Aphids are closely related to other common garden pests such as whiteflies and mealybugs. They feed on the plant sap and excrete a sticky substance called honeydew, which is actually aphid pee—and aphids pee a lot.
Plant sap, which is high in sugar, is a poor source of proteins and other nutrients, so aphids have to eat an incredible amount of it to get enough nutrients.
To assist in this process, aphids have a specialized digestive system that allows them to pull water out of their liquid diet and excrete it much more rapidly and efficiently than other insects. They also have special organisms in their gut that help process the plant sugars into food.
What Type of Damage do Aphids Cause?
The massive amount of sap a large infestation of aphids pulls out of plants can stunt plant growth and decrease yields. Furthermore, all of this excreted sugar makes a sticky mess on the plants. This is where the damage associated with aphids most commonly occurs.
The honeydew can mold, causing a reduction in photosynthesis for the plant, and eventual death of the molded leaves. The mold also makes for unpleasant and unhealthy working conditions for anyone handling the crop, and is generally unacceptable on marketable crops.
Basic Control of Aphids in the Garden
When it comes to dealing with aphids, a thorough washing of the plant with a garden hose is a good place to start. A cone nozzle or high-pressure mist nozzle are both good weapons that will knock off aphids but not hurt the plant.
Aphids are relatively soft-bodied insects, so hitting them with water injures them, making it difficult for them to walk back up the plant. The water also helps wash off any honeydew that has already accumulated.
There are also several low-toxicity pesticide options, including horticultural oils and insecticidal soaps, which are effective against aphids. Some growers prefer to use pesticides that work systemically (throughout the plant).
Make sure to read and follow the instructions and safety precautions printed on the product labels whenever you are dealing with pesticides. Also remember that aphids are extremely prolific and can rebound quickly.
Biological Control Options for Aphids
Biological control options are available for aphids and are more environmentally friendly than pesticides. The first, and arguably the most common in outdoor growing, is the use of lacewings (or golden-eyed flies). Green or brown lacewings, which are widely available through mail order, are voracious predators when they are in larvae form.
There is also an aphid parasitoid (Aphidius colemani) that is highly effective at finding low populations of aphids. These minute wasps lay their eggs inside of aphids, where the immature wasps eat the aphids from the inside out, eventually emerging from the hollowed-out aphids as adults.
Lastly, there are midges, which are small flies whose larvae eat aphids. As maggots, (fly larvae) midges, which are blind, will cruise around the plants, searching for a group of aphids.
They will then use a sticky substance from their mouths to grab hold of an aphid and consume it. The maggot grows to several times the size of an adult aphid before it matures into an adult, providing significant control of aphids.
Ladybugs (or ladybird beetles), which are known to eat aphids, are not actually good at reducing aphid populations. This is partly due to where a female ladybug lays her eggs. She will only lay her eggs near large infestations of aphids.
If you only have a few aphids on your crop, ladybugs are likely to keep looking for a better spot, which allows small aphid populations time to grow to levels not curable by ladybugs alone. Another issue with ladybugs is that they tend to want to fly.
The majority of released ladybugs won’t stay where you release them long enough to control your aphid problem.
No matter what biological option you choose for dealing with an aphid infestation, you will have to be mindful of ants, since ants actually farm aphids for their honeydew and will protect the aphids from predators and parasitoids.
For this reason, it is important to have your ants under control at all times, otherwise natural predators and parasitoids will be unable to control your aphid population. In addition, any predators or parasitoids you introduce into your crop will also be ineffective if ants are present.
Aphids are a common problem in the garden and control of them can be frustrating. However, with the proper plan and some knowledge about their biology and natural enemies, aphids can be managed effectively.