Leafminers are important insect pests of many agricultural crops throughout the world (Parrella, 1987), including spinach (Spinacia oleracea L.). The predominant species in the spinach production areas in central California is Liriomyza langei (Scheffer et al., 2001). Leafminer adults are small, shiny black flies that puncture leaves to feed on plant sap, whereas females lay white, oval eggs within the leaf tissue, resulting in “stings” that appear as holes or bumps on the leaves. Larvae hatch from eggs and feed between upper and lower leaf surfaces leaving winding, whitish tunnels (mines) that are initially narrow but increase in width as the larvae grow. After completing three instars, larvae emerge from mines and pupate in cracks in the soil or on the leaf surface, and adult flies emerge from pupae in ≈8 to 11 d. The entire life cycle can be completed in less than 3 weeks 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 are moving targets and any treated field is subject to reinfestation 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 ≈24 d from planting (at the four- to five-leaf stage) with “junior leaves” harvested several days later. Many systemic insecticides for larval control have a requirement of a 14-d preharvest spray interval (the period before harvest when no spray is allowed). The result is that a “baby leaf” field has to be sprayed ≈10 or fewer days after planting when plants are still small and most of the spray is wasted. Some growers try to avoid leafminers by planting spinach in fields where the insect pressure is low, but leafminer infestation is often unpredictable. As a result, spinach is often tainted with the stipples of adult stings or mines from feeding larvae, thus 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 may cut down the costs of chemicals, energy, and labor associated with pesticide spray, reduce workers’ exposure to hazardous chemicals, and decrease insecticide residues on plants, resulting in increased consumer acceptance of spinach. It may also reduce possible pesticide contamination of soil and groundwater. However, spinach cultivars with high levels of resistance to leafminers are not currently available. With the rising demand and increasing production of spinach products, there is an urgent need to develop leafminer resistance in spinach. In this article, the development of a leafminer-resistant spinach breeding line is described.
KeilC.B.ParrellaM.P.1990Characterization of insecticide resistance in two colonies of Liriomyza trifolii (Diptera: Agromyzidae) J. Econ. Entomol.831826
MasonG.A.JohnsonM.W.TabashnikB.E.1987Susceptibility of Liriomyza sativae and Liriomyza trifolii (Diptera: Agromyzidae) to permethrin and fenvalerateJ. Econ. Entomol.8012621266
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