How to Fight Aphids on Lettuce: 7 Pro Grower Tips
If left unchecked, aphids can quickly ruin your lettuce plants. However, these pests don’t mean your plants are doomed. Join farmer Briana Yablonski to learn a few easy ways to keep your tender greens free from these pests.
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Aphids may appear like small, benign insects, but these tiny pests can wreak havoc. Their small size and tendency to hide in the crevices of lettuce heads means they’re often tricky to spot and effectively treat. While a few of these pests aren’t a problem, they can quickly reproduce and reach harmful populations.
Fortunately, small attacks don’t mean your beautiful heads of romaine and butterhead are destined to discoloration and death. Practicing preventative measures, spotting the pests early, and taking the necessary control steps can help keep your lettuce plants safe from these pests.
In this article, I share some aphid control tips I’ve learned throughout my ten years of growing lettuce at scale. Implement these practices (including sowing beneficial companion flowers!) to help limit and remove aphids on lettuce.
Know What You’re Looking For
Knowing what aphids look like is the first step in fighting these pests! They are small insects with soft, oblong or pear-shaped bodies that remain under an eighth of an inch long. They have six long legs and a pair of elongated antennae. Most adults are wingless, but some have clear wings that extend behind their bodies.
Most aphids have a pair of small, tube-like structures pointed out of their backs. These structures, called cornicles, only appear on these species and are an easy way to distinguish them from other small insects like scales and whiteflies.
Although all of these pests have a small, soft body with six legs, the different species vary in appearance. If you’re used to seeing one type in your garden, you might not recognize the pests when a different species appears. Knowing the species that regularly attack lettuce plants can help you properly ID the pests.
Aphids that Commonly Attack Lettuce
There are hundreds of aphid species, but not all attack lettuce crops. The following types of aphids are some of the most common lettuce pests. Although control and treatment practices are the same across species, knowing which species you’re dealing with can be helpful when learning about reproduction rates and host plants.
Green Peach Aphid
Although they have peach in their name, these pests commonly attack vegetable crops. Green peach aphids (Myzus persicae) are normally light green, but some individuals may appear pale pink. Their reproduction varies depending on location; they give birth to live young in warmer climates and lay eggs that overwinter in colder areas.
These insects reproduce rapidly, producing up to 30 generations each year! That means treating them as soon as they occur is essential to maintain healthy plants.
Potato Aphid
Potato aphids (Macrosiphum euphorbiae) are one of the largest aphid species; adults can grow just over an eighth of an inch long. The adults are often bright green, but they may also be pink. Their large size and fast movement are the easiest ways to distinguish them from other species.
Currant-Lettuce Aphid
Currant and gooseberry plants are the primary target of these pests, but they’ll also attack lettuce. They prefer tender lettuce leaves, so they often remain deep in the plants where they’re difficult to spot.
Monitor Regularly
You may have heard that these insects can reproduce quickly, but knowing just how fast they multiply will help you understand why early detection is so important.
Aphids can reproduce sexually and asexually, allowing them to adapt to changes in temperature and grow their populations regardless of the time of year. They typically reproduce asexually during the warmest times of the year, giving birth to live young and creating a rapid increase in population.
For example, green peach aphids can produce multiple nymphs per day during the summer, and these nymphs mature to adults in as little as a week. That means a dozen adults can balloon to thousands in a few weeks.
How to Monitor
Since these pests multiply rapidly, spotting them soon after they appear is the best way to keep them under control. These tiny pests can easily hide, so a casual walk through your garden might be enough to notice if they are present. Instead, set aside at least one day a week to scout your lettuce beds intentionally.
Different aphid species prefer various parts of the lettuce crop, so make sure to look at all parts of the plant. Inspect the outer leaves, then peel them back to examine the tender core.
Note the extent of the infestation in terms of the number of plants infected and the number of aphids on each plant. These observations can help inform you what control method is most appropriate.
Avoid Applying Excessive Fertilizer
Lettuce requires a moderate amount of nitrogen to produce healthy leaves, so adding fertilizer is sometimes necessary. However, adding too much fertilizer can cause harm, especially if you’re using a high-nitrogen product.
Pests are more likely to become problematic in lettuce that is growing in soil with excessive nitrogen. Therefore, you should only apply the amount of nitrogen that plants need.
A slow-release product like a well-aged compost or feather meal is a better alternative than synthetic nitrogen sources. Since the organic products contain forms of nitrogen that are unavailable to plants, they rely on microbes to convert the nutrients into a plant-available form. This happens slowly over time, so your plants are less likely to end up with an overabundance of nitrogen.
Grow Plants that Support Natural Predators
One of the best ways to keep aphids in check is to foster an environment that encourages natural predators. Many insects feed on the soft-bodied insects, so supporting these predators helps keep pest populations low and limits the need to control them with sprays.
Ladybugs, green lacewings, hoverflies, soldier beetles, minute pirate bugs, and parasitic wasps all feed on aphids. However, not all life forms of these predators feed on them. For example, hoverfly and green lacewing larvae are voracious predators, but the adults mainly feed on pollen and nectar.
Growing a diversity of flowering plants will help support the adults, encouraging them to stick around and lay more eggs in your garden. Many predator insects are especially drawn to plants with small flowers. Therefore, the following plants are good options for supporting beneficial insects that keep pest populations in check.
- Sweet alyssum
- Dill
- Yarrow
- Cilantro
- Tansy
- Buckwheat
- Sunflowers
- Queen Anne’s lace
Choosing plants that bloom from spring through fall will keep the adult predators well-fed throughout the growing season. You can also plant multiple successions of dill and cilantro to provide a continuous supply of flowers.
Avoid Broad-Spectrum Insecticides
If you’re lucky enough to have a healthy population of beneficial insects in your garden, the last thing you want to do is kill them. Unfortunately, many common insecticides don’t differentiate between good and bad insects.
If you decide that pest outbreaks warrant the use of insecticides, choose targeted options and apply them carefully. Products like bifenthrin (Sevin) and spinosad can kill a wide range of insects, so you should only use them when a better option doesn’t exist. If you choose to use an insecticide that kills many species, spray it so it only comes into contact with the pests of concern.
Since broad-spectrum insecticides can inadvertently kill beneficial predatory insects that feed on pests, avoid using them when possible. A better option is to choose products that only affect a certain group of pests. For example, Bt only kills caterpillars that feed on sprayed foliage.
Keep a Close Eye on Covered Growing Areas
Since covered growing areas are often warmer than outdoor gardens and exclude natural predators, they often allow aphid populations to explode. I recommend keeping a close eye on high tunnels, cold frames, and plants under row cover.
Check the plants growing in these covered areas at least twice a week. If you spot any of the pests, take care of them ASAP.
Regularly venting these spaces will help prevent temperatures from increasing too much, which will slow insect reproduction rates and keep your crops happier. It will also allow beneficial insects to enter and feed on the pests.
Treat as Necessary
If you notice aphids on your lettuce, taking swift action is the best option. Although natural predators may eventually kill the aphids, spending a few minutes removing them from your garden will discourage larger outbreaks.
The best control method depends on the size of the outbreak and your personal preferences. Here are a few options.
Release Biocontrols
Although beneficial insects may be naturally present in your garden, you can also bolster their presence with plantings. Biocontrols won’t remove large populations immediately and typically work best as a preventive measure. However, pairing these biocontrols with other treatment methods can help keep the pests from returning.
General predators like green lacewings, assassin bugs, and ladybugs slowly curb aphid populations, so they work best for small outbreaks or prevention. Parasitic wasps target more pests, making them a better choice for larger outbreaks. However, they work best when the temperature is between 60-80°F (16-27°C).
Remove Infected Leaves or Plants
If you notice a few aphids on an outer leaf, you can remove the leaf and the pests from your garden. Similarly, you can remove infested lettuce plants if only one or two plants are covered.
This control method works best if the sap-sucking insects are localized to a few plants. If the pests are on the majority of your lettuce plants, choose another control method.
Spray With a Hose
Aphids’ small and fragile bodies make them easily damaged. Spraying the pests with a steady stream of water can wash them off your plants and prevent them from feeding. You want the water to be strong enough to remove them but not so strong that it damages the plant.
You’ll likely need to spray your plants repeatedly, so continue to check them and repeat as necessary. Spraying in the morning is ideal since it will give your plants time in the sun to dry.
Apply Organic Insecticides
If you’re dealing with a large outbreak, applying insecticides may be your best approach. Neem oil and insecticidal soaps are two effective options.
Neem oil works by stopping feeding and disrupting insect hormones. They must feed on or come into contact with the oil in order for it to be effective. Since it doesn’t kill them immediately, don’t be alarmed if you see them a few days later.
Insecticidal soap is a knock-down spray that kills these pests right away. It works via suffocation, so it must contact the pests to be effective.
Both of these products can intensify the sun’s rays, potentially causing burnt leaves. Therefore, you should always spray in the evening or on a cloudy but dry day. No matter which product you’re using, make sure to read the package instructions and apply as intended. Avoid spraying when pollinators are most active.
You may need to reapply the product if it doesn’t kill all of the aphids on the first try. Spraying seven to ten days later is a good approach.
