Common Plant Pests & Organic Control Options
We’ve posted several in-depth articles on pest control here on our Seed Hopper blog before, and you can find all of them archived in the “Plant Pests” category in the left-hand navigation column. They include coverage of Striped Cucumber Beetles, Colorado Potato Beetles, Japanese Beetles and Bloat Nematodes. Our “What’s Wrong with My Garden?” series goes over many common plant pests including: Aphids, Cabbage Maggots, Cabbage Worms, Earworms, Hornworms, Slugs, Snails, Squash Bugs and Vine Borers. Also in our blog post archive, Kate Spring of Good Heart Farmstead offers sound advice on how to take a holistic approach to pest management in her article, Pest Management on the Small Farm.
This post will go into some detail on pests that we have not covered before – some of which are extremely common and widespread, and others that are just beginning to emerge and affect certain parts of the U.S. and Canada. To jump to a certain section, click on the name of the pest you’re looking to learn about: Cutworms · Whiteflies · Mexican Bean Beetle · Leaf Miner · Leek Moth
Cutworms are some of the sneakiest bugs out there. They take cover just under the soil surface during the day, only emerging at night to feed on your crops. They especially like tender, newly planted seedlings. Their tell-tale mark is a clean cut at the base of a plant, toppling the seedling entirely (thus the name), but you can also see them munch along the foliage if the stem of a plant is too thick to gnaw through. The term “cutworm” is a general reference to several different species of night-flying moths. The larval stage of these moths feed on different agricultural crops, depending on their species.
Species: Variable; different species target different crop types.
Crops: Brassicas (broccoli, Brussels sprouts, cabbage, cauliflower, kale, kohlrabi), corn, peas and beans
Life cycle: One generation per year. Eggs or young larvae overwinter in the soil and emerge in early spring to feed. Development to moths occurs in August or early September, when they mate and lay eggs just below the surface of the soil before freezing temperatures occur.
Diagnosis: Look for cut stems, holes in the foliage, or edge damage on foliage.
Prevention: Crop rotation is key to avoiding attacks from cutworms, since the eggs and larvae overwinter in the soil. Cover cropping can also help starve a cutworm population if the cover crop is a plant species that the cutworm does not eat. If you practice an animal rotation, putting pigs or chickens in an area where plants have been targeted by cutworms will help eliminate them when the animals root up the area searching for food.
Mitigation: If cutworms are already present in your fields or garden, there are a few organic controls you can attempt. As with all pest mitigation, damage is best controlled when the pest is detected as early as possible, so effective and frequent scouting is key. For gardeners growing organically on a small scale, coffee grounds and eggshells scattered around the base of the plant will deter cutworms. Cutworms can also be stopped from reaching the tender stems of young seedlings by wrapping each stem with damp newspaper or placing a piece of cardboard around each plant stem. (This may not be practical for those growing on a larger scale.) For commercial operations or larger scale growers who are certified organic, diatomaceous earth is an OMRI-approved application that will kill cutworm larvae. Nighttime applications of the biological control Bacillus thuringiensis has also been shown to be an effective organic control. Parasitic nematodes like Steinernematidae carpocapsae are a successful control agent for cutworms as a natural predator. Adequate conditions, especially appropriate soil moisture levels, should be in place before attempting to colonize a population of beneficial nematodes as an organic control measure.
Whiteflies are related to aphids and are similar in size and scope of damage. Like aphids, they are sap feeders that suck plant nutrients from the stems and foliage, reducing the overall vigor of the plant and sometimes making photosynthesis impossible. Also like aphids, their life cycle is short and without a dormant stage that can withstand cold temperatures, so their populations can build quickly, especially under favorable conditions like greenhouses which provide protection, warmth and humidity.
Species: Variable; the most common species that cause agricultural crop loss include Aleyrodes proletella (cabbage whitefly), Bemisia tabaci (silverleaf whitefly) and Trialeurodes vaporariorum (greenhouse whitefly).
Crops: Nightshades (tomatoes, peppers, eggplant, potatoes), sweet potatoes and lettuce
Life cycle: Approximately 3 weeks (multiple generations per season); adult females can produce up to 400 eggs which hatch between 1 and 4 weeks after laying.
Diagnosis: Because whiteflies are so small, you may not notice them on your plants at first if the population has not exploded. They tend to like to hang out on the underside of leaves, so physically moving the leaves around and brushing them from below can help expose a population of whiteflies, which will fly up as soon as they are disturbed. They also leave a sticky substance behind when they feed, which is easy to feel on plant foliage. Their eggs appear as tiny white dots on the undersides of leaves.
Prevention: Crop rotation with fallow years, cover cropping, and diligent sanitation techniques to avoid cross-contamination can help prevent populations of whiteflies from establishing. Greenhouses in particular are susceptible to whitefly infestations and should be rotated and cleared of all infected plant debris from season to season.
Mitigation: Gardeners with whitefly populations can try blasting the whiteflies off the affected plants with a garden hose, followed by an application of an OMRI-approved insecticidal soap. For certified organic growers, currently there are some parasitic insects that can be used to suppress whitefly populations through natural predation including Encarsia Formosa, Eretmocerus eremicus, and Delphastus catalinae. These biological controls are only effective on whiteflies when applied at the whiteflies’ egg or larval stage, so they should be released as early as possible for the greatest success.
MEXICAN BEAN BEETLE
Species: Epilachna varivestis
Crops: Legumes (beans, peas, soybeans, alfalfa, clover)
Life cycle: Adult beetles overwinter in areas with foliage cover, usually on the edges of fields and gardens, under brush or leaves. Emergence coincides with warm weather in the spring. Females lay their eggs on the underside of legume plant leaves, and the eggs hatch within 1-2 weeks. Larvae feed for 2-5 weeks while they mature to the adult stage. Adults are strong fliers and can travel significant distances to find new feeding areas. The full life cycle takes 30-60 days, depending on daytime temperatures. Depending on climate, there can be 2-3 generations per season.
Diagnosis: Scouting early and often is key to diagnosing a bean beetle infestation early. If you can catch the damage from overwintered adults first, you’ll have the best chance of suppressing a potentially massive population of Mexican Bean Beetles. Leaf damage will likely be seen as soon as daytime temperatures are consistently above 60°F. The beetles feed on the underside of the leaves, causing a lacey or skeletonized appearance. The beetles’ eggs are recognizable and noticeable as long as you’re looking for them. They are bright orange and attach to the underside of leaves by a single point on the end of the egg so they stand vertically upright. Larva are also highly recognizable due to their distinctive bright yellow color and spiny outer shell. Fully mature adults look very similar to common ladybugs (they are related), but with a more rusty, auburn color instead of bright red.
Prevention: Crop rotation helps with preventing bean beetles from finding their preferred crops when they emerge from overwintering. Avoid planting legume crops in the same area in consecutive years. Cover cropping with a rotation of non-legume cover crops will also help starve an overwintered population of bean beetles. You can also take measures like keeping the edges of your fields cleaned and cut short to avoid hosting overwintering beetles, and planting legume crops later in the spring to avoid the first emergence.
Mitigation: There are a few options for biological control that are safe to use in organic systems. A parasitic, non-stinging wasp, Pediobius foveolatus, will suppress Mexican Bean Beetle populations by laying its eggs in the beetle larvae. The tachinid fly, Paradexodes epilachnae, has also been found to reduce numbers of Mexican Bean Beetles. For best results, these biological controls should be released as soon as crop damage from the beetles is observed. Hand-picking adult beetles, larvae and eggs off the leaves and killing them is also an option for organic pest control, although may only be practical on a small scale.
Species: Variable; leaf miner are the larval stage of many species of flies, often difficult to distinguish in the field. The most common species that affect vegetable crops are spinach leaf miner (Pegomya hyoscyami), beet leaf miner (Pegomya betae) and vegetable leaf miner (Liromyza sativae).
Crops: Spinach, beets, Swiss chard, lettuce, tomato, cucumber, celery, bean, eggplant, pepper, potato, squash, water melon, cucumber and peas.
Life cycle: The pupae of all leaf miner flies overwinter in the soil and emerge as mature flies when the weather warms enough in the spring. From this first emergence, several generations cycle through in one season at a rate of about 30-40 days per generation from egg to mature adult fly.
Diagnosis: Because the eggs are so small and are inserted into the plant tissue when they are laid, they are difficult to scout. The first sign of leaf miner presence is usually the tell-tale damage of serpentine “mines” that look like white or transparent lines winding their way through leafy green foliage. This is evidence of the larvae eating the mesophyll between the surfaces of the leaf, giving it a see-through or colorless appearance.
Prevention: Many common weeds act as host plants for leaf miner, including lamb’s quarter and amaranth, so weed control is essential to prevent harboring leaf miner populations. Crop rotation is the other primary defense for avoiding an infestation of leaf miner. If you are certain your susceptible crops have been planted in an area free of leaf miner pupae, row cover is also an option for protecting crops from leaf miner attacks. (However, if you use row cover on an area where pupae are present in the soil, it will not prevent damage to the plants.)
Mitigation: Once leaf miner are already in the leaf of a plant, they are very difficult to control because insecticides and other applications cannot reach them within the leaves. Removing affected leaves from your growing area entirely can help control populations. Disrupting or tilling the soil in early spring before planting can eliminate any pupae that have overwintered in the fields. Some parasitoids or naturally occurring enemies can be released, but their effectiveness is often determined by which leaf miner species you are trying to target. Although leaf miner damage generally creates only an aesthetic issue, it can make crops unmarketable. Because of this potential crop devaluation, preventative measures should be taken first and foremost to avoid damage in the first place.
Species: Acrolepiopsis assectella
Crops: Alliums (leeks, onions, garlic)
Life cycle: Adults overwinter in plant debris or on the edges of fields. In the spring, females lay eggs at the base of a host plant. Larvae grow and feed over the course of two weeks, then pupate for 10 days before emerging as adults. There are usually two to three generations per season. Females lay up to 100 eggs over a 3-4 week period, so generations can be staggered. The larval stage is the most damaging to allium plants, when the larvae move toward the center of their host plants where young leaves are formed to feed.
Diagnosis: Leek moths are also called onion leaf miners, as their form of attack on plants is similar to those of fly leaf miners. Damage is done by the larval stage of the moths, when they are in the form of a caterpillar. Feeding damage can be seen at the growth point of allium plants, often accompanied by the caterpillar’s excrement and plant debris. This damage stunts the plants’ growth and can introduce rot. Although plants can survive this feeding damage, storage life of onions and garlic is often greatly reduced. Regular scouting by checking the growth points of allium plants can help determine if a population of leek moths is present in your growing space. University researchers may also provide leek moth traps (which attract the moths with a pheromone) if you are in a region of potential risk, which will help track the spread of this pest. Currently, leek moths have been observed in Ontario, Quebec, upstate New York and Vermont.
Prevention: Crop rotation is critical to avoid establishing and maintaining populations of this pest. Removing plant debris from your fields and gardens to avoid hosting the moths over the winter will also help diminish risk. Using row cover immediately after planting can also help protect allium plants from the pest if you’re in an area of concern (since moths are nocturnal, the cover can be removed during the day for weeding and cultivation).
Mitigation: Because the spread of this pest is fairly recent (the first observed leek moth in the continental U.S. was found in Plattsburgh, NY in 2009), there are not many biological or organic controls that have proven effective at mitigating the damage of leek moths once they are present. Hand picking and crushing the larvae and pupae of the pest is an option for smaller scale operations. Broad spectrum OMRI-approved sprays and Bt have been used with some success if applied at the larval stage. Pheromone traps for leek moths can be helpful in determining the proper timing of these applications. There is some research in Europe and Canada to find a parasitoid that is a natural enemy to the leek moth, but none have been released in the U.S. yet.
Now, if you want a real scare, read about the Brown Marmorated Stinkbug which targets, well, just about everything.
Do you have advice on how to prevent or control these or other pests? Share your insights in the comments below!