GardenLine | Common Saskatchewan Pests | The Beet Leafminer

Have you noticed brown spots on the leaves of your beets, spinach and swiss chard? Do the spots have small black specks inside? Do the spots appear blister-like? If you answered "yes" to the above questions, your leafy vegetables might be infested with beet leafminer.

Description:

The adult beet leafminer is a grey fly, approximately 7 mm in length, resembling the common housefly. Their bodies are covered with long, stiff, bristle-like hairs and have abdomens that are bent and curve downward.

The eggs are white, elongate and about 1 mm in length. The eggs are laid singly or in rows of 2-10 on the underside of the leaf surface of the host plants.

The puparia are brown, oval structures approximately 5 mm in length. The larvae, which are found inside the leaf surface are yellowish to white in colour. The larvae are headless and legless and tapered from front to back. The frass or waste material that is excreted by the larvae, appear as small, black specks inside the leaf.

Life Cycle:

The beet leafminer, also known as the spinach leafminer, overwinter as pupae in the soil, beneath the host plants. The adult fly emerges from the soil in late May. After mating, the female lays her eggs on the underside of the leaf surface. Once the eggs hatch and the larvae emerge, they chew holes through the leaf surface and proceed to feed on the interior tissue of the leaf. Once the larvae reach maturity, they drop to the soil and burrow down to a depth of 5-8 cm, where they change into pupae. There is usually more than one generation per year.

Hosts:

In addition to sugarbeets, beets, spinach and swiss chard, several weeds also serve as hosts. Lamb's quarters is the most common weed host of the leaf miners, but several other weed species will serve as hosts. In light of this, it's best to ensure proper weed control measures are carried out in the garden.

Control:

As previously mentioned, good weed control should be an integral part of cultural practices. Planting early, maintaining adequate soil moisture and soil fertility will help a plant survive an infestation. The oldest leaves on the plant are those which are typically infested first. The oldest leaves are usually shaded by newer ones and are therefore less photosynthetic, so removal of older infested leaves should not be detrimental to the overall health of the plant. Removing the leaves as soon as the first signs of leaf miner are visible, is a safe, non-chemical means of control if not too many leaves are removed.

Contact insecticides are usually not effective on leaf miners, since the insect is protected by the outer leaf surface. This leaves hand picking as the only recommended control since systemic insecticides like Cygon (dimethoate) are not registered for the control on leafminers. Cygon is however registered for the control of aphids and leafhopper on crops such as spinach, beet greens and swiss chard.

REFERENCES:

University of Alberta, Insect Pests of the Prairies, by Hugh Philip and Ernest Mengersen; 1989.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.