The term leafminer is commonly used to describe flies, moths, sawflies or beetles in the larval stage. However, leafminers that feed on vegetables most commonly belong to the order Diptera – the flies.
Leafminers feed on the mesophyll tissue between the upper and lower surfaces of leaves. This region of the leaf is where the plant converts light to energy through the process of photosynthesis.
Economic crop damage occurs most often in vegetables harvested for edible foliage, such as spinach or chard. There are three primary garden leafminer pests in the state of Wisconsin: pea (Liriomyza huidobrensis, Blanchard), vegetable (Liriomyza sativae, Blanchard) and the spinach (Pegomya hyoscyami, Panzer). Increasingly a fourth, serpentine (Liriomyza brassicae, Riley), has become more common in greenhouse settings. Growers should note that many of these leafminer species do not persist at economically damaging levels in the state and may be a sporadic pest on vegetables. Significant problems may arise when transplants are sourced from southern regions. Many regions of the southeastern United States have large, persisting leafminer populations with significant insecticide resistance problems. Growers in Wisconsin should take great care to inspect all transplant material sourced from outside the Midwest ensuring plants are healthy and free of leafminer, other arthropod pests and plant pathogens.
Life cycle and Appearance
Fly (Dipteran) leafminers have a very similar life cycle among several species. Leafminers have a relatively short life span that is temperature dependent. In Wisconsin, overwintering species pupate in the soil or in leftover crop residue. Adult flies emerge in the spring and lay eggs below the leaf surface of susceptible host plants. When eggs hatch, the larvae immediately enter the leaf and begin to consume the mesophyll tissue between the upper and lower leaf surfaces. Leafminer larvae are generally cylindrical in shape, tapering to a point at the head end. Larvae are typically white to yellowish-white in the most species. When larvae reach physiological maturity, they may remain in the plant or drop to the ground to pupate. Numerous generations of leafminers occur per year. Depending on timing and crop first generation typically causes the majority of damage.
The damage that results from leafminer activity may appear as blisters, blotchy mines or serpentine tunneling. Frass (feces) of the larvae can contaminate leafy tissue intended for human consumption. Stunting, due to a reduction of photosynthetic leaf surface area, can also be a problem in vegetable crops not exclusively sold for foliage consumption. Spinach leafminer produce serpentine mines initially but later produce large, blotchy feeding areas. Larvae of the vegetable leafminer may feed on multiple leaves prior to completing the larval portion of their life cycle.
It is critical to identify leafminer infestation before the marketability of the crop is affected; this threshold differs greatly among crops. Effective control of leafminer occurs early in the pest’s larval life cycle. Many leafminer species deposit eggs on the lower leaves often avoiding new growth. Focus upon these surfaces when looking for early leafminer damage. Yellow sticky cards may be helpful for monitoring adult leafminer flights. Challenging adult identification may make this scouting tactic impractical for most producers.
Threshold levels for leafminer control have not been established for many crops due to sporadic nature of the pest in Wisconsin. Based upon the crop adjust infestation tolerances to the end product. For instance, when growing beets for direct consumption of greens demand a lower threshold for control than beet for roots. Adjustments may also be made based upon the spatial distribution of the pest in the plant. If the majority of leafminers are found in older wrapper leaves of chard, which are commonly discarded, control may not be necessary.
Younger plants are generally more susceptible to damage than older ones. On leafy green crops such as spinach, lettuce, and chard, a 5% damage threshold is commonly used. Basic Integrated Pest Management (IPM) principles such as accurate identification, infestation monitoring and realistic action thresholds are all critical components for adequate management of this pest complex.
Deep plowing in early spring to destroy infested weeds and plant material from the previous season can reduce the severity of leafminer outbreaks. Covering highly susceptible crops with floating row cover to exclude adult flies from laying eggs may also help. Alternate weedy hosts such as pigweed, lamb’s-quarters (Chenopodium album), plantain, chickweed, and nightshade should be destroyed to reduce overwintering populations.
In many systems parasitic wasps are known to be effective biological control agents for leafminer. Identification of pest leafminers is essential for management as many biological control agents are species specific to the host.
Because leafminers are protected within the plant, foliar insecticidal control is often difficult. Foliar protectants must be applied prior to egg deposition on the crop. Window of activity is a concern and may require several applications for adequate control of asynchronous emergence of leafminer. Newer reduced risk insecticide groups, such as the diamide class (chlorantraniliprole, IRAC MoA 28), may provide excellent systemic control of the leafminer complex in several crops, especially where insecticide resistance to older chemistries is suspected.
Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.
UW-Extension fact sheets
- Commercial Vegetable Production in Wisconsin (A3422)
- Biological Control of Insects and Mites (A3842)
- Biological Control of Greenhouse Pests (NCR58)
- Managing Insects in the Home Vegetable Garden (A2088)
- Agromyzidae – BugGuide