Orange Oleander aphids on a small milkweed plant. Also pictured are two young monarch caterpillars. Photo by FOL taken in The Serene Forest.
*** 7/28/23 This post has been updated reflecting new information that was shared on the Univeristy of Florida's IFAS blog. Scroll to the bottom to see this updated information.
Recognize these little guys on the milkweed in this photo? No, not the two little caterpillars. All those little yellow guys? If you said those were monarch eggs I can't be one to judge. At one point when my thumb had no business even thinking it was green, I too thought those were monarch eggs. However, unfortunately they are not fun little monarch eggs that will soon become gorgeous butterflies. Instead, these are aphids. Aphis nerii common name Oleander Aphids to be exact.
The good news - oleander aphids rarely cause significant damage to their host plant. Chances are, you are seeing these aphids on your milkweed. Caterpillars are going to destroy the milkweed before aphids ever have a chance to. Long story short.... leave the aphids be. There is no need to worry about these little yellow milkweed aliens. Besides, you don't want to accidentally kill or remove monarch eggs and small caterpillars do you?
Species: A. nerii
What exactly is an aphid?
Aphids are soft bodied, "sap sucking" insects. In Florida we have several types. Each type can be identified easiest by identifying the host plant. For you butterfly enthusiasts, please notice how the term "host plant" was used. Aphids, like butterflies, have specific plants and plant species that they prefer for food and reproduction.
During the spring and summer, female aphids do not require males to reproduce. Interestingly enough, they actually don't lay eggs and their young are born live. Some people have compared their reproduction to cloning although I don't know the exact science behind it. At the end of summer, males and females will mate. Females will then lay eggs that will survive the colder winter months in order to ensure their return to your garden the next year.
Ants and aphids have a fascinating relationship. A byproduct of aphids digestion is honeydew which is left deposited on the host plant. This honeydew itself can encourage bacteria and fungi to grow and destroy plant foliage. The honeydew also attracts ants as their food source. Ants themselves are brilliant insects. They further demonstrate this through a process called "aphid farming". In order to ensure a constant food source, ants will actually protect aphids in exchange for the honeydew. Ant activity on your plants can be a key indicator that your struggling plant is infested with aphids.
So how do aphids find their host plants? Research is still ongoing. At one point it was highly believed that their sense of smell attracted them to their host plants. Currently, scientists believe that both visual and olfactory senses are involved. Additionally, aphids have been seen flying from plant to plant as if in a trial and error approach. They land on a plant and give it a taste. If it is not right, they move to another.
While scrolling gardening groups and pages on social media, I often see people asking how to remove aphids from their garden. Commonly I see people recommending chemicals such as BT, Sevin Dust, and neem oil. First of all.... BT is not going to have any affect on aphids but will kill all the caterpillars. So go ahead and get rid of that idea. For oleander aphids on milkweed, chemical treatments are going to be completely unnecessary. Oleander aphids rarely are able to cause enough damage to slow down the rapid growth of milkweed plants. Milkweed tends to be fairly disease resistant and I have never lost a plant secondary to aphids. The caterpillars WILL beat them to it!
Other types of aphids that target vegetable plants may require more attention. Plant health from aphids can directly affect production of the plant which then reduces the gardener's harvest. Not only do aphids cause damage to their host plant, they can also transmit viruses that will further destroy host plants. This is primarily seen in strawberry aphids, green peach aphids, melon aphids, and cabbage aphids. Additionally, their honey dew biproduct can encourage fungal and bacterial growth. At this point it may be necessary to intervene before aphid's natural predators arrive. Instead of starting with chemicals, I recommend starting with simple removal methods.
Citrus Aphids in different life stages covering a citrus tree branch. Photo by FOL taken in The Serene Forest.
In my opinion mechanical removal of aphids is best. Mechanical removal is done primarily by pruning off areas of infected plants and then proper disposal of clippings into the trash. If this method is not feasible, such as in young plants with limited foliage, then aphids can be sprayed off with a regular garden hose. Removal by hand can also be done although I recommend using a glove if you choose this option. Placing yellow sticky pads around the base of plants has proven to work quite well for preventing aphids from finding your plants.
Another approach is "companion planting". This term describes a method of organized planting of varying types that benefiting each other for several reasons. These include: providing soil nutrients such as nitrogen fixation, stabilization, and even pest prevention. Since aphids find their host plant through senses, companion planting has been highly successful in distracting aphids from unwanted infestations. Incorporating highly fragrant plants to your garden near aphid host plants can help with aphid prevention. Examples of fragrant plants that have proven to be successful are mints, basil, rosemary, and marigolds. In addition to fragrant plants, having multiple "distractor" plants around host plants has proven to aid in aphid prevention. Since there are more plants in the area, it may take aphids longer to find their host plants through the trial and error process. The more time spent searching, the more time their natural predators have to find and eliminate them.
Appearance of Hoverfly and Ladybeetle larvae - aphid predators. Photo by FOL taken in The Serene Forest
A key principle in gardening and landscapes is establishing an ecosystem. The definition of an ecosystem for anyone who brushed off science class is a biological community of interacting organisms and their physical environment. Basically, successful gardeners win over unwanted pests through a balance of harmful insects and beneficial insects. There are several natural predators of aphids. They do a pretty jam up job of keeping them in check too. However, chemical and mechanical removal of aphids at first site do not allow these predators the chance to introduce themselves into your garden. By allowing aphids to stick around for a short time, natural predators will arrive and actually help you with your infestations. Here are a list of natural aphid predators:
Long Legged Flies
Some species of Beetles
Some species of Spiders
Some species of flies
If all hope seems to be lost, the next step is chemicals. Start with a natural ingredient approach such as neem oil. People also have success with insecticidal soaps and essential oils such as peppermint. This may be the best solution for aphids whose host plants are also your food producing vegetable garden plants.
While oleander aphids may be causing you to question every ounce of your passion for gardening, there is definitely hope. I believe that once your garden is more than a garden - an actual ecosystem - you will find ultimate success in avoiding more than just aphids.
**** UPDATE ****
The University of Florida's IFAS blog has published new information on the effects of oleander aphids and Monarch Butterfly caterpillars. To summarize, it seems that there is evidence showing that the presence of oleander aphids on Tropical Milkweed, Asclepias curassavica, not only deters female Monarchs from laying eggs on them, but also that oleander aphid presence leads to less developed Monarch caterpillars. Scientists have not proven why this happens. However, the assumption is that aphid presence on the milkweed causes the plants to produce higher amounts of cardenolides in plant tissue. Cardenolides are the toxic chemical in milkweed that help protect the plant from predators. Monarch caterpillars are able to digest this toxin and then they themselves become toxic to their predators. Tropoical Milkweed already has a very high concentration of cardenolides. If the plant increases that concentration, the caterpillars are negatively impacted.
To see the blog post with more detailed information... CLICK HERE