Feeding Preference and Survival of Budworm and Bollworm on Mixtures of Transgenic and Nontransgenic Cotton
Grant # 93-34103-8443
Jonda L. Halcomb and John H. Benedict
Texas A&M University Research and Extension Center
Route 2, Box 589 Corpus Christi, Texas 78406
The transfer of truncated genes, producing CryIA(c) -endotoxin from Bacillus thuringiensis, into Upland cotton, Gossypium hirsutum (L.), has produced insect resistant plants possessing a high level of efficacy against certain Lepidoptera such as tobacco budwonn, Heliothis virescens and bollworm, Helicoverpa zea. Transgenic cotton plants, (BTK cotton) exhibiting this type of host plant resistance espouses many of the essential components of integrated pest management (IPM). Usage of BTK cotton provides a higher level of efficacy than afforded by conventional host plant resistance methods and can result in higher yields for the grower while decreasing insecticide usage for lepidopteran control. BTK cotton is compatible with alternative IPM methods, such as use of natural enemies and other pest control tactics (e.g., biorational pesticides) aimed at non-lepidopterous insects. However, with this high level of efficacy, > 90% and continuous expression of the toxin, resistance management becomes a primary concern in order to maintain long term effectiveness of BTK cotton. Commercialization of BTK cotton offers a unique opportunity to develop and test resistance management strategies because the EPA requires a plan for this plant produced insecticide. The utilization of mixtures of BTK and non-BTK seed as a "built in"refuge to produce susceptible homozygous tobacco budworm and bollworm to develop so as to breed with resistant insects, was advocated as one feasible resistance management strategy. In 1992-1994 experiments were performed on feeding preference, survival and growth of tobacco budworin and bollworm in response to mixed plantings, to determine the practicality and usefulness of planting seed mixtures as a resistance management strategy. These studies provided us information on feeding behavior and what would occur if tobacco budworm and bollworm larvae, at different ages moved, after feeding and developing on a non-BTK plant to a BTK plant in a mixed stand in the field. The specific objectives of these experiments were to determine: (1) the effect of BTK cotton on age-specific larval and pupal survival, and developmental times for tobacco budworin and bollworm; and (2) if 3rd instar tobacco budworm and bollworm show a preference for non-BTK cotton as compared with BTY, cotton plants in mixed stands of BTK and non-BTK plants.
    Experiments were conducted at the Texas A&M University Agricultural Research Center, Corpus Christi, TX In the age-specific survival and growth experiment a uniform cohort of tobacco budworm and bollworm neonates was reared on artificial diet to the 2nd, 3rd, 4th, or 5th instar. At the appropriate instar, I st, 2nd, 3rd, 4th or 5th, tobacco budworm or bollworm were fed freshly excised flower buds from BTK or non-BTK plants. The experimental design was a randomized complete block with 10 treatments (150 insects per treatment) per insect species (2 cotton genotypes x 5 instars) replicated 5 times. The cotton isolines used were: transgenic MON 1076 and MON 249 and non-transgenic 'Coker 312'. Data collected were daily survival, weight of 10-d-old larvae, time to pupation, pupal weight, number of days to adult emergence and sex.
    In the feeding preference experiment 5 treatments (5 plant stand ratios of BTK and non-BTK plants were evaluated): (1) 100:0, all resistant BTK plants; (2) 75:25, BTY,:non-BTY, plants; (3) 50:50, BTK: non- BTK plants; (4) 25:75, BTK: non-BTK plants; and (5) 0:100, all susceptible non-BTK plants. Each cluster of 4 plants per cage was infested with 12 (3 per plant), 3rd instar tobacco budworin or bollworin. The cotton isolines used were transgenic MON 1076 and nontransgenic 'Coker 312'. Data collected were the number of larvae and the numbers of injured flower buds and capsules per plant 24 and 48 h after infestation.
    Results of the survival and growth experiment showed that tobacco budworm and bollworms fed BTK flower buds as 1st, 2nd, 3rd, or 4th instars died in approximately 1- 4 days. Most importantly, a high number of 5th instars of both insect species survived to the pupal and adult stages. When fed BTK flower buds significantly less 5th instar bollworm pupated, 46.0%, and emerged as adults, 27. 1 %, compared with those fed non-BTY, cotton, 76.0 and 51.3%, respectively. No significant differences in the number pupating, 75.3%, and emerging as adults, 48.0%, were found for 5th instar tobacco budworm fed BTK flower buds and those fed non-BTK flower buds, 73.3 and 41.3%, respectively. Pupal weight, length of pupal stage and the percentage of males and females was not different between the BTK and non-BTK treatments for either insect species that were fed flower buds as 5th instars.
    Results of the feeding preference experiment showed that fewer tobacco budworm and bollworm 3rd instars were found on BTK plants than non-BTK plants 24 h after infestation. At 48 h after infestation, fewer tobacco budworm but not fewer bollworm larvae were found on the BTY, plants. However, the initial density of 3 larvae per plant did not increase on the non-BTK plants over time. In addition, there was a trend for fewer tobacco budworm injured flower buds and capsules in all plants stands containing BTK plants 24 and 48 h after infestation. The higher number of larvae on the non-BTK plants was probably the result of larval intoxication, reduced feeding, increased mortality, and plant abandonment rather than a classical feeding preference.
Both experiments indicate that the movement of tobacco budworm and bollworm from non-BTK to BTK plants in mixed plantings occurred frequently and could result in injury to non-BTK and BTK plants. Moreover, larvae of both species would feed indiscriminately on both BTK and non-BTK plants in mixed plantings and be selected for resistant individuals.
    The impact of these experiments has been multifaceted. The most apparent outcome was the influence on major BTK product developers and selection of the first resistance management plan for BTK cotton. Our findings, in part, resulted in the elimination of the mixed seed strategy as the primary way to provide a refuge that would produce susceptible insects. During the course of these experiments, their discussion, and presentation of publications, many people have been exposed and influenced by the thoughts, concepts and results of these studies. The exposure of scientists, industry developers, growers, educators and the public to these studies has provided them with new knowledge and understanding of this revolutionary technology and an increased knowledge base not only of this technology but of the interactions and behavior of Lepidoptera in response to plant toxins. A collaborative effort between a heterogeneous group of people involved with the experiments included: (1) 4 research scientists at Texas A&M University and Monsanto company; (2) 2 post doctoral research associates; (3) 5 graduate students; (4) 12 student workers; (5) 3 research technicians; (6) 2 entomology extension agents; and (7) faculty of 2 universities, serving on graduate committees. Several persons as listed above have advanced to become: (1) involved with product development of pesticides and transgenic material within major chemical companies; (2) entomology extension agents; (3) research scientists; (4) faculty; and (5) graduate students. The overall influence of these experiments on a diversity of people should help make BTK cotton better understood and managed as an important tool in sustainable IPM programs. In addition, these experiments generated written and oral presentations including: (1) 1 graduate thesis; (2) a peer reviewed publication in the journal, Environmental Entomology; (3) 1 manuscript submitted to a peer reviewed ESA j ournal; (4) 2 presentations at Beltwide Cotton Conferences; (5) 1 presentation at the International Plant Resistance to Insects Workshop; (6) 2 non-peer reviewed articles in Annual Plant Resistance to Insects Newsletter; and (7) several refuge management-product development problem solving meetings. Participants included researchers, major seed companies, and academia. Also, the results of these experiments lead to more experimentation and ftinding for this lab on the movement and suppression of tobacco budworm and bollworm in mixed stands of BTK and non-BTK cotton and the survival, growth and fecundity of 5th instar tobacco budworm fed BTK and non-BTK cotton. More importantly, these studies have played a part in the development and application of a new insect control technology that has proved successful, by any measure, to producers and society.