PLANT RESISTANCE Host Plant Resistance in Romaine Lettuce Affects Feeding Behavior and Biology of Trichoplusia ni and Spodoptera exigua (Lepidoptera: Noctuidae) AMIT SETHI, 1 HEATHER J. MCAUSLANE, 1,2 RUSSELL T. NAGATA, 3 AND GREGG S. NUESSLY 3 J. Econ. Entomol. 99(6): 2156Ð2163 (2006) ABSTRACT Lettuce quality and yield can be reduced by feeding of several lepidopterous pests, particularly cabbage looper, Trichoplusia ni (Hu ¨ bner), and beet armyworm, Spodoptera exigua (Hu ¨ bner) (Lepidoptera: Noctuidae). Host plant resistance to these insects is an environmentally sound adjunct to conventional chemical control. In this study we compared the survival, development, and feeding behavior of cabbage looper and beet armyworm on two romaine lettuce cultivars, resistant ÔValmaineÕ and susceptible ÔTall GuzmaineÕ. Larval mortality of both species was signiÞcantly higher on resistant Valmaine than on susceptible Tall Guzmaine. The average weight per larva after feeding for 1 wk on Tall Guzmaine plants was 6 times (beet armyworm) and 2 times (cabbage looper) greater than that of larvae feeding on Valmaine plants. SigniÞcant reduction in larval growth on Valmaine compared with that on Tall Guzmaine resulted in a 5.9- (beet armyworm) and 2.6-d (cabbage looper) increase in larval duration and almost a 1-d increase in pupal duration. Average pupal and adult weights and successful pupation of cabbage looper and beet armyworm were reduced on Valmaine compared with Tall Guzmaine. The sex ratio of progeny did not deviate from 1:1 when larvae were reared on either Valmaine or Tall Guzmaine. The fecundity of cabbage looper and beet armyworm adults that developed from larvae reared on Valmaine was about one-third that of adults from Tall Guzmaine, but adult longevity did not signiÞcantly differ on the two lettuce cultivars. The two insect species showed different feeding preferences for leaves of different age groups on Valmaine and Tall Guzmaine. Cabbage loopers cut narrow trenches on the leaf before actual feeding to block the ßow of latex to the intended site of feeding. In contrast, beet armyworms did not trench. The different feeding behavior of the two species on Valmaine may explain the superior performance of cabbage looper compared with beet armyworm. KEY WORDS host plant resistance, cabbage looper, beet armyworm, fecundity, development Over the past 15 yr, romaine lettuce, Lactuca sativa L., has been the fastest growing vegetable in terms of production, consumption, and exports in the United States. During the period 2002Ð2004, romaine lettuce accounted for 22% of all lettuce produced in the United States, and per capita use of romaine lettuce has tripled (3.7 kg) since 1992Ð1994 (USDA 2005a). Lettuce is vulnerable to attack by several insects, in- cluding lepidopterans, which can be responsible for yield losses of 100% if populations are not managed (Inglis and Vestey 2001). In Florida, the cabbage looper, Trichoplusia ni (Hu ¨ bner) and the beet army- worm, Spodoptera exigua (Hu ¨ bner) (Lepidoptera: Noctuidae), are serious pests of lettuce (Nuessly and Webb 2003). Economic pests are managed chemically on 89 and 85% of head and other lettuce acreage, respec- tively, in the United States (USDA 2005b). Florida ranks Þrst among lettuce-growing states in the use of insecticides, and growers apply insecticides on 98 Ð 100% of the stateÕs lettuce acreage to manage these insect pests (Mossler and Dunn 2005). For example, restricted insecticides such as lambda-cyhalothrin (34 and 32% of head and other lettuce acreage, respec- tively) and methomyl (32 and 30% of head and other lettuce acreage, respectively) are extensively applied on lettuce (USDA 2005c). Rapid development of in- secticide resistance has been reported for Liriomyza spp. (Diptera: Agromyzidae) against chlorinated hy- drocarbons, organophosphates, and the pyrethroid permethrin (Genung 1957, Leibee 1981, Parrella and Keil 1984). The high dependence on chemicals poses a threat to agricultural workers and natural enemies of these insect pests and increases production cost. Therefore, the implementation of alternative strate- gies, such as host plant resistance, for the management of economic insect pests should be explored. ÔValmaineÕ romaine lettuce (Leeper et al. 1963) was the major cultivar grown in Florida before the adop- 1 Department of Entomology and Nematology, P.O. Box 110620, University of Florida, Gainesville, FL 32611Ð 0620. 2 Corresponding author, e-mail: hjmca@uß.edu. 3 EREC, University of Florida, 3200 East Palm Beach Rd., Belle Glade, FL 33430 Ð 4702. 0022-0493/06/2156Ð2163$04.00/0  2006 Entomological Society of America Downloaded from https://academic.oup.com/jee/article/99/6/2156/2218626 by guest on 12 May 2023