There’s something flying around in your tomato seedlings! And what are those little white dots?

Plants we grow indoors, whether permanently like houseplants or temporarily like seedlings we start indoors, are often bothered by little insect pests. It can seem a bit of a mystery. Where do they come from? Watching helplessly as a flat of seedlings succumbs to fungus gnats can drive you nuts. 

Thrips, fungus gnats, and aphids are common pests, and all can be a problem for our plants. Here’s a quick guide to these three pests, including a little about each one, how to recognize them, the damage they do, and how to minimize or treat the issue.

Remember, the best solution is often monitoring and prevention, not post-problem treatment. 

Thrips

Thrips are a group of small, slender, winged insects with over seven thousand species. With a group that large, there’s a thrip to attack nearly any kind of plant from cotton fields to orchids and vegetable crops. They’re quite tiny, often only 1/20th of an inch long, and can be hard to spot. More often, we first notice the damage from their feeding.

Thrips leave a damage pattern called stippling, which looks like a series of pale dots. Thrips pierce the exterior of the leaf and suck out the good stuff inside the plant cells, leaving them pale and, of course, damaged. While thrips don’t often kill shrubs and ornamental trees, they can do quite a bit of damage to young plants and garden crops in high numbers. 

Identifying

Look for thrips using bright yellow sticky card traps or shaking suspected infested plants over a white piece of paper. They frequently leave white streaked feeding scars and black specs of feces, called frass. Larvae are often translucent, white, or yellowish. 

Adult thrips look like typical insects, are often brown or black, and while they don’t feed on plants, the adults are often caught in sticky traps. The larvae are what does most of the damage to the plants, so catching adults before they lay eggs is still worthwhile. Females lay cylindrical or kidney-shaped eggs on leaves, buds, and shoots where the hatched larvae can easily find their first meal.

Controlling Thrips

Horticultural oils and insecticidal soaps work well on the larvae but must be applied to the underside of the leaves. These products need contact with the insect to work. Hang sticky card traps to capture adults. Inspect plants before bringing them indoors for the first time.

Thrips in the greenhouse or outdoors can sometimes be controlled by natural predators, including lacewings, pirate bugs, and parasitic wasps. 

Fungus Gnats

Fungus gnats can be a real pain, especially at seed-starting time. The larvae feed on young plant roots, often weakening or killing seedlings. The high peat moss content in many seed-starting mediums is particularly attractive to fungus gnats, and an adult female can lay up to 200 eggs. 

Identifying

Use yellow sticky traps to capture adult gnats and monitor population levels. Look for adults which are about ⅛ inch long, with long legs and antennae. With a magnifying glass, you may be able to spot a Y-shaped pattern on their wings. Fungus gnat larvae look like translucent little worms with black heads.

Controlling Fungus Gnats

Fungus gnats become a problem in spring when we are sprouting seeds indoors for later transplanting. The warm, moist environment we create for our seedlings is perfect for fungus gnats. 

For seedlings, water trays from the bottom when possible, and allow the top of the soil to dry out slightly between waterings. A thin layer of vermiculite can also discourage egg-laying females.

In houseplants, allow the top 1-2 inches of soil to dry between waterings to discourage egg laying and survival of eggs. It’s not the adults which cause the problem for the plants, but the larvae. 

Aphids

Another small insect, usually measuring about 1/8th of an inch long, aphids can do a significant amount of damage. There are about 1350 species of aphids in North America, but only a few of them are significant problems for our plants.  

Aphids eat the sap from our plants and excrete the excess as a mostly clear, sticky material called honeydew. This sometimes shiny but always sticky substance is a telltale sign of aphid infestation. When numbers are high, it can be quite a mess, even dripping off to cover chairs, cars, or anything else underneath the tree or plant. A black mold sometimes grows on the honeydew, which can block light needed by the plant for photosynthesis. 

Aphids damage plants by feeding on them, but can also be vectors for introducing diseases including many mosaic viruses of cauliflower, lettuce, and cucumber.

Identifying

Many aphids are light green in color, but whites, pinks, yellows, and pretty much every other color are also present. They are soft-bodied and wider at the rear than the front, like a pear. They have sucking mouthparts and feed on sap. At the rear of the insect, protruding from the rump, are short appendages looking like sticks called cornicles. Some adults have wings, and others do not.

While inspecting the leaves for eggs, nymphs, and adults is good practice, you’ll likely notice the honeydew first. That shiny, clear, sticky substance is a giveaway and is much easier to spot than the actual aphids. 

Controlling Aphids

In the garden, natural predators like lady beetles, lacewings, and predatory wasps often do a good job keeping aphid populations in check. Give those beneficial insects some help by planting a diverse array of native plants nearby and eliminating broad spectrum pesticide use. 

Whether in the house or out in the garden, a strong stream of water is often very effective at controlling aphids. They are knocked off the plants and climb back up slowly, if at all. Indoors, horticultural oils and insecticidal soaps can be used for severe cases. 

A note about predatory insects

The idea of releasing a natural predator to feast on our problem insects in the garden is intoxicating to many gardeners, and supply companies have listened to our demands. However, the results often don’t align with our hopes. Many unhappy reviews explain how the gardener released all these lady beetles or other insects, and after a couple of days, they couldn't find any. Presumably they flew away. That’s not necessarily bad; it means the lady beetles went looking for more food. In other words, they ate all their food in your garden (the bad bugs) and now they went in search of more somewhere else.

Purposely releasing predatory insects to control garden pests can work, but it often requires several applications to build up a base level.  Providing a proper habitat so they’ll stay in the area is the key. You may find that this year’s application becomes next year’s army of insect helpers. 

Lacewings, parasitic wasps, or lady beetles won’t stick around if the habitat doesn’t support them. A generous border of native plants near your veggie patch will help. Many beneficial insects visit flowers for nectar and pollen. Beneficial insects also need a water source and will benefit from shallow wet areas like saucers and puddles. Keep in mind that a deeper water source can drown them. Use something with shallow sides more akin to the children’s end of the pool, not the diving end. And remember, many insecticides labeled as “safe” for pollinators are still unsafe for beneficial insects.