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.