The milkweed aphid, also known as the oleander aphid, is a source of great frustration for people who raise Monarch and Queen butterfly caterpillars. Let’s look at some options to controlling these plant pests.
First, we should realize that aphids are attracted to milkweed the same way Monarchs are attracted to milkweed. Their offspring depend upon the plant for survival. (The difference is that milkweed aphids, Aphis nearii, will also host on oleander and a couple of other plants. There are very few other plants that are their host plant. Milkweed is the most common host plant for these aphids.)
Although they don’t normally kill a plant, they can stunt its growth, attract ants to the plant, and cause other frustrations. Sometimes they will kill a plant, depending on size, species, and number of aphids. (Ants take eggs and kill caterpillars as well as chrysalises.)
Aphis nerii, the milkweed aphid, is yellow with black appendages. It sucks the sap from milkweed, dogbane, hoya, oleander, and a few other plants. It sometimes spreads plant viruses from one plant to another.
Males have never been found in the wild although they have been produced by lengthening nights and shortening days in a lab. Females give live birth without mating. They have five nymph (young) stages, molting between each stage. Without producing a pupa, the last nymph stage molts into an adult. Adults may be wingless or may grow wings.
There are many methods used to control aphids that cause little to no damage to butterfly eggs or caterpillars.
Beneficial insects are great for controlling numbers but they rarely eliminate all aphids unless they are continually added to a garden. When food becomes scarce, they either die or leave a garden. When they eat/kill all aphids, some beneficial insect species must leave or die.
There are many species of beneficial insects. By learning which of the critters are beneficial insects and which are not, we can avoid killing the ones that are on our side of this battle. Before killing aphids, it is good to remove beneficial insects from the plant. Ladybugs, hover flies, lacewings, Aphidius wasps, and a few more types of beneficial insects devour aphids. We have shared some photos of various beneficial insects here. The book Milkweed, Monarchs, and More is a great addition to any Monarch enthusiast’s library. Some species of predatory beneficial insects may eat a young caterpillar or two. For most of us, the trade-off of a lower number of aphids is worth the loss, creating a greater benefit than a loss.
Ladybug nymphs and adults eat aphids. Most people that grow milkweed for caterpillars are aware of the nymph’s ‘alligator’ appearance of many species of ladybugs.
There are other species of ladybug that have nymphs that closely resemble mealy bugs.
The adult form of these ladybugs resembles little dark beetles, nothing like the red classic ladybug image we know.
Hoverflies, also called flower flies and syrphid flies, are another great beneficial insect. Although the adults drink nectar, larvae feed on aphids. Larvae may be brown, green, or multicolored, varying greatly in appearance. Adult hoverflies lay eggs near aphids.
Lacewing adults drink nectar and honeydew (excrement from aphids). Lacewing nymphs eat aphids. The green lacewing (species of lacewing as well as the color of the adult) takes old cast-off aphid exoskeletons and attaches them to tiny hooks on its back. The nymph looks like a clump of dirt on a leaf. Lacewing eggs are on the end of tiny white strings that stick out from a leaf.
Parasitoid wasps, Aphidius & Aphelinus species, lay eggs in aphids. The wasp larva hatches inside the aphid and eats the inside of it, turning the aphid exoskeleton into a hard bloated mummy.
To emerge, the adult wasp cuts a round hole in the abdomen of the brown aphid mummy and exits it. The brown mummy aphids are good. They should be left intact. These parasitoids do not lay eggs in Monarch eggs, caterpillars, or chrysalises. They only lay eggs in aphids.
There are many products recommended for aphids. Some are oils and some are not. Some are organic and some are not. Remember, organic does not mean safe for insects. Most of these products will kill butterfly eggs and caterpillars. Even if the label does not mention caterpillars directly, many will still kill them. Some kill at pupation, some when the adult butterfly emerges, and some by affecting how their mouth parts work. Moth caterpillars are sometimes mentioned as a target for the pesticide. If it kills moth caterpillars, it will kill butterfly caterpillars. Watch for words that are used in place of ‘caterpillar’ such as leaf-roller, worm, and ‘other insects’. Remember that not all pesticides kill instantly. Some take time to kill, days or weeks.
Remember: Aphids are natural plant pests and we cannot get rid of them for good. Other aphids will find our plants and begin reproducing. Making a plant aphid-free is a continual process. A constant watchful eye is handy to catch new intruders before they have a chance to multiply beyond a certain point.
We recommend these methods to control aphids:
~ First, remove any eggs and larvae of butterflies and beneficial insects. Temporarily place them on another plant or in a sealed container.
~ If there is enough milkweed for caterpillars, cut of any part of the plant that is infected with aphids and dispose of the stems in a sealed bag, off site (or freeze them).
~ The next step is hand-squishing. Disposable gloves are nice for this task. Although they congregate on new growth tips, they can often be found under lower leaves and on the stem at soil level.
~ An optional choice is to blast the plant with water. The negative side is that it does not remove all the aphids or kill the aphids. Some of the aphids may find their way back to the plant.
~ Soapy water is the next step we recommend if the above doesn’t work. We recommend a mixture of 1 tablespoon liquid soap to 1/2 gallon of water. Soapy water should be sprayed either in the early morning or late evening. In bright sunlight, soap acts as a magnifying glass and the light burns the leaves, sometimes causing the plant to drop all leaves. After spraying with soapy water, allow the plant to sit for 10-15 minutes, then rinse well. Both the top and bottoms of the leaves should be sprayed and rinsed. Soap is a deadly diet for caterpillars. The plant should be rinsed well (several times a day) for a couple of days before critters are returned to the plant.
We avoid neem oil (and any other oil) because it tends to stay on the leaf. Neem oil can also be systemic. One label states that “neem oil does not harm beneficial insects, only sucking and chewing insects”. Caterpillars are chewing insects. What we butterfly enthusiasts often fail to remember is that caterpillars are plant pests. Although they are beneficial insects that pollinate, they are also plant pests.
When experiments with different items to kill aphids, remember that all it takes is a drop of alcohol to kill a caterpillar.
Butterfly Fun Facts 