The Pea Leafminer, Liriomyza huidobrensis, in Israel

by Phyllis Weintraub

he pea leafminer, Liriomyza huidobrensis (Blanchard), was described originally as Agromyza huidobrensis from South America (Blanchard 1926). There, it was under natural biological control until it was secondarily subjected to massive amounts of insecticides in the 1970s directed at a lepidopterean pest in potatoes (Chavez and Raman 1987). As documented in Weintraub and Horowitz (1995), the pea leafminer was introduced to, and spread throughout, Europe as a chemically resistant pest, and subsequently was introduced to Israel. The first outbreak in Israel occurred in February 1992 in the Jordan Valley, when chrysanthemum growers suddenly encountered a leafminer that could not be controlled chemically. The leafminer was formally identified as L. huidobrensis and probably had entered Israel from Europe a year or two before the outbreak in the Jordan Valley.

Liriomyza huidobrensis is easily distinguished from the other major agromyzid pest in Israel, L. trifolii. In general, L. trifolii is smaller and distinctly yellowish in appearance; whereas the overall appearance of L. huidobrensis is dark. See the diagram (Fig.1) for specific differences.

Both species cause similar damage which reduce photosynthesis and aesthetic appearances: females perforate leaf surfaces with their ovipositor to feed on plant juices.

eggs are laid on the underside of leaves where larvae then create "mines" or "tunnels" as they burrow in the leaves.

Liriomyza huidobrensis huidobrensis is a pest in open fields from autumn, but most notably in spring; it is not found in the summer. Liriomyza huidobrensis adults are resistant to conventional insecticides. At present, the only effective insecticides are translaminar insecticides, which penetrate the leaves to affect the leafminer larvae (abamectin, cyromazine, neem and spinosad). Although L. huidobrensis is highly polyphagous, the two most important field crops affected in Israel are potatoes and celery.

Potato Crops:
Initial studies were concerned with monitoring methods and diel activity (Weintraub and Horowitz, 1996). Optimum catches are when yellow sticky traps are placed at, or slightly above, plant height. Vacuum sampling is effective, and has the additional benefit of sampling parasitoids species. Leaf samples are the best means to monitor larvae and the effects of insecticides.

Leafminers are found in potatoes from autumn to spring, adult populations in commercial fields have peaked during the first to second week of April. Growers, responding to this peak of adult activity, first begin applying insecticides to control the leafminers and made make multiple applications until the end of the season in May or June. Trials were designed to reduce the number of insecticide applications by utilizing knowledge of the biology of the leafminer with the known mode of action of the available insecticides, thus treating the larval population that eventually will cause this peak of adult activity. The single application of these insecticides, 10-14 d before the anticipated adult peak, was intended to kill the larvae present in potato leaves. Both abamectin and cyromazine were highly effective in suppressing larval populations (Fig. 2) (Weintraub, 2001).

A series of trials were performed: a) plots were treated with conventional insecticides to kill natural enemies of the leafminer, thus promoting its activity, b) plots were treated with translaminar insecticides to kill the leafminer larvae, and c) plots were left untreated. These trials were intended to determine economic threasholds. Although these trials were repeated for 3 years, no yield losses were ever observed. In fact, based on growers yield records for 3 years before and 5 years after the leafminer was observed in the Negev region, yields have slowly increased since the leafminer arrived (Fig. 3). Apparently the year before the leafminer arrived in the Negev, record yields were obtained; the arrival of the leafminer coincided with a return to normal yields, hence the appearance of a decline in yield.

Celery Crops:
Celery is grown extensively in the western Negev and northern regions of Israel. In the 1994-95 season, about 90 hectares of celery was planted in the Bekah Valley, and the leafminer caused losses of about 50% of this crop. This occurred because the growers were waiting too long before treating, and making up to 25 insecticide applications (abamectin, cyromazine, dichlorovos, methamidophos, methomyl, pyrazophos, thiocyclam hydrogene oxalate, and combinations thereof) by spray or through the drip irrigation systems. Some of these insecticides actually promoted leafminers by killing natural enemies. It was demonstrated that six scheduled translaminar insecticide applications from the beginning of the season would effectively control the leafminer (Weintraub and Horowitz, 1998) (Fig. 4.). Additional trials, to check the duration of a single insecticide application, have shown that a single spray application suppresses larval populations for at least two weeks; applications through the drip irrigation systems are ineffective (Weintraub, 2001).

The effect of the leafminer on celery is different from potatoes in that the mines are in the marketable portion of the produce, the leaves and stalks. To be exported, there must be no visual evidence of the leafminer; all affected leaves and stalks must be stripped off, thus significantly reducing yields. Hence, the importance of early insecticide treatment to avoid large leafminer populations.

Literature Cited:

1. Blanchard, E. 1926. A dipterous leaf-miner on Cineraria, new to science. Rev. Soc.
   Entomol. Argent. 1:10-11.
2. Chavez, G.L. and Raman, K.V. 1987. Evaluation of trapping and trap types to reduce damage to potatoes by the leafminer, Liriomyza huiidobrensis (Diptera,
   Agromyzidae) Insect Sci. Appl. 8:369-372.
3. Weintraub, P.G. and Horowitz, A.R. 1995. The newest leafminer pest in Israel,
   Liriomyza huidobrensis. Phytoparasitica 23:177-184.
4. Weintraub, P.G. and Horowitz, A.R. 1996. Spatial and diel activity of the pea
   leafminer (Diptera: Agromyzidae) in potatoes, Solanum tuberosum. Environ. Entomol. 25:722-726.
5. Weintraub, P.G. and Horowitz, A.R. 1998. Effects of translaminar versus
   conventional insecticides on Liriomyza huidobrensis (Diptera: Agromyzidae)
   and Diglyphus isaea (Hymenoptera: Eulophidae) populations in celery. J.
   Econ. Entomol. 91:1180-1185.
6. Weintraub, P.G. 1999. Effects of cyromazine and abamectin on the leafminer
   Liriomyza huidobrensis and its parasitoid Diglyphus isaea in celery.
   Ann. appl. Biol. 135: 547-554.
7. Weintraub, P.G. 2001. Effects of cyromazine and abamectin on the pea leafminer
   Liriomyza huidobrensis (Diptera: Agromyzidae) and its parasitoid Diglyphus isaea (Hymenoptera: Eulophidae) in potatoes. Crop Protection. 20: 207-213.
8. Weintraub, P.G. 2001. Changes in the dynamics of the leafminer, Liriomyza huidobrensis, in Israeli potato fields. Intern’l J. of Pest Manage. 47: 95-102.

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