Bean Fly

The bean fly, also known as Ophiomyia phaseoli, is an insect that inflicts damage to the stems of plants and has the ability to cause early death, particularly in seedlings.

Hosts

The bean fly targets a wide range of leguminous crops, including common bean, cowpea, French beans, and various others.

Life Cycle

The female flies of the bean fly species lay their eggs in close proximity to the leaf stalks of delicate and young leaves. These eggs subsequently hatch into maggots, progressing through three stages over a span of approximately 10 days. Afterward, the maggots enter the pupal stage, typically occurring at the junction between the stem and roots in younger plants, or at the junction between the leaf blade and leaf stalk in more mature plants. The pupation period lasts around 10 to 12 days, varying based on temperature conditions.

Identification

The adult bean flies are relatively small, measuring approximately 3mm in length. They have a shiny black-bluish appearance, with clear wings that are about 5mm wide.

The eggs of the bean fly are minuscule and possess an oval shape, appearing white in color.

As for the larvae or maggots of the bean fly, they are cream-colored and have dark mouthparts. Their length is around 3mm.

The pupae of the bean fly are small and cylindrical, exhibiting a brown hue. They have rounded ends.

The following factors contribute to the prevalence of bean fly infestations:

  • The presence of crop debris from previous infestations.
  • The absence of crop rotation, leading to a continuous environment suitable for bean flies.
  • Inadequate sanitation practices, specifically the improper removal of legume and weed debris from previous seasons.
  • The use of contaminated seeds that carry bean fly eggs or larvae.
  • Planting susceptible varieties of leguminous crops that are more prone to bean fly attacks.

Feeding & Damage

Both adult bean flies and their larvae are responsible for causing damage.

Crops are most susceptible to bean fly infestations for a period of approximately three to four weeks after emergence. However, there are instances where later crops can also be attacked.

After hatching, the larvae begin to feed and create numerous larval mines. These mines are more visible on the underside of the leaves, just beneath the epidermis, appearing as silvery and curved stripes. On the upper side of the leaf, only a few tunnels are noticeable. As time passes, both the egg holes and larval mines darken and become distinctly visible, turning a dark brown color.

In severe cases of infestation, the affected leaves become patchy and eventually droop. These leaves may dry out and may even fall off.

When mature plants are infested, the damage caused by the insects is limited to the leaf petioles, resulting in swelling and, at times, wilting of the leaves.

The larvae of the bean fly, during their second and third stages of development, create tunnels downward into the cortex, just beneath the epidermis. The third-stage larvae continue feeding by moving further down into the taproot. Eventually, they return to the stem to pupate, typically near the soil surface. These feeding tunnels are clearly visible on the stems.

In cases where the larval population is high, their feeding activity can lead to the destruction of the cortex tissue around the junction where the root and shoot meet. This initial damage causes the leaves to turn yellow, inhibits plant growth, and can even result in plant mortality.

If the damage inflicted by the larvae is less severe, the area around the root-shoot junction appears swollen. In some instances, the host plant responds by producing adventitious roots above the swollen area on the stem.

When the adults feed and lay eggs, they create holes in the young leaves. The female bean fly pierces the leaves to deposit eggs and also feeds on the sap that oozes out. These activities leave yellow blotches on the leaves, which serve as the initial signs of bean fly infestation. These yellow blotches can be useful early symptoms for monitoring the presence of the pest in the field.

The severity of the damage caused by bean fly infestations is amplified in plants growing under unfavorable conditions, such as infertile soils and drought.

Control

Chemical method

For achieving effective control of bean flies, the utilization of systemic insecticides is recommended.

Some examples of systemic insecticides suitable for bean fly control include:

  • KINGCODE ELITE 50EC
  • LEXUS 247SC
  • SINOPHATE 750SP
  • EMERALD 200SL
  • LOYALTY 700WDG
  • PROFILE 440EC
  • PRESENTO 200SP

These systemic insecticides can aid in managing bean fly infestations and protecting the crops from damage.

Please take note of the following:

  • Planting seeds that have been treated with SHIELD 600FS helps protect them from bean fly attacks.
  • When applying foliar sprays, it is advisable to mix the insecticide with INTEGRA at a rate of 3ml per 20 liters. INTEGRA acts as a sticker, spreader, and penetrant, enhancing the effectiveness of the insecticide.
  • To prevent the development of insecticide resistance in bean flies, it is recommended to alternate between several insecticides throughout the crop’s season, rather than relying on a single one.
  • Timely application of the chosen insecticide(s) is crucial for effective control of bean flies.
  • It is recommended to monitor seedling crops at least twice a week to promptly detect any signs of bean fly infestations.

Non-chemical methods

  • Opt for early planting as bean fly populations are typically low in the early stages of the growing season, increasing over time.
  • Maintain field hygiene and sanitation practices to minimize favorable conditions for bean flies.
  • Create favorable growing conditions to enhance plant vigor and increase tolerance to insect attacks and damage.
  • Implement mulching techniques, such as using straw or cut grasses, to conserve moisture, stimulate adventitious root development, and improve tolerance to maggot damage.
  • Employ crop rotation with non-host crops to disrupt the bean fly lifecycle and reduce infestation risks.
  • Ensure proper weed control to eliminate potential alternative hosts for bean flies.
  • Promptly remove and dispose of crop residues and any plant parts displaying signs of bean fly damage.
  • Perform earthing/building up of soil around the plants approximately 2-3 weeks after emergence, covering the roots. This practice supports faster growth of adventitious roots.
  • Select resistant varieties of leguminous crops that exhibit natural resistance to bean fly infestations.
  • Use sticky traps to capture adult flies and monitor their presence in the field.

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