E-1288

Mark Boetel Research and Extension Entomologist

▲ Figure 1. Adult of beet leafminer.

The beet leafminer () as pupae in the soil and emerge Feeding continues until the larvae and spinach leafminer (Pegomya as adults in late May to early June. are fully grown (0.3 to 0.38 inch long). hyoscyami) sporadically infest Mating typically occurs four to seven They then molt into pupae and drop sugarbeets in northern production days after adult emergence. Females to the soil immediately below the plant areas of North America, including the lay tiny (0.04 inch long) white cigar- canopy. Pupae are brown and have a Red River Valley. Pockets of damaging shaped eggs, often in neatly arranged somewhat hardened outer covering. infestations developed in the central rows in groups of two to 10, on the They molt once more and emerge as and southern valley factory districts undersides of sugarbeet leaves adults. This begins another genera- from mid- to late-June of the 2001 (Figure 2). tion. Leafminers can produce up to growing season. Since then, leaf- three generations per year in the Red Most of the leafminer egg laying miners have been observed at varying River Valley, although first-generation that causes economic concern for levels in sugarbeet fields throughout larvae are more likely to cause sugarbeet production typically occurs the production area from the Fargo, economic damage than later genera- during the first two weeks of June. N.D./Moorhead, Minn., vicinity to as tions. This is may be due to plants Eggs hatch into tiny (0.03 inch long) far north as Cavalier, N.D. being smaller and more vulnerable to larvae four to 10 days after being attack when first-generation larvae are deposited. The timing of their hatching causing mining injury. Also, predators Description and Life History depends on prevailing air tempera- and parasites probably build up later tures. Larvae are legless maggots Beet and spinach leafminers are in the season and help keep leafminer and initially are translucent white. similar in appearance at each life populations at subeconomic levels. stage. Adults are small (0.25 inch They quickly burrow into the leaf long), clear-winged with dark surface from which they hatched and gray to greenish gray bodies. They gradually become pale to lime green look like small hunchbacked house as they feed on host plant tissues flies with numerous hairlike spines (Figure 3). and finer-textured hairs on their backs. They hold their wings above their backs while at rest (Figure 1).

Beet and spinach leafminers also are very similar in their biology, life cycles and the damage they cause in sugarbeets. Both overwinter ▲ Figure 2. Leafminer eggs on leaf.

▲ Figure 3. Full-grown leafminer larva and mining damage. JULY 2005 Plant Injury and Symptoms Leafminer feeding injury first appears as individual, bleached light green to white mines that result from young larvae burrowing tiny tunnels into the mesophyll zone between upper and lower surfaces of leaves. The winding mines expand as larvae ▲ Figure 4. grow and may merge with prolonged Leafminer pupa feeding. Older larvae continually and necrotic feeding in a concentrated area leads blotch on to the development of larger tan to sugarbeet leaf. dark brown necrotic leaf blotches (Figure 4). Extensive mining damage on multiple leaves can inhibit the Insecticide products without spe- Scouting: Scouting is an important plant’s photosynthetic activity and cific labeling for leafminer control practice because leafminer infestation reduce yield. or suppression in sugarbeets can levels can vary within and among be applied to manage these fields. Early detection of a leafminer as long as they are registered infestation will increase the likelihood Management for foliar application to the crop. of successful control. Mines and eggs Cultural Control: Destruction of alter- However, manufacturers cannot be are visible readily with the unaided nate weed hosts, including common held liable for losses or unsatisfac- eye. To see leafminer larvae and lambsquarters, redroot pigweed and tory performance associated with pupae inside leaf layers easily, hold other related Amaranthus species, control failures if the label does the leaf toward the sun. To scout a may reduce the likelihood and severity not explicitly offer leafminer control field, sample several sets of 10 plants of leafminer problems in sugarbeets. in sugarbeets. each in several representative areas within a field. The more samples Chemical Control: Although chemical Damaging leafminer infestations taken, the more reliable the estimate control of leafminers rarely is neces- can coincide with the need for will be. Sampling in a manner that sary in the Red River Valley growing postemergence control of weeds in represents the whole field, and not area, foliar insecticides are effective sugarbeets. Tank-mixing the insecti- just field edges, is important. management tools if applied within cide for leafminer control with herbi- a few days of the first appearance cides for weed management is an Treatment threshold: An insecticide of mines or before most eggs hatch. attractive option in this scenario application may be justified if the Applying insecticide shortly after most because it saves time and input costs combined number of eggs and live larvae have tunneled into leaves also associated with an additional pass larvae exceeds the square of the can achieve favorable performance. across the field. Combining an oil- number of true leaves on plants. For Planting-time application of systemic based insecticide with a conventional example, a field with most plants in soil insecticides, a common practice rate of a postemergence herbicide the four-leaf stage would need to used by growers for protection from can result in considerable crop injury average more than 16 eggs and soil pests, probably provides and yield reductions in sugarbeets, larvae per plant before the insecticide incidental control of early season especially if the crop is at an early treatment would be necessary. leafminer infestations. Experts do not (seedling to four-leaf) stage of recommend this practice for leafminer development. Microrate herbicide Photo credits: Deleplanque & Cie© management because economically combinations will be safer to tank- (Figs. 1 & 2) and M. Boetel (Figs. 3 & 4). damaging infestations are rare and mix with oil-based foliar insecticides Reference: Gratwick, Marion (ed.). the insects can be controlled easily for application to younger sugarbeet 1992. Crop Pests in the U.K. pp. 233- with a curative foliar insecticide fields. Six-leaf and older beets should 236. In Collected Editions of MAFF treatment. be tolerant to tank mixtures that Leaflets. Chapman and Hall, London. include herbicides at conventional rates. For more information on this and other topics, see: www.ag.ndsu.edu NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating. Duane Hauck, director, Fargo, N.D. Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all people regardless of race, color, national origin, religion, sex, disability, age, Vietnam era veterans status or sexual orientation; and are an equal opportunity employer. 5M-7-05 This publication will be made available in alternative format, upon request to people with disabilities, (701) 231-7881.