants thoroughly with water to wash off the mites. Insecticidal soap helps if the infestation elevates. Dust that settles on the leaves encourages mites, so it is best to continually spray the plants with water.
ThripsThese are near microscopic pests that rasp the leaf tissue and drink the juices the plant secretes. Infestation causes leaves to twist, stick together, and discolor. Plants are more vulnerable in a greenhouse than outdoors. Milkweed plants with thrips are not a good diet for monarch larvae because of nutrients taken out of the leaves.
Thrips are very difficult to control. Spraying the plant with water helps and neem oil or insecticidal soap works to some degree. Using a systemic insecticide spray is best if the plant is not used as food for one month.
Most leaf miners that use milkweed are very small wasp larvae. They feed between the two layers of a leaf material making a swirling design. Their consumption of the leaf parts eventually kills the leaf. They are extremely difficult to control when large numbers of milkweed plants are cultivated.
The only cure for leaf miners is to continually remove infected leaves or use a systemic insecticide when the infestation is high.
Snails and Slugs
Snails and slugs are great fans of milkweed plants, especially young tender plants. They are rated as the overall worst garden pests. They hide by day and feed at night. It can be devastating to check on your milkweed plants in the morning and they are only sticks. These pests are difficult to get rid of because they come from all surrounding areas.
There are really only two choices. Go outside after 10:00 PM and smash the critters, or use snail bait. The bait will not harm the monarch larvae if they are on the plants. When the plants get large there is not much of a problem except for very small snails.
Common Milkweed Diseases
Leaf Spot (fungus)
Leaf spots on milkweed plants are usually red, brown, or black. The spots often enlarge and coalesce infecting the entire leaf which drops. Severe infections can defoliate the plant. The fungus spores that cause leaf spot are airborne or waterborne. The disease is far less serious in dry climates with low rainfall.
Growing milkweed in an area with low humidity can help. Trim infected leaves and twigs to eliminate sources of future infection. Dispose of carefully since the spores are airborne. There are many insecticides available that help control the problem.
Excess water can suffocate plant roots if there are not enough spaces in the soil. Most plants need well drained soil and good aeration. Overwatering causes water-mold fungi if the water stands too long around roots, especially with warm soil.
To control problems caused by water-mold fungi, take steps to improve soil quality and drainage in the ground and in potted plants. Do not overwater the plants.
Verticillium Wilt (fungus)
Verticillium fungus invades and plugs the water-conducting tissues in the roots and stems. It is one of the most widespread and destructive plant diseases in California. A common symptom is a wilting of one side of the plant or the entire plant. The leaves begin to turn yellow and then brown, and the plant dies. The fungus can survive in the soil for years.
No measures will kill the fungus once it has invaded a plant. Soil from potted plants must be fumigated to eliminate the fungus.
To learn more, go online and Google the pests and diseases listed on this page.