The term “corn flies” is a common, non-scientific label used by gardeners and small farmers to describe flying insects that damage corn, mainly the adult Western Corn Rootworm and the Seedcorn Maggot fly. These pests can cause significant damage, ranging from poor stand establishment to reduced nutrient uptake and plant lodging. Effective management requires a two-pronged approach: immediate action to control current infestations and long-term planning to prevent future outbreaks. This guide provides practical strategies to eliminate these pests and safeguard your corn crop.
Identifying the Culprit
The two most likely culprits are the adult Western Corn Rootworm (WCR) and the adult Seedcorn Maggot fly (SCM). Correct identification is necessary because their life cycles and damage patterns dictate the best treatment.
The adult WCR is a beetle, about a quarter of an inch long, typically yellow-green with three distinct black stripes. Active from mid-summer through early fall, WCR adults primarily feed on corn silks and pollen, leading to poor kernel set. Their larvae are the most destructive stage, feeding directly on the corn roots below ground.
The Seedcorn Maggot fly is a slender, light gray fly, smaller than a housefly, peaking in activity during the cool, wet conditions of early spring. The adult fly lays eggs in the soil, especially where fresh organic matter, like manure or newly tilled cover crops, is present. The resulting maggot larvae are legless, yellowish-white, and burrow into corn seeds and seedlings, often causing missing plants or slow, spotty emergence.
Immediate Non-Chemical Control
Immediate non-chemical methods can provide quick relief for active infestations. Yellow sticky traps are effective for monitoring and capturing adult flies and beetles. Placing these traps near damaged plants helps reduce the flying population and indicates the severity of the problem.
Adjusting the soil moisture level disrupts the life cycle of the Seedcorn Maggot, which thrives in damp, cool environments. Allowing the topsoil to dry out slightly between waterings makes the environment less hospitable for egg-laying and larval development. For young plants, physical barriers like fine-mesh row covers can be placed over seedlings immediately after planting to prevent adult flies from accessing the soil. In small garden settings, handpicking adult WCR beetles from corn silks can reduce silk clipping and subsequent egg-laying.
Utilizing Targeted Treatments
When physical controls are not sufficient, targeted treatments focus on biological and low-toxicity product applications. Beneficial nematodes, such as Steinernema feltiae, are a biological control option applied to the soil to target the larval stage of pests like the Seedcorn Maggot and the rootworm. These microscopic roundworms naturally seek out and parasitize the soil-dwelling larvae, offering a long-term reduction in the next generation of pests.
For active adult flies and beetles, low-toxicity insecticides can be used judiciously. Products containing pyrethrins, derived from chrysanthemum flowers, offer quick knockdown of adult WCR beetles and SCM flies upon contact. Spinosad, a naturally derived product from a soil bacterium, is effective against chewing insects like the adult WCR beetle with a low impact on many beneficial insects. These foliar sprays should be applied in the late evening or early morning to minimize risk to foraging pollinators. Application timing should coincide with the peak emergence of the adult pests.
Long-Term Prevention Strategies
Preventing future infestations requires implementing cultural practices that disrupt the pest’s life cycle before planting. Crop rotation is one of the most effective strategies against the Western Corn Rootworm, which relies on continuous corn planting for its survival. Introducing a non-host crop, such as soybeans or alfalfa, for at least one year breaks the rootworm’s life cycle, as the larvae hatch but cannot find corn roots to feed on.
Managing plant residue and debris is another preventive measure, particularly for the Seedcorn Maggot, which is attracted to decaying organic matter for egg-laying. Incorporating cover crops and manure at least two weeks before planting allows the material to break down sufficiently, making the soil less appealing to the adult SCM flies. Planting corn seeds when soil temperatures are warm and conditions favor rapid germination helps the seedlings outgrow the vulnerable stage before maggot feeding can cause severe damage.