Progress 10/01/04 to 09/30/05
Outputs
Two romaine lettuce varieties, Valmaine (resistant to banded cucumber beetle and serpentine leafminer) and Tall Guzmaine (susceptible to both species) were evaluated for survival and development of two lepidopterans: beet armyworm (BAW), Spodoptera exigua (Hubner) and cabbage looper (CL), Trichoplusia ni (Hubner). Larval mortality of BAW and CL was significantly higher on resistant Valmaine than on susceptible Tall Guzmaine. Average weight per larva after one week of feeding (neonate to 8 days old) on Tall Guzmaine was six times (BAW) and two times (CL) that of larvae on Valmaine. There was a significant reduction in larval growth on Valmaine compared to that on Tall Guzmaine so that larval duration from neonate to pupation was increased by 5.9 days (BAW) and by 2.6 days (CL) on Valmaine. Pupal development took approximately one day longer on Valmaine than on Tall Guzmaine for both species. The average pupal weight of BAW and CL was 1.38 and 1.19 times less on
Valmaine than on Tall Guzmaine. There was a significant reduction in successful pupation and adult emergence of BAW and CL on Valmaine, with surviving adults weighing significantly less and being less fecund (producing only 32 to 37% the number of eggs that insects on susceptible Tall Guzmaine produced). The sex ratio of emerging adults was male-biased on resistant Valmaine (BAW, 1.20:1; CL, 1.18:1), and female-biased on susceptible Tall Guzmaine (BAW; 1:1.22; CL, 1:1.20). The feeding behavior of the two species differed on Valmaine, which may explain the relatively better performance of CL compared to BAW. Cabbage looper trenched leaves and fed on the leaf tissue distal to the trench, on tissue whose laticifers had been depressurized. Beet armyworm did not trench but instead scraped the upper epidermis of leaves as early instars progressing to making holes through the leaf as older instars. A bioassay to assay chemical components of the latex as possible feeding deterrents was
developed. Latex was collected from lettuce plants and a known quantity was applied to the surface of a disk of artificial diet. The amount of diet eaten in a 24-hr period by banded cucumber beetles from disks coated with Valmaine latex and disks coated with Tall Guzmaine latex was compared. Beetles ate half as much diet from the Valmaine coated disks. Latex is being extracted in various solvents to determine where the active factors reside so that HPLC fractionation and further bioassay and characterization can proceed.
Impacts
Valmaine has significant resistance to several important insect pests (banded cucumber beetle, serpentine leafminer and beet armyworm). The resistance seems to reside in the latex. If this resistance can be characterized and transferred to horticulturally acceptable varieties of romaine lettuce, the need for insecticide applications would be greatly diminished.
Publications
- No publications reported this period
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Progress 09/15/03 to 09/14/05
Outputs
Inheritance of resistance to banded cucumber beetle, Diabrotica balteata in Valmaine lettuce was evaluated in crossing experiments. Reciprocal crosses were set up among Short Guzmaine, Tall Guzmaine and Valmaine. Based on beetle weight gains and feeding damage on F1 populations, the data suggested resistance is inherited as a recessive character. Reciprocal crosses indicated no maternal effect in the F1 generation. Beetle weight gains used to assess resistance/susceptibility in F2 generation plants resulted in a 3:1 segregation ratio of resistant to susceptible when Tall Guzmaine was the seed parent and Valmaine was the pollen parent. However, the reciprocal cross resulted in segregation ratios of 1.2:1 and 2.4:1 resistant to susceptible for the two populations tested. The F2 results indicated resistance was the dominant factor, contrary to conclusions based on the F1 generation. F3 and F4 generations were produced with selected plants to generate true-breeding
resistant and susceptible lines. The breadth of resistance in Valmaine to other insects was investigated in greenhouse studies. Larvae of beet armyworm (BAW), Spodoptera exigua and cabbage looper (CL), Trichoplusia ni experienced more mortality when confined as neonates on Valmaine compared to Tall Guzmaine. Larval growth rate was significantly slower on Valmaine than on Tall Guzmaine. Pupal development took approximately one day longer and pupal weight was reduced on Valmaine compared to Tall Guzmaine for both species. There was a significant reduction in successful pupation and adult emergence of BAW and CL on Valmaine, with surviving adults weighing significantly less and being only one-third as fecund. The sex ratio of emerging adults was male-biased on Valmaine and female-biased on Tall Guzmaine. The feeding behavior of the two species differed on Valmaine. Cabbage looper trenched leaves and fed on the leaf tissue distal to the trench, on tissue whose laticifers had been
depressurized. Beet armyworm did not trench but instead scraped the upper epidermis of leaves as early instars, progressing to making holes through the leaf as older instars. Studies on suitability of Valmaine to phloem-feeding pest insects indicated partial resistance to potato aphid, Macrosiphum euphorbiae and Uroleucon pseudambrosiae but no resistance to the sweetpotato whitefly, Bemisia tabaci. A bioassay to assay chemical components of the latex for banded cucumber beetle feeding deterrence was developed. Latex was collected from lettuce plants and applied to the surface of disks of artificial diet. The amount of diet eaten in a 24-hr period by beetles from disks coated with Valmaine latex and disks coated with Tall Guzmaine latex was compared in both choice and no-choice tests. Beetles were strongly deterred by latex from Valmaine but not by latex from Tall Guzmaine. Latex was extracted in solvents of varying polarity and extracts were applied to disks of artificial diet.
Feeding deterrents were extracted from the 80:20 methanol:water extract of Valmaine latex. Isolation and identification is proceeding through bioassay-directed fractionation under a continuing project.
Impacts
Leaf-chewing and phloem-sucking insects are serious economic pests of lettuce in the United States and the Caribbean region. Producers routinely spray large amounts of expensive and toxic insecticides to control these pests; however many insecticides have become less effective due to insecticide resistance. Identification of compounds with feeding deterrent effects on a broad range of insects will be tremendously useful to lettuce breeders, and ultimately to producers. Breeders will be able to incorporate Valmaine germplasm into superior lettuce varieties with desirable horticultural traits to reduce pest insect damage and the resulting need for intensive insecticide applications. We are close to identifying the compound(s) responsible for feeding deterrence. Monitoring levels of these compounds during lettuce selection using HPLC will ensure that resistance to insects is not lost during the variety improvement process.
Publications
- Sethi, A., H. J. McAuslane, R. T. Nagata, and G. S. Nuessly. 2006. Performance of cabbage looper and beet armyworm on resistant romaine lettuce, Lactuca sativa L. Journal of Economic Entomology 99: 2156-2163.
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Progress 10/01/03 to 09/30/04
Outputs
Inheritance of resistance to banded cucumber beetle, Diabrotica balteata in Valmaine lettuce was evaluated in crossing experiments. Reciprocal crosses were set up among Short Guzmaine, Tall Guzmaine and Valmaine. The resistance bioassay consisted of caging a pair of beetles on 2-week old plants for 6 days. Based on weight gains and feeding damage by banded cucumber beetles on F1 populations of lettuce, the data suggest resistance is inherited as a recessive character. All F1 plants had similar feeding damage ratings and female weight gains as the susceptible control, Tall Guzmaine. Reciprocal crosses did not change response, which indicated no maternal effect in the F1 generation. Banded cucumber beetle weight gains used to assess resistance/susceptibility in F2 generation plants resulted in a 3:1 segregation ratio of resistant to susceptible when Tall Guzmaine was the seed parent and Valmaine was the pollen parent. However, in the reciprocal cross, Valmaine (seed) x
Tall Guzmaine (pollen) resulted in segregation ratios of 1.2:1 and 2.4:1 resistant to susceptible for the two populations tested. The F2 results indicated resistance was the dominant factor. These results were contrary to those of the F1 which indicated susceptibility was inherited as a dominant character. F3 and F4 generations were produced from selected crosses to generate true-breeding resistant and susceptible lines. These will be used in later studies to determine the biochemical nature of the resistant factor(s) in Valmaine. The breadth of resistance in Valmaine to other insects was investigated in greenhouse studies. Larvae of both beet armyworm, Spodoptera exigua and cabbage looper, Trichoplusia ni experienced mortality when confined as neonates on Valmaine compared to Tall Guzmaine. Beet armyworm was more severely affected (reduced developmental rate and increased mortality) than was cabbage looper. Studies on susceptibility of Valmaine to Florida flower thrips, Frankliniella
bispinosa are ongoing.
Impacts
The production of true-breeding insect resistant and susceptible romaine lettuce lines will enable us to more efficiently examine biochemical variation in these lines and its potential role in insect resistance. The broad range of activity against insect pests of the resistance in Valmaine will make it a useful trait that can be bred into horticulturally superior modern varieties.
Publications
- No publications reported this period
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