Lettuce is the most popular ingredient of salads or salad mixes consumed in the United States. The U.S. growers planted ≈278,900 acres of lettuce and produced 90 million cwt of lettuce (53 million cwt of head lettuce, 26 million cwt of romaine lettuce, and 11 million cwt of leaf lettuce) in 2009 (U.S. Department of Agriculture, 2011). Lettuce production practices in Florida are unique as the growing season begins in early October and harvest finishes in the mid-May. The subtropical climate of Florida facilitates lettuce production from fall through spring, but the warm and moist conditions are also favorable for insect proliferation and damage.
Banded cucumber beetle, serpentine leafminer, and aphids are among the major pests causing significant economic damage to lettuce in Florida. BCB is a phagous insect with a host spectrum of more than 50 plant species in 23 families (Saba, 1970). Economic losses caused by BCB have been reported in other crops, including cucumber [Cucumis sativus (Da Costa and Jones, 1971)], common bean [Phaseolus vulagaris (Cardona et al., 1982)], sweetpotato [Ipomoea batatas (Schalk et al., 1986)], and soybean [Glycine max (Layton et al., 1987)]. Foliar feeding on lettuce by BCB adults leads to decreased photosynthetic area, increased vulnerability to diseases, and reduced market grade (Nuessly and Nagata, 1993). SL is also polyphagous and attacks lettuce and other vegetable crops (Drees and Jackman, 1999). Plant leaves are damaged as SL larvae tunnel through the inner leaf tissue producing so-called whitish “mines” that reduce the photosynthetic area, and lettuce becomes unmarketable if infestation is severe (Nuessly and Nagata, 1994). Several species of aphids affect lettuce, including green peach aphid (Myzus percicae), potato aphid (Macrosiphum euphorbiae), the aphid Uroleucon pseudambrosiae, and lettuce aphid (Nasonovia ribisnigri). The lettuce aphid is a problem worldwide and has recently become problematic in the western United States and Canada (Liu and McCreight, 2006; McCreight, 2008), but the first three species of aphids are most economically damaging in Florida (Nuessly and Webb, 2010). Although heavy aphid pressure can stunt plants, the major problem caused by aphids is head contamination, which makes lettuce unmarketable. In addition, aphids are the vectors for transmission of lettuce mosaic virus and bidens mottle virus (Nuessly and Webb, 2010).
Control of BCB, SL, and aphids in lettuce production is historically dependent on application of pesticides. However, there are disadvantages associated with pesticide uses for insect control, including increased production costs, adverse environmental and ecological effects, and development of pesticide resistance in insects. One alternative for effective control of insects is the use of host plant resistance, which is an environmentally friendly method compatible with other approaches used in IPM (Smith, 1989).
Host plant resistance in lettuce to BCB and SL has been identified, as Nuessly and Nagata (1994) reported that ‘Valmaine’, a romaine lettuce cultivar, had a high level of resistance to SL. This cultivar was later found to be resistant to BCB (Huang et al., 2002; Sethi et al., 2008) and two lepidopterans, Trichoplusia ni and Spodoptera exigua (Sethi et al., 2006). ‘Valmaine’ is an obsolete cultivar that was used in lettuce production in 1970s and then used as a parent for crosses for development of romaine cultivars in the University of Florida (UF) lettuce breeding program (Guzman, 1986; Guzman and Zitter, 1983). The cultivars Short Guzmaine, Tall Guzmaine, Floricos 83, and Floriglade released in the 1980s (Guzman, 1986; Guzman and Zitter, 1983) contain ‘Valmaine’ in their pedigrees but have little SL resistance (‘Short Guzmaine’) or no resistance to either SL or BCB, indicating that the high level of resistance in ‘Valmaine’ was not incorporated into these cultivars. Sources of resistance in lettuce to aphids were also documented (Reinink and Dieleman, 1989; Reinink et al., 1989). Arend et al. (1999) reported development of an aphid-resistant butterhead lettuce ‘Dynamite’.
Responses of lettuce to BCB or SL were generally assessed under the controlled laboratory or greenhouse environment (Huang et al., 2002; Nuessly and Nagata, 1994). This approach can accurately distinguish resistant cultivars/lines from susceptible ones and determine if resistance is due to antibiosis or antixenosis. However, only a limited number of lettuce lines are evaluated and one insect species is studied at a time, which can hinder quick identification of resistance sources from a large collection of germplasm lines. Field test with natural insect infestation has been used in other crops such as maize (Butrón et al., 1998) and wheat (Sherman et al., 2010) but not in lettuce for evaluation of responses to BCB or SL. The established technology for studying reaction of lettuce to aphid infestation includes counting aphids (Palumbo et al., 2000; Reinink and Dieleman, 1989). The process of counting aphids is tedious and time consuming, and especially when a highly dense population of aphids develops on plants rendering it very difficult to count the individual aphids.
Cultivars that are currently being used in Florida's lettuce production were developed by the UF breeding program or introduced from other U.S. states. Information is limited on responses of these cultivars to BCB, SL, and aphids. It is desirable to investigate their performance in fields under attack from insect pests and to understand if any of the cultivars have resistance that can be incorporated into IPM tactics for insect control. Objectives of this study were to: 1) investigate population dynamics of BCB, SL, and aphids on lettuce; 2) evaluate responses of the cultivars to infestation of the insects under field conditions; and 3) identify resistance in cultivars that are useful as a component in IPM.
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