Yellow bugs on milkweed

Secret to killing milkweed aphids, not monarch butterfly eggs

Los Angeles reader Millicent Stoller wrote to our SoCal Garden Clinic saying she was trying to plant for monarch butterflies. Stoller said she was growing two milkweeds, but both had become inundated with aphids.

“When I try to eliminate the bugs with a water spray or a soap-water spray, I end up destroying butterfly eggs,” she wrote. “Is there another way?”

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For an answer, we turned to Barbara Eisenstein, research associate at Rancho Santa Ana Botanic Garden in Claremont, founder and manager of a nature park stewardship program in South Pasadena and horticulture chairwoman for the San Gabriel Mountains Chapter of the California Native Plant Society.

Eisenstein’s response:

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The bright yellow aphids found on milkweeds are destructive, non-native pests. It is important to remove and dispose of them at first appearance or they will quickly infest the plant, making it difficult for monarchs to use the plant.

Monarch eggs are more tightly attached to leaves than aphids, so with just the right amount of pressure you may be able to wash off aphids without destroying the eggs.

Sprayed water may only dislodge the pests, which can climb back up on their own or be returned by aphid-harvesting ants. Soapy water may dislodge and kill more aphids, but it also is more damaging to the monarchs and can build up on the plant.

Though tedious, dabbing aphids with cotton swabs dipped in isopropyl (rubbing) alcohol is most effective. That kills them outright. Alcohol, however, also is lethal to monarch eggs and larvae, so care must be taken when dabbing.

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Natural controls for the pests, sometimes called oleander aphids, include the ladybug, especially in the larval stage, as well as the lacewing, syrphid fly larvae and the tiny wasp Lysiphlebus testaceipes. Some of these are available in stores and online.

The Garden Clinic welcome questions and comments at [email protected] Include “Garden Clinic” in the subject line. Because of the volume of mail we receive, we can respond only to select questions.

How do I treat milkweed plants that have aphids?

Like many insects that forage on milkweed, Oleander Aphids have adapted the ability to tolerate the toxins in milkweed plants.

The good news is that aphids are not a direct threat to monarch eggs or larvae. Aphids will feed on the milkweed plant only; they won’t spread to your other plants. They only tend to be problematic is the plant is very small or weak. In these cases, they may weaken the plant even further and greatly decrease the nutritional value for your caterpillars. We have had larger caterpillars eat the plant aphids and all!

It is nearly impossible to get rid of aphids. Lady bugs are nice because they eat the aphid but not the larvae. Of course catching enough lady bugs to solve the problem is more than time consuming- and it can be pricey to purchase them from a biological control supplier. Also, the lady bug larvae do eat the monarch eggs.
The easiest way to control aphids is to use the hose to blast them off every couple of days. You won’t completely get rid of them, but it helps. You can spray them with hot, soapy water. This should kill them on contact.

Try this “Contact Only” mix (This was shared with us by Vic Jost @ Jost Greenhouses through Elliott Duemler at Taylor Creek Nursery).
• 1 part (e.g. 1 oz) Blue Dawn
• 1 part Isopropyl Alcohol
• 1 part white vinegar
• 128 parts (e.g. 1 gal) water
“Contact only” means that the insects have to have the mixture applied to their body for it to work.

Once you see that the mixture is working, as an extra precaution you can then rinse the plants so that they are safe for monarch larvae.

As a last resort, you can wear rubber gloves to smash the aphids. The gloves keep your fingers from getting stained orange.

Don’t smash the aphid mummies! They contain beneficial wasps!

Posts Tagged: oleander aphids

Will Milkweed Bugs Eat Aphids?

Yes, they will! Milkweed bugs gained the nickname of “seed eaters” for primarily eating the seeds…

Yes, milkweed bugs feed on oleander aphids. This is a large milkweed bug (Oncopeltus fasciatus) with an aphid. (Photo by Kathy Keatley Garvey)

Yes, milkweed bugs feed on oleander aphids. This is a large milkweed bug (Oncopeltus fasciatus) with an aphid. (Photo by Kathy Keatley Garvey) Posted on Thursday, September 19, 2019 at 4:19 PM

  • Author: Kathy Keatley Garvey

Tags: aphids (37), Hugh Dingle (16), lady beetles (39), ladybugs (42), milkweed bugs (2), oleander aphids (6), Oncopeltus fasciatus (1), UC Davis Department of Entomology and Nematology (318)

Hey, I’m Eating as Fast as I Can!

Have you ever seen the larva of a lady beetle (aka ladybug) dining on an aphid? Lights! Camera!…

An immature lady beetle (larvae) chowing down on an oleander aphid. This photo was taken on a milkweed plant in Vacaville, Calif. (Photo by Kathy Keatley Garvey)

An immature lady beetle (larvae) chowing down on an oleander aphid. This photo was taken on a milkweed plant in Vacaville, Calif. (Photo by Kathy Keatley Garvey)

A well-fed adult lady beetle (aka ladybug) ignores a fat Oleander aphid. (Photo by Kathy Keatley Garvey)

A well-fed adult lady beetle (aka ladybug) ignores a fat Oleander aphid. (Photo by Kathy Keatley Garvey) Posted on Wednesday, May 22, 2019 at 5:27 PM

  • Author: Kathy Keatley Garvey

Tags: Coccinellidae (7), lady beetles (39), ladybugs (42), milkweed (34), monarchs (43), Oleander aphids (6), pollinator garden (15) Focus Area Tags: Agriculture Food Health Natural Resources Pest Management Yard & Garden

The Enemy of the Gardener

Aphids, don’t you just hate them? Especially those oleander aphids that suck the very lifeblood…

Oleander aphids clustering on a milkweed stem. (Photo by Kathy Keatley Garvey)

Oleander aphids clustering on a milkweed stem. (Photo by Kathy Keatley Garvey)

Aphids magnified on a Leica DVM6 microscope, operated by Lynn Epstein, UC Davis emeritus professor of plant pathology.

Aphids magnified on a Leica DVM6 microscope, operated by Lynn Epstein, UC Davis emeritus professor of plant pathology. Posted on Friday, September 28, 2018 at 4:00 PM

  • Author: Kathy Keatley Garvey

Tags: aphids (37), Lynn Epstein (1), milkweed (34), oleander aphids (6), soap (1), UC Davis Department of Plant Pathology (1), UC IPM (34), UC Statewide Integrated Pest Management (1) Focus Area Tags: Environment Pest Management Yard & Garden

Wash Their Mouths Out With Soap

We’re not the only ones “celebrating” the first week of spring. The oleander aphids are doing a…

Oleander aphids absolutely love tender milkweed plants. (Photo by Kathy Keatley Garvey)

Oleander aphids absolutely love tender milkweed plants. (Photo by Kathy Keatley Garvey)

Close-up of an army of aphids on milkweed. (Photo by Kathy Keatley Garvey)

Close-up of an army of aphids on milkweed. (Photo by Kathy Keatley Garvey) Posted on Friday, March 25, 2016 at 4:20 PM

  • Author: Kathy Keatley Garvey

Tags: cha-cha-cha (1), merengue (1), milkweed aphids (1), monarchs (43), oleander aphids (6), salsa (1), tango (2), UC Statewide Integrated Pest Management Program (11)

How Small Is Small?

Sometimes in a world of towering skyscrapers, jumbo jets and warehouses big enough to hold a small…

A lady beetle, a monarch caterpillar and an infestation of oleander aphids. (Photo by Kathy Keatley Garvey)

A lady beetle, a monarch caterpillar and an infestation of oleander aphids. (Photo by Kathy Keatley Garvey)

An oleander aphid on “the nose” of a monarch caterpillar. (Photo by Kathy Keatley Garvey)

An oleander aphid on “the nose” of a monarch caterpillar. (Photo by Kathy Keatley Garvey)

An oleander aphid on the back of a monarch caterpillar. (Photo by Kathy Keatley Garvey)

An oleander aphid on the back of a monarch caterpillar. (Photo by Kathy Keatley Garvey)

An oleander aphid crawling on a tentacle of a monarch caterpillar. (Photo by Kathy Keatley Garvey)

An oleander aphid crawling on a tentacle of a monarch caterpillar. (Photo by Kathy Keatley Garvey) Posted on Wednesday, September 2, 2015 at 6:36 PM

  • Author: Kathy Keatley Garvey

Tags: lady beetles (39), lady bugs (5), monarch caterpillars (6), oleander aphids (6), small (1), tentacle (1)

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Question and Answer

How do I get rid of pests, like aphids, from my garden or habitat? Are they hurting my monarchs?

Many people are concerned about infestations of insects in their garden or habitat; we get the most questions about aphids. There are a few key points to remember when thinking about pest control. First, with good quality habitat comes a higher diversity of insects using the site. This diversity is providing resources for a wide variety of wildlife using the habitat and overall contributes to the health of the ecosystem. Some of these insects may compete with monarchs for milkweed, or even kill monarchs, but we can trust that in a healthy and diverse ecosystem/habitat, some monarchs will evade predators and contribute to the population. Second, remember that almost all insecticides will kill more than just the target species. If you spray an area with insecticide to kill aphids, other insects (including monarchs) that are in the area will be affected. For this reason, we don’t recommend using insecticides in your monarch or pollinator habitat.

If you have a severe issue with aphids on your milkweeds, the safest way to remove them is manually. While a high concentration of aphids on your milkweed may look “bad”, these insects are not necessarily causing harm to monarchs; unless they are in extremely high density, there are usually not enough aphids to kill the plant. You can squish the aphids and then rinse the plants with water to dislodge them from the plant. Make sure to check for monarch eggs and caterpillars first! A mild solution of dish soap and water can also be used to kill aphids on milkweed plants (again, after monarchs have been removed). Spraying this solution directly onto the aphids effectively kills the insects. Rinse the plants about a day after they’ve been treated with this insecticidal soap to remove any residue or dead aphids. This method is only effective when the solution directly hits the target insects; it acts by blocking the spiracles, so the residue will not kill insects. You can find a variety of recipes for home made insecticidal soap online, using simple ingredients typically found in your home.

Back to Frequently Asked Questions

  • The dreaded oleander aphids have arrived here and are trying to wreak havoc in my gardens. At a friend’s house today, I noticed that all of her milkweed is infested beyond hope….Are there any new ideas on how to deal with them? Thanks. –Jan LeVesque, Minneapolis

    Jan LeVesque is not alone in her exasperation at the hands–rather, mouth parts–of plant sucking aphids. Anyone who raises milkweed in an effort to attract Monarchs is familiar with the soft-bodied, squishy orange insects that seemingly take over anything in the Asclepias family.

    If you raise milkweed and Monarchs, you’re well acquainted with oleander aphids. Here they are on Tropical milkweed. Note the sticky, slick looking substance on the leaves. That’s honeydew. Photo by Monika Maeckle

    Jan, like many before and after her, posted the above query on the Monarch Watch DPLEX list, an old school listserv that goes to hundreds of citizen and professional scientists and butterfly fans who follow the Monarch migration. And as usual, the community had plenty of ideas.

    But before we explore how to kill them, let’s take a look at the interesting life cycle of these ubiquitous, annoying insects, known as oleander aphids, milkweed aphids, or by their Latin name, Aphis nerii.

    First off, they are parthenogenic, which means they clone themselves and don’t require mates to reproduce. In addition, the clones they produce are always female. Yes, that’s right–all girls. “No boys allowed.”

    All female aphid colonies undermine our milkweeds. Photo via University of Florida

    According to the University of Florida’s Department of Entomology and Nematology “Featured Creatures” website, “The adult aphids are all female and males do not occur in the wild.” Instead, the aphid moms deposit their all-female nymph broods on the stalks of our milkweed plants. That generation morphs four more times until they grow up to become aphid moms who repeat the process.

    Even more interesting, under normal conditions, adult female aphids do not sport wings, but get this: if conditions are crowded, or the plant is old and unappetizing (which happens as the summer progresses in our part of the world), the girls grow wings so they can fly away to greener pastures–or in this case, fresher milkweed. Aphids live 25 days and produce about 80 nymphs each.

    This brilliantly efficient method of reproduction, says Featured Creatures, is one of the reasons “large colonies of oleander aphids…build quickly on infested plants.”

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    Small populations of aphids are pretty harmless to the plants, but when you get a large colony, the milkweed suffers. The aphids insert their piercing mouthparts into the milkweed, literally sucking the life out of it as they enjoy the sweet liquid that courses through the plant. The high concentration of sugar in that liquid means the aphids have to eat a lot of it to get the protein they need.

    That results in a profuse amount of excrement, called honeydew. It is prolific and forms a thin, sticky layer on the leaves of your milkweed, choking the absorption of essential nutrients. It can also cause sooty mold, an ugly dark fungi that can cover your milkweed.

    We have a fair amount of milkweed near the house and it’s been aphid free so far – except for one incarnata shoot on which a 3 inch long colony had formed with probably more than 500 aphids of all sizes. Not having mixed up any fancy aphid remedy and not having insecticidal soap, I looked under the sink for the handy spray bottle of 409. It wasn’t there – so, I grabbed the Windex instead. Last evening a quick inspection showed that all the aphids were blackened and dead. I checked again this morning and spotted one aphid. The plant seems unaffected by the treatment. –-Dr. Chip Taylor, Founder of Monarch Watch

    Once you have well-established infestation of aphids, the plant just goes downhill. The aphids themselves are also highly appetizing to Ladybugs, wasps and syrphid flies–all insects that eat aphids and Monarch or Queen eggs with equal abandon.

    When I get a serious aphid infestation, I typically use a high pressure spray of water to blow the bugs off the plant. That simultaneously washes the honeydew off the milkweed, which will deter the arrival of ants and also clean the leaves so they can absorb sun, air, water and nutrients to fuel their growth.

    My other method is to simply squish the aphids between my thumb and fingers and wipe them off the plant. Your thumb and fingers will turn bright gold, but will wash off.

    A Ladybug’s favorite treat: aphids. Photo by Monika Maeckle

    Some gardeners like to use alcohol or other additives with the water spray, but I prefer to keep that stuff off my plants.

    As Dr. Karen Oberhauser of the University of Minnesota and founder of the Monarch Larvae Monitoring Project wrote to the DPLEX list in July of 2015, “Detergent treatments will kill any live insects on aphid-infested plants, including Monarch eggs, Monarch larvae, and aphid predators like syrphid fly larvae, ladybug larvae, and lacewing larvae.”

    Alas, even Dr. Oberhauser, who has decades of experience with aphids, admits “There is really no good way to kill aphids without killing everything else, except by trying to lower the population by carefully killing them by hand.”

    If Monarch cats are not present, I spray with insecticidal soap and then rinse later with water hose. If that is ineffective, I move up to Neem oil and rinse it off also later. One and/or the other are very effective…. It is easy to go from 1 to 1,000s of aphids in a very short period of time.
    — David Laderoute, Coordinator, Missourians for Monarchs

    Another option is biological control. Lady beetles or Ladybugs feed primarily on aphids. Somehow they seem to magically find their way to our milkweed gardens to feast on the yellow critters. Ladybugs can be purchased in bags at some garden centers and released to do their jobs. But remember–they also eat butterfly eggs.

    Hover flies and wasps also eat aphids. Wasps have a bizarre practice of laying their eggs on the aphids, then eating them from the inside out, leaving a brown shelled carcass in their wake. We often find these hollow corpses on our milkweed plants.

    Pesticides will kill aphids, but they often remain in the plant for months and will also kill Monarch caterpillars. Photo by Sharon Sander

    One good thing about aphids: if you see them on a plant in a nursery, you know the milkweed is clean, and has NOT been sprayed with systemic pesticides. We all want perfect looking plants, but the occasional aphid is a good sign that your plants are pure. See this post for more on that topic.

    Need more ideas for getting rid of aphids? Check out this useful post from The Monarch Butterfly Garden. If you have new tips not covered here, please leave a comment below. Good luck!

    Aphids Got your Milkweeds

    (Oleander aphids)

    (Aphids and Monarch caterpillar)

    Milkweeds are under assault from an alien insect, the oleander aphid, Aphis nerii. These little orange insects suck the sap out of stems, leaves, can cause flowers and pods to abort, and can even kill plants. They concentrate milkweed toxins in their tissue more effectively than native milkweed aphids, and studies have shown that beneficial insects are less effective at controlling them. As a milkweed gardener, what are your options?

    • Wait for beneficial insects to arrive – sometimes this approach works and sometimes it is an abysmal failure.
    • Right plant right place – reduce stress on your milkweeds and they are less likely to be plagued by aphids. Swamp milkweeds require constant moisture, butterfly milkweeds require excellent drainage.
    • Don’t fertilize milkweeds – in general, aphids are attracted to plants with higher nitrogen content.

    When patience and prevention have been exhausted, it’s time to get out the big guns. The following options will kill aphids, but will also take some beneficial insects and even monarch caterpillars out, so use the utmost caution:

    • Squish – place affected parts of the milkweed plant between thumb and forefinger and drag along the stem.
    • Squirt – use water from a hose or strong spray bottle to blast the aphids off the stem (can be combined with previous option).
    • Spray – spray aphid colonies with either horticultural soap or oil, both break down quickly but the soap is more likely to burn foliage. To prevent damage to insects elsewhere on the plant, consider cupping the part of the plant you are spraying in the palm of your hand.

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