Why Aphid Control in the Greenhouse Starts With Knowing Your Genus
July 18, 2021 
Green Peach Aphid, Myzus persicae.
Photo by David Cappaert
It seems appropriate in a discussion about battling aphids to paraphrase a quote by Sun Tzu from his book The Art of War: “If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat.”
Though he was referring to military strategy, in reality, there’s not much difference when the enemy is six-legged and threatening the health of a crop. This is especially true with aphids, which can have an exponential growth rate and cause significant harm to plants.
One of the first things I ask a grower when he or she tells me they have an aphid issue is “What type of aphid is it?” Usually, I get averted eyes, a shrug, or some sort of non-committal grunt. Aphids, though they can vary widely in color and relative size, are fairly easy to distinguish from other plant pests.
A unique feature of these sap-stealing foes are the tail pipe structures on their rear abdomen, called cornicles, which are used to excrete a warning chemical when attacked. A 10x magnification can allow you to easily see this unique feature. You may also notice shiny honeydew on leaves or sooty mold growing on the honeydew — a sign that aphids have been feeding. An increase in the number of ants crawling up pots and benches can also clue you in to aphid populations, since ants are known to farm and protect aphids for the sugary, sticky waste aphids produce as a byproduct of feeding on plant phloem. But, with 5,000 different possible aphid species worldwide, and about 20% of North American aphid species considered invasive, the task of differentiating them becomes a bit trickier.
Why Proper ID Is Critical
Many people assume that because they have identified their pest as an aphid that the work is done. Not so! Identifying the genus (at a minimum) can save you a lot of time and money. Here is a list of reasons why it could be important to properly identify the genus or species of the aphid on your plant:
- If you want to use parasitic wasps as biological control, you must know the aphid species, as many parasitic wasps are fairly host specific. This is not nearly as important with predators such as green lacewings or lady beetles.
- It can help determine where you should focus your spray efforts on the plant. Some aphids prefer to colonize certain areas of the plant (e.g. New growing tips vs. internal stems or lower down in a plant canopy)
- It can help determine if all different plants in an area require management or if just one or two types of plants need treatment, since some aphids are more host specific than others.
- Different species can cause different levels of damage, such as significant leaf curling and distortion, and so have a different threshold for treating. Some aphids are also known to transmit viruses. The threshold for these aphids is lower than for aphids that don’t vector viruses.
- It will help in differentiating aphids that you have on banker/insectary plants vs. pest aphids.
The good news is that there are only about six or so species of aphids common to greenhouses. The top four are melon aphid, green peach aphid, potato aphid, and foxglove aphid. By looking, with good magnification (10x-30x) and lighting, from a top rear view, at the notch on their head between the antennae call the tubercles, you can fairly easily differentiate these four species (see the chart below for the identifying characteristic and notes on their biology).
If you can’t identify the aphid, it is recommended that you reach out to your local Extension agent or other person knowledgeable in insect identification so you have the best chance of success with your pest management. Bring in a branch with live aphids, or put multiple specimens of the aphids in a container with 70% isopropyl alcohol.
Fortunately, many predators and parasites like to feed on aphids, and successful biocontrol is possible. Biorational products such as the entomopathogen Beauveria bassiana, and the natural insect growth regulator azadirachtin as well as soaps and oils can all be used, along with good scouting, into a highly effective integrated pest management program. But, first, you must ‘Know thy aphid’!
How to Know Which Aphids You’re Battling
| Aphid Name | Host Plants | Location on Host | Tubercle Notching | Image of Tubercule, and link with more information | Other Feature(s) |
|---|---|---|---|---|---|
| Melon/cotton (Aphis gossypii) | Many (more than 135 plant families); very common aphid | Towards plant interior, on stems or flower buds; implicated in transmission of more than 50 plant viruses | No notch between two antennal tubercles | http://aphid.aphidnet.org/Aphis_gossypii.php | Cornicles are dark |
| Green Peach (Myzus persicae) | Many (more than 130 plant families); very common aphid | Tend to be high on plants, near the growing tips; implicated in transmission of more than 180 plant viruses | Has a notch; tubercles converge | http://aphid.aphidnet.org/Myzus_persicae.php | Cornicles same color as body, but dark on tips |
| Potato (Macrosiphum euphorbiae) | More than 90 plant families, but common on: Abutilon, carnation, cineraria, lettuce, rose, tomato, tulip, etc. | Implicated in the transmission of nearly 70 plant viruses | Has a notch; tubercles diverge | http://aphid.aphidnet.org/Macrosiphum_euphorbiae.php | Often have a slightly dark line visible down the back; various body colors |
| Foxglove (Aulacorthum solani) | More than 82 plant families but common on: Anemone, arum, calceolaria, carnation, cineraria, dahlia, geranium, gloxinia, lettuce, nasturtium, etc. | Implicated in transmission of at least 45 plant viruses | Has a notch; tubercles nearly parallel | http://aphid.aphidnet.org/Aulacorthum_solani.php | Body usually dark and shiny, often has dark patches near base of cornicles |
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