What’s Eating My Lettuce? 7 Common Lettuce Pests
Lettuce is easy to grow in the ground, in raised beds, and in pots, indoors or out. However, the crispy crunch of a fresh leaf is not immune to several pests who may beat us to the goodness. Gardening expert Katherine Rowe explores what to scout for and how to stave off common lettuce invaders.
Contents
The fresh, tender, tasty leafy greens of lettuce are vulnerable to several pests that find them an easy food source. Fortunately, the crop is easy to grow and quickly produces new leaves for the picking.
Because the whole leaf is a food crop, we’ll cover how best to contend with the creatures while ensuring safe eating. To set up the best foundation, grow Lactuca sativa in full sun to partial shade in moist, well-draining soils. Lettuce does best in cool conditions and as underplanting for taller crops that shield it from afternoon sun in warm temperatures. Sow successive rounds for an ongoing supply of leafy greens.
Interplant companion plants and flowering selections to attract beneficial insects and complement the best growth. If you experience pest problems, rotate the lettuce crop for two to three seasons to decrease recurrence. Keep the pests guessing for the best lettuce aesthetics, crunch, and flavor.
Caterpillars
Caterpillars are the most common lettuce pest and a big nuisance when it comes to leaf integrity. You’ll notice holes or gashes in the leaves when they’re present, and depending on their numbers, damage can be extensive. The main invaders are armyworms, loopers, and cutworms, which all contain different species that have palettes for a range of edibles and ornamentals.
Armyworms
Armyworms, like many lettuce pests, prefer young leaves. They settle into the crown to feed, destroying leaves up to the midribs and growth points.
The beet armyworm is a common offender, which is green or black with three light stripes and a black dot on either side. Often, you won’t see the damage until harvesting and unfurling a mature head.
Loopers
Loopers arch their backs as they crawl, creating a looped motion. The caterpillars go for tender leaf tissues, often leaving the midribs and veins intact.
The cabbage looper is common to the crop and is green with long white stripes. Young larvae feed on the undersides of leaves, while mature caterpillars burrow deep within the head.
Cutworms
Cutworms are plump gray or black caterpillars that are active at night. When young, they feed on leaves. As adults, they sever the stalks as they feed, debilitating the plant. They damage the crown and sometimes the outer wrapper leaves.
Treatment for Caterpillars
As a biological control, incorporate a diversity of blooming herbs, annuals, and perennials to draw predatory and parasitic insects. Parasitic wasps, tachinid flies, damsel flies, assassin bugs, and beetles feed on eggs, larvae, and adults. Grow flowering perennials like asters, yarrow, and coreopsis, and herbs like dill, parsley, and chamomile to attract them. These natural predators feed on the eggs, larvae, and adult caterpillars.
Keep weeds at bay to reduce the spread of caterpillars. Remove plant debris at the end of the harvest to avoid lingering larvae.
Scout for larvae and egg masses on lettuce leaves as they form, and on surrounding plants and weeds. Handpick small numbers off the crop, dropping them in a bucket of soapy water. In larger infestations, organic pesticides help control populations.
Bacillus thuringiensis (BT) is a biological control that keeps soft-bodied insects like caterpillars from being able to feed. They die off when they ingest the bacteria. Spinosad, also derived from a bacterium, is an organic treatment with the same effect.
Leaf Miners
Leaf miners are fly larvae that burrow and tunnel into foliage. Larvae are yellow, cylindrical, and seedlike. Adult leaf miners are small black flies, often with a yellow triangle at the base of their wings.
Leaf miner larvae feed beneath the leaf surface, leaving winding trails. The visible pathways may begin in the early leaves of seedlings and continue up to maturity. They become yellow and blotchy.
Adult leaf miners pierce the leaves to access juices and to lay eggs. The punctures turn white and lend a speckled appearance on surfaces.
In heavy infestations, leaf miners can slow growth, cause leaf drop, and create openings for other infections.
Treatment
Lacewings and predatory wasps prey on leafminers. Remove damaged leaves to easily control a light infestation. Horticultural soap, BT, and spinosad foliar sprays control infestations.
To prevent leaf miners, aim for consistently moist soils, avoiding overwatering and prolonged periods of saturation. Irrigation control isn’t so much for the leaf miner but for associated fungal infections like root rot that easily spread via the insect.
Slugs and Snails
Slugs and snails slide across lettuce leaves and feed on soft new growth. The creatures are most active in the evening and early morning, when you may spot them on a leaf. Or, you’ll notice deep holes, serrations, and their sluggy trail.
Treatment
Scout at dusk and dawn to hand-pick slugs and snails from the leaves. In the middle of the day, they’ll tuck in under leaves or on the mulch and soil around the crowns. If numbers increase, bait them with beer or soda traps by filling a shallow dish and placing it at soil level.
Remove mulch from around the crop to reduce protective hiding spots. Target irrigation to minimize inviting damp conditions. Drip irrigation at the base delivers water directly to the roots without wetting the overall surface area.
Diatomaceous earth scattered around the base of the head contains sharp particles that harm soft-bodied creatures as they crawl. It needs reapplication after heavy rains to maintain the boundary.
Aphids
There are a number of aphid species that impact lettuce, from green peach to potato to lettuce aphids. Aphids reproduce quickly and congregate in numbers on the undersides of leaves.
Aphids are soft-bodied and pear-shaped in light yellow, green, pink, brown, or black. They leave behind a sugary “honeydew” waste, and the sticky substance can lead to black, sooty mold across the leaves.
In dense populations, aphids stress and stunt head growth. They’re also vectors, spreading diseases and viruses as they travel between plants, including problematic lettuce disorders like mosaic viruses and yellows. Foliage may curl and yellow, and mottling and distortion may follow as signs of disease.
To prevent aphids, water consistently for healthy growth that resists infection. Aphids have many natural predators, and growing a variety of blooming plants to attract a diversity of insects offers natural pest control. In high numbers, predators won’t be enough to combat the damage. Use them in combination with other treatments.
Treatment
If you see aphids or their residue, try spraying them with a strong stream of water to detach their mouthparts from leaves (killing the insect). Take care not to damage or sever tender leaves in the process.
Horticultural soaps are effective for larger outbreaks. Diatomaceous earth dusted over leaves and the surrounding soil may lessen the spread. Entomopathic fungi (parasitic fungal spores) like Entomophthora aphidis may limit large populations, but these are not available commercially.
Crickets and Grasshoppers
These hopping insects are occasional pests that can quickly clip lettuce seedlings. When present, crickets damage seedlings, while grasshoppers eat the leaves at any stage.
To prevent crickets and grasshoppers, clear weeds from the surrounding areas to prevent migrating.
Treatment
If you spot high numbers of crickets or grasshoppers, pyrethrin sprays can aid in reduction. Biocontrol agents like Nosema locustae (pathogens that infect the insects) can be effective.
Whiteflies
Like aphids, whiteflies feed on tissues and cause leaves to yellow, and sometimes a smaller head size. Adult whiteflies are tiny and white, and they flutter around foliage with disturbance.
Adult female whiteflies lay eggs on leaf undersides, and nymphs hatch to feed on tissues. The adults pierce foliage to feed on sap. They, too, leave behind a honeydew that promotes sooty mold.
Whiteflies most often crop up in areas with low air circulation or in the hot, dry portions of summer before fall’s cooling temperatures. They’re prone to walled gardens, greenhouses, and indoor growing situations.
Treatment
Remove and destroy leaves in decline to stave off further damage. Sticky traps can help catch them as they fly, and horticultural soap is a treatment in extreme cases. Plant the fall rotation when temperatures cool.
Thrips
Thrips are tiny insects with long, narrow bodies. The adults have fringed wings, and their color ranges from translucent white to yellow, brown, and black. They most often occur on lettuce in cool spring temperatures.
Thrips feed on foliar tissues and can carry viruses. Seedlings may distort, and lettuce leaves become stippled and papery.
If you suspect thrips, gently shake stalks over a piece of paper and look for dislodged pests. They’ll show as small dark spots on the page.
Treatment
As with other pests, predatory insects like parasitic wasps, predatory thrips, and green lacewings naturally control thrips. Clip off any affected leaves. Spinosad and horticultural soaps treat outbreaks.
