Maintaining a garden is challenging enough with soil, fertilizer, and weather factoring into overall plant health. Another issue which can affect a well-kept garden is an infestation of pests, such as pea aphids. This small insect can invade a garden, damage plants, and pave the way for mold formation. One way to control pea aphids without harming your plants is to use organic pest control methods.
What are Pea Aphids?
Pea aphids, scientifically known as Acyrthosiphon pisum, are small green insects featuring long legs, antennae, a segmented body, and a pair of excretion tubes, or cornicle, at the end of the last segment. They are similar in appearance to the blue alfalfa aphid, except for one noticeable difference. The pea aphid's antennae features narrow dark bands, while the blue alfalfa aphid's antennae is completely brown.
How do Pea Aphids Affect Plants?
Pea aphids feed on a number of plants including alfalfa. They usually suck the sap out of the leaves or stems and inject a toxin which affects the growth of the plant. The pea aphid also secrets a sticky substance on the surfaces of the plant called honeydew. By itself, honeydew appears as a shiny residue and isn't too harmful to the plant. It is however a great growth medium for the sooty mold fungus. Spores of the fungus are usually transported by wind and stick to the honeydew. Eventually, entire leaves and stems are coated with a layer of black mold. If the infestation is large enough, the plant may wither and die.
Controlling Pea Aphids with Organic Insecticides
Organic control of pea aphids ensures the safety of the plant, while effectively reducing the infestation. Sprays that consist of azadirachtin, neem oil, or pyrethrin are generally effective against pea aphids. These organic sprays are recommended when the infestation becomes concentrated. For example, for a plant that is less than 10 inches in height, spraying is recommended when there are 40 to 50 pea aphids per stem. Plants that are over 20 inches should be sprayed with organic insecticides when there are over 100 pea aphids per stem. Frequent monitoring will allow for intermediate control methods before the use of insecticides becomes necessary.
Using Predatory Insects and Parasites to Control Pea Aphids
Introducing insects that prey on pea aphids into the garden is one way to control the population before it threatens the plants. Lady beetles and green lacewings are aggressive towards pea aphids. Hippodamia convergens and Coccinella septempunctata are two species of lady beetles that eat pea aphids.
The pea aphid population is naturally controlled by parasites. If this is the case, golden-brown remains of pea aphids typically appear on the upper surface of the leaf. Aphidius smithi is the primary predatory parasite of the pea aphid.
Planting Resistant Varieties and Using Border-Strip Harvesting
To keep the predatory insect population at adequate levels, use the border-strip harvesting technique. In the case of alfalfa crops, this involves leaving an uncut strip of alfalfa at regular intervals across a field. The predatory insects will converge on these plants, leaving other crops undamaged.
Another organic control method involves introducing plant species that are resistant to pea aphids. These plants are not damaged by pea aphids, but there is one type of pea aphid, known as the pink biotype, that has overcome the barriers of many resistant varieties. Prolonged periods of below average temperatures reduces the resistance of many plants and leaves them susceptible to damage from the blue alfalfa aphid.
Organic control of pea aphids involves several techniques including planting resistant varieties, introducing predatory insects, and applying organic insecticides. These methods effectively control the pea aphid population without harming the plants. The plants are less likely to attract the sooty mold fungus and more likely to grow healthily.
1. "Alfalfa – Blue Alfalfa Aphid and Pea Aphid." UC Pest Management Guidelines. https://www.ipm.ucdavis.edu/PMG/r1300211.html
2. "Alfalfa – Border-Strip Harvesting." UC Pest Management Guidelines. https://www.ipm.ucdavis.edu/PMG/r1900411.html
3. "Sooty Mold Factsheet." Plant Disease Diagnostic Clinic Cornell University https://plantclinic.cornell.edu/FactSheets/sooty/sootymold.htm