Leafminers are serious insect pests of many agricultural crops throughout the world (Parrella, 1987). The predominant leafminer species in spinach (Spinacia oleracea L.) production areas in central California was identified to be Liriomyza langei Frick (Scheffer et al., 2001). Leafminer adults are small, shiny, black flies with a bright yellow triangular spot on the upper thorax between the wings. Damage occurs when adult flies puncture leaves to feed on plant sap and females lay white, oval eggs within the leaf tissue, leaving “stings” that appear as holes or bumps on the leaves. Larvae hatch from eggs and feed between upper and lower leaf surfaces. The winding, whitish tunnels or mines they create are initially narrow but increase in width as the larvae grow. Larvae emerge from the mines after completing three instars and pupate in cracks in the soil or on the leaf surface. Adult flies come out of pupae in ≈ 8–11 d. The entire life cycle can be completed in less than 3 weeks in warm weather, and many generations are produced each year in California. Damage incited by adult stings and larval mining of leaves reduces photosynthetic capacity, renders spinach leaves unmarketable, and provides an entrance for disease organisms (LeStrange et al., 1999).
Chemical control of leafminers usually lasts only a short time. Adult control with contact insecticides is especially unsatisfactory because flies can easily move, and the treated field is subject to re-infestation from adjacent untreated crops and weeds (LeStrange et al., 1999). Many studies have shown that leafminers can develop a high degree of resistance to a broad range of insecticides (Keil and Parrella, 1990; Mason et al., 1987; Parrella and Trumble, 1989). In California, chemical control is often not an option for spinach. Fresh market “baby leaves” are harvested in about 24 d from planting (at the four- to five-leaf stage). Many systemic insecticides for larval control have a requirement of a 14-d preharvest interval (the period with no chemical sprays before harvest). The result is that “baby leaf” fields have to be sprayed about 10 d after planting or earlier, when plants are still small and most of the spray hits the ground and is wasted. Consequently, growers try to avoid leafminers by planting spinach in fields where the insect pressure is low, but growers may not have that option and leafminer infestation is unpredictable. As a result, spinach is often tainted with the stipples of adult feeding or tunnels (mines) from larva feeding, reducing quality, appearance, and value. Therefore, it is essential to develop alternative management strategies for leafminers.
Resistant cultivars remain the most economical means of insect control. Their use will reduce the costs of chemicals, energy, and labor associated with pesticide spraying. In addition, resistant cultivars will reduce worker exposure and pesticide residues on food and in the environment. However, spinach cultivars with high levels of resistance to leafminers are not currently available. With the rising demand and increasing production for spinach products, there is an urgent need of leafminer resistance in spinach. In this report, a leafminer-resistant spinach germplasm is described.
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ParrellaM.P.TrumbleJ.T.1989Decline of resistance in Liriomyza trifolii (Diptera: Agromyzidae) in the absence of insecticide selection pressureJ. Econ. Entomol.82365368
SchefferS.J.WijesekaraA.VisserD.HallettR.H.2001Polymerase chain reaction-restriction fragment-length polymorphism method to distinguish Liriomyza huidobrensis from L. langei (Diptera: Agromyzidae) applied to three recent leafminer invasionsJ. Econ. Entomol.9411771182