Lettuce (Lactuca sativa) grown in your backyard tastes better than lettuce bought in a grocery store -- but you're not the only one who enjoys the crisp flavor of this hardy annual vegetable. Several kinds of pests may target your lettuce plants and will quickly decimate the entire harvest if they're not immediately controlled and eradicated.
Aphids are one of the most common pests among lettuce plants. They appear as tiny green, red, black or yellow dots, sucking on the lettuce plant's juices and producing a sticky, sweet substance known as honeydew.
For heavy infestations on just a few lettuce stems, prune away the affected area — wipe down any pruning equipment with rubbing alcohol to sterilize it first — and discard the infested leaves. Alternatively, shoot down the lettuce plants with a blast of water from a garden hose. The water knocks the aphids off of the lettuce leaves and rinses away their honeydew. The washed-off aphids have a difficult time returning back to the lettuce plant.
If these options don't sufficiently curtail the aphid infestation, use an insecticidal soap. Mix a tablespoon of liquid dish soap in a pint of water and spray the soapy solution directly on the aphids to kill them. Repeat once a week until problems subside.
Lettuce is an attractive food source for leafhoppers. These green, flying insects measure just 1/8 inch long, but heavy infestations can cause stunted growth, yellowing leaves and leaf loss in lettuce plants.
Remove affected lettuce leaves with sterilized pruning shears. Then, dust the lettuce plants with a coating of food-grade diatomaceous earth. This natural powder is made from crushed, fossilized diatoms. The sharp edges of the powder cut and kill leafhoppers, effectively defending the lettuce foliage from future attacks. Avoid inhaling the powder, as it can irritate your lungs. Reapply diatomaceous earth whenever it rains, as the rain will wash away the powder.
Vegetable weevils also rank as one of the most common pests in lettuce beds. The brown beetles eat lettuce shoots and roots.
Because vegetable weevils cannot fly and move very slowly, hand-picking effectively controls the problem. Go out at night when the weevils are their most sluggish. Pluck off the insects and either crush them or drop them into a bucket of soapy water. Additionally, since the weevils can only crawl slowly from plant to plant, sticky barriers around infested lettuce plants will keep them from spreading to unaffected lettuce plants. Finally, a dusting of diatomaceous earth will also kill the pests.
If you notice a slime trail on and around the lettuce plants, with missing foliage on the lower ends of the lettuce leaves closest to the ground, you have a slug or snail problem. Lettuce's tender, low-hanging foliage makes it a popular target with these pests.
Regular handpicking is one of the most effective control methods. Wearing gloves, pick up the slugs or snails and either drop them into a bucket of soapy water or put them into a sealed plastic bag and discard them. A strip or band of copper metal around the edges of the vegetable garden will also effectively repel slugs and snails. Copper shocks the pests whenever they touch it because the metal reacts to the slug and snail slime.
Cabbage loopers, common lettuce pests that look like tiny green worms, feed on the underside of lettuce leaves and create a patchwork of holes. Stands of weeds typically host these pests, which then migrate to nearby vegetables. Reduce risks of looper infestations by keeping lettuce gardens free of weeds.
Inspect the underside of lettuce leaves. Handpick larvae and any egg masses. This can keep populations at levels too low to cause long-lasting harm. If infestations get out of control, spritz the lettuce plants with an insecticidal soap. In a spray bottle, combine a pint of water with a tablespoon of dish soap. Spray the soapy water on affected lettuce plants to kill the cabbage loopers.
- Old Farmer's Almanac: Lettuce
- VirginiaTech Extension: Pests Commonly Found on Lettuce
- University of California Integrated Pest Management Program: Aphids
- Virginia Cooperative Extension: Leafhoppers
- Colorado State University Extension: Insect Control - Soaps and Detergents
- University of California Integrated Pest Management Program: Vegetable Weevil
- Virginia Cooperative Extension: Vegetable Weevil
- University of California Integrated Pest Management Program: Snails and Slugs
- University of California Integrated Pest Management Program: Cabbage Looper
- Missouri Botanical Garden: Cabbage Looper