October 13, 2012

1 - Aphid insect pests may come indoors with your houseplants for the winter.

Yard and Garden October 13, 2012


I was preparing to bring my houseplants inside for the winter. As I was looking at them, I noticed some plants have a lot of small greenish and gray insects on them. These insects are very fragile, as I tried to collect some to look at, they easily smashed. I have also seen some similar insects among the others that have wings and a few in the middle of the others that are larger, very spherical, and tan colored. I have sent you some pictures. I hope you can identify these insects and recommend a treatment.

Mary J. R.


The photographs you e-mailed were helpful. The insects on your plants are aphids. These are small insects that rapidly reproduce to create large populations in a very short time. They feed by piercing the plant cells and sucking the liquids and nutrients from the cells. This can be quite harmful to your plants. The winged insects appear to be the winged form of the aphid. This one can fly to other plants and infest other plants.

The tan colored ones are good news! They are aphids that have been parasitized by a parasitoid wasp (does not sting you). The female wasp has laid eggs inside the aphid where the egg hatched to form the wasp larva. This larva feeds on the inside of the aphid until it matures. During this feeding period the aphid enlarges and grabs tightly onto the plant. It then dies. Soon after the aphid dies, the wasp larva matures and emerges to attack other aphids. This seems cruel, but it is an important part of the natural process of insect pest management.

If you treat the pests with some insecticides you may kill the parasitoid wasps and remove this natural pest control mechanism. However, since you are preparing to bring your plants inside, you may choose to spray to avoid bringing the small wasps indoors with the aphids. These wasps will not harm or sting you, but they will be a nuisance as they fly around indoors. You may choose to allow them to remain if you wish, or you may treat with insecticides. Once your houseplants are indoors, the pool of wasps to manage the aphid population will be diminished, requiring that you manage the aphids anyway.

Aphids may be treated by spraying with a strong stream of water that causes the aphids to drop from the plants. As they crawl back up the plant, they are not feeding on the plant, but they are accessible to the parasitoid wasps and many other natural predators that help manage their population. Insecticidal soaps may also be used quite effectively to kill aphids without doing much harm to the predators that help you. Stronger insecticides labeled for aphid control may kill the wasps, this includes some of the “organic” insecticides.

Any aphids that survive the treatment (hiding in the leaf bases of the plants or just below the soil line) will result in a rebound of the aphid population once after the plants are indoors, so you must remain vigilant even after the plants are inside. Since the natural predators are not active indoors, you may need to spray your plants with water or insecticidal soap during the winter. You can move the plants to the bath tub to do this to avoid damaging fabrics in the house and to remove the pests from the area where the plants are maintained. If the plants are too large to move, plastic sheets under and over nearby fabrics will allow you to safely manage the aphids. If you want to use stronger insecticides, choose products that are labeled for indoor use to manage aphids, or move the plants outdoors to spray them.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


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