When spring begins, we eagerly head outside to plant all the crops we love. As we care for our plants, we start to envision the harvest to come. But some pests can ruin our crops and shatter our dreams.
One such pest are bean stem maggots or bean flies.
Arm yourself with knowledge about this devastating crop intruder. Read on.
What Are Bean Stem Maggots?
Bean stem maggots, or bean flies, are common pests that invade many horticultural and agricultural spaces throughout North America during the spring and fall. They stick to legumes, which are plants in the family Fabaceae or Leguminosae.
These pests are flying insects in the Ophiomyia genera, which is in the Agromyzidae family. There are three main species that you must watch for. These are O. centrosematis, O. spencerella, and O. phaseoli. They look similar and cause similar damage.
These are some of the most damaging pests to bean crops across the globe and cause a huge amount of yield and income loss.
The various species are drawn to specific species of legumes more than others.
O. phaseoli is considered to be the most damaging of the bean stem maggots. It likes to invade many legumes, but in particular cowpeas (Vigna unguiculata), common beans (Phaseolus vulgaris), and soybeans (Glycine max).
O. spencerella attacks common beans (Phaseolus vulgaris) as well as lima beans (P. lunatus), rice beans (V. umbellata), and mung beans (V. radiata).
O. centrosematis isn’t fussy. It’s able to feed on most of the Fabaceae family species. Should butterfly peas (Clitoria ternatea) or cowpeas be on offer, they will colonize those first
How To Identify This Pest
The best way to identify bean stem maggots and flies is to examine your bean or pea crops.
They typically do not travel far from their main food source and reproductive grounds unless there are insufficient host plants available. So if they’re around, you’ll find them on your beans, peas, clover, lupins, lentils, or vetch.
As an adult, the bean stem fly has a blue-to-black body with opaque wings. In certain light, these seemingly unremarkable wings appear to be iridescent or rainbow-streaked. They are shiny, with compound eyes, and two antennae, which are used for navigation.
Bean stem maggots are approximately 5/16 inches in size and can appear creamy white or off-yellow in color.
Typically, the white, round eggs are hard to see with the naked eye; they average 1/16 inches in diameter and are placed on the underside or in crevices of the plant for protection.
Lifecycle
The bean stem fly’s lifecycle is categorized into four stages of development. Each growing season, the bean stem fly can produce from three to six generations.
If hatching or metamorphosis should occur in less favorable weather conditions, this pest can overwinter and kick-start the new generations when the timing is right in the spring.
1. Egg Stage
This pest’s life journey begins with the female adult fly laying her eggs in clusters on the host plant’s foliage and stems. The female adult fly can make indents on the foliage, or will take advantage of any damaged areas in which to better secure the eggs
These tiny eggs are white and elongated. Within two to four days, the eggs hatch, and the larvae (maggots) start feeding on plant material.
2. Larval Stage
The larval stage is the most problematic stage, as the maggots feed heavily during this period in order to mature and develop through three instars.
Factors such as the weather, health, and age of the bean plant may influence the duration of the larval stage. If food and temperatures are right, this stage can last one to two weeks.
Once they mature, the maggots’ outer skin hardens and forms a protective shield. Now they’re entering the pupal stage.
3. Pupal Stage
Depending upon the species of bean stem fly, the color of the pupal outer shell can vary. This is a reliable way of distinguishing between the species.
Ophiomyia phaseoli turns dark brown, and O. spencerella is shiny and black.O. centrosematis is more red-to-orange-toned.
Typically, these pupae or cocoons can be found just under the soil surface. If the health and age of the host plant allows, they may be attached to the physical structure of the plant.
Cold weather triggers the larvae to find cracks in the soil or dead plant material to burrow in and wait out the colder months in their pupal casing.
This stage lasts for a week to ten days in prime conditions and early on in their breeding season. Here, the maggots metamorphosis into fly form through cellular reconfiguration, developing wings, and the fly characteristics of bulbous eyes, sectional body, and six legs.
4. Adult Stage
Even though the bean stem fly’s life span is only a few weeks, they are active reproducers during this time.
After all, it is imperative for the species’ survival to lay as many eggs as possible, increasing the maggot’s potential for survival into adulthood and further reproduction.
Soon after emerging from their pupa, they actively seek out a mate. As active flyers, they are equipped to travel to locate not only mates but also new prospective host plants for their offspring.
Adult flies also feed on plant sap, excretions from other pests, and honeydew.
Signs Of An Infestation
Recognizing the key signs and symptoms of a bean stem maggot or fly infestation is crucial for early detection and prompt management.
1. Leaf Mining
Visible winding tunnels or mines through the leaf layers is a sure sign of problems. This tunneling can also give the foliage a bruised look, with discolored or darkened patches. This is a clear sign of the feeding activities of the maggots.
2. Stippling or Yellowing of Leaves
The presence of fine stippling or small yellow spots on the leaf surface is another common sign. This damage is also caused by the feeding of the maggots. As nutrition is depleted from the plant foliage, it is unable to photosynthesize and retain vigor.
3. Reduced Plant Growth and Yield
Depending upon the age and size of the host plant, the maggots can rapidly interrupt new growth and crop development. Any plant under sustained attack will eventually be unable to put enough energy into new shoots or produce.
The younger the plant and the less developed it is, the quicker the stunting will become evident. Since its roots are still developing and its resources are limited, young plants are more dramatically impacted.
4. Wilting or Drooping Leaves
Wilting, curling, or drooping of leaves is a common symptom of a severe infestation. As the maggots mine through the layers of the plant it leads to cellular collapse.
5. Pest Presence
The presence of adult bean stem flies around any plants belonging to the Fabaceae family is a warning that there are likely to be eggs or maggots present.
Check the underside of leaves to confirm the presence of eggs or pupae and take immediate steps to limit further damage.
6. Plant Defoliation
When feeding has been extensive, the internal connective tissue may be so severely damaged that the leaf will drop to the ground.
Similarly, sparse foliage or leaves that appear to be skeletonized are all hallmarks of a severe infestation.
Preventing an Infestation
Several strategies can be implemented to protect susceptible host plants. Due to this pest’s disruptive nature, you may want to implement several techniques to curb the problem and break the pest cycle.
1. Monitoring
It all starts with monitoring. Regularly inspecting your Fabaceae plants will give you the warning you need to take action before an infestation sets in.
Look for all the symptoms we talked about above, but by the time symptoms show up, it means the pest is already present. To monitor for their arrival, use yellow sticky traps and check them daily.
The second you see bean flies in the traps, it’s time to start treating.
2. Crop Rotation
Rotate bean and peas with non-host crops every other year to break the life cycle of the bean stem maggot and fly. Anything that isn’t in the bean or pea family is fair game. Corn, potatoes, tomatoes, cucumbers, alliums, and peppers are all safe.
3. Sanitation
Proper garden sanitation is vital to reduce the risk of this pest moving in. Remove and destroy crop debris after harvesting to eliminate potential breeding grounds and overwintering sites.
If a severe infestation has occurred, it may be prudent to dig out the entire plant and dispose of it or destroy it.
4. Protection
It’s possible to protect your plants from the adults in the first place. Floating row covers are an important tool in the gardener’s toolkit. Put them in place when planting.
Don’t use floating row covers if there’s a chance that the pest is overwintering in the soil. If you put row covers over infested soil, the pests will just hatch and hang out under the cover.
That’s why crop rotation is important. If you rotate your crops, then you can be sure the pest isn’t living in the soil, and your row covers will be successful.
Buy row covers at Amazon.
5. Plant Resistant Cultivars
Purchase and plant varieties of beans or peas that have been developed to resist the bean stem maggot.
‘Provider’ is a bush type known for its resistance to several diseases and bean stem maggots. It’s a popular choice among growers for its productivity. Find a packet of heirloom seeds at Amazon.
‘Strike’ is another bush bean variety. This one is an early cropper and high-yielding heirloom. Snag a pound of seeds at Amazon.
A black-seeded bush bean, ‘Raven’ is favored for its dark, glossy pods and great flavor.
‘Eureka’ offers the growers a different color option. It’s a yellow wax bean cultivar that produces long, straight pods. It’s known for it’s high yields.
‘Slenderette’ is a traditional, bush-type bean. It provides slender, stringless pods and has an excellent texture and flavor. Bring home a packet from Amazon.
Remember that resistance doesn’t mean immunity. These plants can still be infested but tend to tolerate an infestation better than more susceptible types.
6. Trap Cropping
Plant trap crops such vetch or clover around the perimeter of the Fabaceae beds. This will hopefully attract bean stem maggots, reducing their population within the main crop.
The idea is that they won’t try to travel further into your garden because they found food readily available on the perimeter. Once you see an infestation in your trap crop, dig them up and dispose of them.
Treatment Options
You’ve done your best to avoid an infestation, but despite your efforts, bean stem maggots found your legumes. Now it’s time to treat the problem. Start with environmentally friendly, organic options like biological control.
The good news is that both the bean stem maggot and adult fly are soft-bodied, making them susceptible to a range of treatments.
Attracting natural predators to the garden to control the number of bean stem maggots and reproductive adult flies will help reduce impact and benefit the garden as a whole.
Parasitic wasps (braconid wasps, Ichneumonid wasps), predatory beetles like ground beetles, rove beetles, predatory flies such as hoverflies and dance flies are effective.
You can also encourage hungry predators like birds, lacewings, and ladybugs. Or, purchase and introduce beneficial nematodes, which will kill the larvae in the soil.
You can also use neem oil or insecticidal soap to kill the adults and maggots, but the pest needs to come in contact with the product, so you’ll need to reapply often.
If none of that works, you can use a product that contains pyrethrin or spinosad. Only spray these when bees and other pollinators aren’t present and on a calm day. Keep in mind that these products will also kill beneficial insects.
Avoid the use of neonicotinoids because they cause more harm than good in the home garden.
