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EFFICACY OF BACILLUS THURINGIENSIS AND CABBAGE
CULTIVAR RESISTANCE TO DIAMONDBACK MOTH (LEPIDOPTERA: YPONOMEUTIDAE)
Paul W. Ivey and Seth J. Johnson
Department of Entomology
Abstract
Population density estimates were used to determine
the effectiveness of a commercial formulation of Bacillus thuringiensis
Berliner subspp. kurstaki and aizawai (Agree 50 WP®) and host
plant resistance in three cabbage cultivars against the diamondback
moth, Plutella xylostella (L.). Cabbage plots treated with Agree
50 WP® had significantly fewer larvae per plant compared with
untreated ones. The ranking from most to least susceptible of
the three main cabbage cultivars grown in Jamaica was ‘KY Cross’
> ‘Early Jersey’ > ‘Tropicana’. These findings provide evidence
that a new cabbage hybrid, ‘Tropicana’, and products containing
effective strains of B. thuringiensis may be successfully used
for P. xylostella management in Jamaica.
Key Words: P. xylostella, Agree 50 WP®, plant resistance,
Jamaica
Resumen
Fueron usados estimados de la densidad poblacional
para determinar la efectividad de una formulación comercial
de Bacillus thuringiensis Berliner subspp. kurstaki y aizawai
(Agree 50 WP®), y la resistencia de tres cultivares de col a
la polilla Plutella xyllostella (L.). Las parcelas de col tratadas
con Agree 50 WP® tuvieron significativamente menos larvas por
planta que las no tratadas. El rango de susceptibilidad en orden
creciente de los tres cultivares más usados en Jamaica
fue “KY Cross” > “Early Jersey” > “Tropicana”.
Estos hallazgos muestran que un nuevo híbrido de col, “Tropicana”,
y productos conteniendo cepas efectivas de B. thuringiensis podrían
ser exitosamente usados para el manejo de P. xylostella en Jamaica.
Cruciferous vegetables grown in Jamaica and other
Caribbean islands are susceptible to damage by many insect pests:
armyworms, Spodoptera spp., cabbage looper, Trichoplusia ni (Hubner),
cabbage white butterfly, Pieris rapae L., and diamondback moth,
Plutella xylostella (L.). A complex of these pests occurs whenever
these crops are grown for commerce, and their control is a prerequisite
for meeting quality standards for damage- and pest-free produce.
Populations of P. xylostella frequently account for 75% of the
insect pest population and cause crop loss of up to 90%, making
it the key insect pest from an economic standpoint (Salinas 1986,
Alam 1992).
Historically, farmers have relied primarily on multiple
applications of broad-spectrum insecticides for control of P.
xylostella in Jamaica. Alam et al. (1987) recommended an action
threshold of six larvae per plant at the post-transplanting stage
of cabbage. Over 18 different insecticides have been used since
1972 (Walton 1989), and between 20-22 insecticide applications
over a growing season are not uncommon. As a result, many insecticides
from the organophosphate, carbamate, and pyrethroid groups are
now ineffective because of insecticide resistance (Alam 1992,
Robinson et al. 1995). Reports of low efficacy of Biotrol® and
Thuricide® (products containing the kurstaki strain of Bacillus
thuringiensis Berliner) in controlling P. xylostella, presumably
due to insecticide resistance, led Alam (1992) to question their
reliability. Because of pest management problems, environmental
degradation, and occupational and public health risks associated
with insecticides, it is imperative to find an integrated pest
management (IPM) approach for P. xylostella management which utilizes
tactics such as host plant resistance and microbial control.
The utility of host plant resistance as an insect
pest management tactic is well established (Painter 1951, Lim
1992). Dickson et al. (1984, 1986) reported the release of four
cabbage breeding lines possessing resistance to P. xylostella.
Results of genetic and other studies indicated that the host plant
resistance exhibited by these cabbage breeding lines was associated
with the glossy dark-green leaf found in the cauliflower Plant
Introduction (PI) 234599 (Dickson et al. 1990, Stoner 1990, Eigenbrode
& Shelton 1992). However, P. xylostella-resistant cabbage
cultivars are not generally commercially available. Use of microbial
agents for controlling P. xylostella has been most successful
with B. thuringiensis; certain strains of this bacteria are highly
effective against early instars (Hofte & Whiteley 1989). Furthermore,
B. thuringiensis is environmentally benign and non-toxic to beneficial
organisms, many of which are natural enemies of P. xylostella.
Because the kurstaki strain of B. thuringiensis is already of
questionable reliability in Jamaica (Alam 1992), and field resistance
in P. xylostella has been reported in the Philippines (Kirsch
& Schmutterer 1988), Hawaii (Tabashnik et al. 1990), Malaysia
(Syed 1992), mainland USA (Shelton et al. 1993), and in Central
America (Perez & Shelton 1997), it is unwise for farmers to
rely solely on this microbial insecticide. To thwart the development
of resistance to this insecticide and preserve its longevity and
effectiveness, a more integrated approach should be developed.
Combining the use of B. thuringiensis and host plant resistance
for P. xylostella control is plausible. A new product, Agree 50
WP® (wettable powder), containing both kurstaki and aizawai
strains of B. thuringiensis and a new cabbage cultivar, ‘Tropicana’,
reputed resistant to P. xylostella, has recently become available
to farmers. Before the advent of this new cultivar, two hybrids
(‘KY Cross’ and ‘KK Cross’) and an open-pollinated cultivar (‘Early
Jersey’) were available to growers in Jamaica. In 1995, marketing
of ‘Tropicana’ began by the leading distributor of agricultural
inputs with claims of resistance to P. xylostella. However, the
relative resistance of these cultivars to P. xylostella, under
local conditions, has not been empirically studied. Therefore,
toward achieving our long range goal of IPM of P. xylostella,
the objective of this study was to determine the efficacy of Agree
50 WP® and evaluate the relative resistance of cabbage cultivars
grown in Jamaica to P. xylostella.
Materials and Methods
In September 1995, a field experiment was initiated
on the farm of the College of Agriculture, Science, and Education,
Port Antonio, Portland, Jamaica. Three cabbage cultivars, ‘Tropicana’,
‘Early Jersey’ (Petoseed, Saticoy, CA), and ‘KY Cross’ (Takii
Seed, Kyoto, Japan), were subjected to two different treatment
regimes: Agree 50 WP® (Ciba-Geigy, Greensboro, NC) applied weekly
and untreated controls. Cultivars and treatments were replicated
four times in a completely randomized experimental design with
a split-plot treatment arrangement. Main plots were cultivar and
sub-plots were Agree 50 WP® and untreated controls. Individual
plots were 2.73 m × 4.45 m and were separated by a distance
of 1.52 m. Four-week-old seedlings of the three cabbage cultivars,
raised in outdoor seedbeds, were planted 0.45 m apart on raised
beds spaced 0.91 m apart. Standard cultivation practices for cabbage
production were employed. Plots were sampled for P. xylostella
once per week for seven weeks, between 18 November and 30 December,
by counting larvae on the leaves of 10 plants in each plot. Agree
50 WP® [3.8% (AI) (25,000 IU per ml) (B. thuringiensis subspp.
kurstaki and aizawai)], was applied to the appropriate plots once
per week, at the rate of 83 g in 15 liters of water using a Solo
475 Knapsack Sprayer (Solo Incorporated, Newport News, VA). The
data were subjected to analysis of variance using PROC GLM (SAS
Institute 1989).
Results
P. xylostella larval population density per plant
was significantly affected by cabbage cultivar (F = 20.94; df
= 2, 18; P = 0.0001) and insecticide treatment (F = 52; df = 1,
18; P = 0.0001). The ranking of cultivars from most to least susceptible
to P. xylostella was ‘KY Cross’ > ‘Early Jersey’ > ‘Tropicana’.
Plots treated weekly with Agree 50 WP® had significantly (F
= 52; df = 1, 18; P = 0.0001) fewer larvae per plant compared
with untreated plots (Table 1).
The interaction between cabbage cultivar and insecticide
treatment was marginally significant (F = 3.38; df = 2, 18; P
= 0.0567). Further examination of this interaction was done using
PROC PLOT (SAS Institute 1989). The cell means for the levels
of the two factors, insecticide treatment and cabbage cultivar,
as shown in Table 1, indicated that the ‘Tropicana’ cultivar supported
fewer larvae per plant across insecticide treatments compared
with the other two cultivars.
Discussion
Based on larval population density estimates, the
‘Tropicana’ cultivar was superior to the other two cultivars in
resisting attack from P. xylostella. To our knowledge, before
this study, cabbage cultivars available to growers in Jamaica
had never been evaluated under local conditions regarding their
relative susceptibility to P. xylostella. The existence of host
plant resistance to P. xylostella in a commercially cultivated
cabbage cultivar, such as ‘Tropicana’, is significant because
of the economic importance of this insect pest related to the
intractable problem of its widespread development of resistance
to conventional insecticides. Several authors have reported on
breeding and the potential for using resistant cabbage cultivars
for P. xylostella management (Dickson et al. 1984, 1986, 1990,
Stoner 1990, Eigenbrode & Shelton 1992), but host plant resistance
in a commercially available cabbage cultivar has hitherto not
been reported.
It appears that the ability of the ‘Tropicana’ cabbage
cultivar to resist P. xylostella is based on leaf texture; the
epidermis of its leaves is relatively thicker, especially as plants
approach the heading stage, compared with those of the other two
cultivars. Importantly, there have not been any reports of complaints
from consumers regarding the texture of ‘Tropicana’. Because first
instars of P. xylostella are leafminers and must tunnel into the
leaf to feed, the thicker epidermis of ‘Tropicana’ may have been
too great a challenge for the mandibles of neonate larvae. These
neonates may starve to death, desiccate, drown or be washed from
leaves, and be vulnerable to predators. Eigenbrode and Shelton
(1992) found that neonate P. xylostella larvae had greater movement
on resistant cabbage breeding lines than on susceptible ones.
Also, Tanton (1962) found that leaf texture affects the number
of nibbles and subsequent leaf area of Brassica rapa L. consumed
by Phaedon cochleariae F. In addition, Iheagwam (1981) reported
that penetrability of the leaf tissue of Brassica oleraceae L.
influences the degree of exploitation by Aleyrodes brassicae Walker.
And, Martin et al. (1975) found a negative correlation between
internode hardness of Saccharum officinarum L. and susceptibility
to attack by neonate Diatraea saccharalis (F.) larvae.
Table 1. Mean numbers (± SEM) of Plutella xylostella
larvae per plant on three cabbage cultivars treated with Agree
50 WP®.
Cultivar
1Treatment means significantly different (ANOVA F
test; a = 0.05).
2Cultivar means significantly different (Waller-Duncan
K-ratio t test; a = 0.05).
Agree 50 WP® proved to be effective in controlling
P. xylostella. This was probably due primarily to the presence
of the aizawai strain of B. thuringiensis. There are differences
in the crystal protein toxin profiles of B. thuringiensis subsp.
kurstaki and subsp. aizawai; the former produces Cry IA (a), Cry
IA(b), Cry IA(c), Cry IIA, and Cry IIB, whereas the latter produces
Cry IA (a), Cry IA(b), Cry IC, Cry ID, and Cry IIB (Hofte &
Whiteley 1989). Shelton et al. (1993) found differential responses
between P. xylostella populations treated with two formulations
containing B. thuringiensis subsp. kurstaki (Javelin WG® and
Dipel 2X®) and populations treated with B. thuringiensis subsp.
aizawai (ZenTari®). Commercial formulations of B. thuringiensis
subsp. kurstaki, for example, Biotrol® and Thuricide®, have
been used in Jamaica for many years to control P. xylostella but
their reliability is now questionable (Alam 1992). However, formulations
of B. thuringiensis subsp. aizawai have only recently begun to
be more widely used. So, P. xylostella populations have no prior
exposure to this strain of B. thuringiensis. In fact, B. thuringiensis
subsp. aizawai was first used against P. xylostella in Jamaica
in 1995.
The existence of a significant interaction makes
it necessary to exercise caution when making statements about
the main effects (cabbage cultivar and insecticide treatments),
even though both were statistically significant with P values
of 0.0001 (Freund & Wilson 1993). The consistently lower numbers
of larvae on the ‘Tropicana’ cultivar, compared with ‘Early Jersey’
and ‘KY Cross’, across plots treated with Agree 50 WP® and in
untreated plots, clearly show that the ‘Tropicana’ cultivar was
superior to the other two cultivars in resisting P. xylostella.
Also, from the interaction between cabbage cultivar and insecticide
treatment, it can be inferred that the ‘Tropicana’ cultivar may
be compatibly combined with use of Agree 50 WP® for successful
management of P. xylostella.
Regarding the effect of these two control tactics
on armyworms and cabbage looper, past experience has shown that
tactics which are successful in controlling P. xylostella simultaneously
also controlled armyworm and cabbage looper populations. Usually,
insecticides are more effective against other insects in the crucifer
pest complex than P. xylostella. In fact, for the duration of
the study, armyworms and cabbage loopers were not encountered.
Considering the low efficacy and tenuous reliability
of commercial formulations of B. thuringiensis subsp. kurstaki
against P. xylostella in Jamaica (Alam 1992), the effectiveness
of Agree 50 WP® (B. thuringiensis subspp. kurstaki and aizawai)
seen in this study makes continued use of toxins of B. thuringiensis
for controlling this insect a viable option, especially when combined
with the ‘Tropicana’ cabbage cultivar. However, we do not recommend
that formulations containing both the kurstaki and aizawai strains
of B. thuringiensis, such as Agree 50 WP®, be used extensively
as this may allow for the development of resistance in P. xylostella
to the aizawai strain without losing its resistance to the kurstaki
strain. It might be a better strategy to alternate both strains.
Acknowledgments
We thank Lloyd Bailey of the College of Agriculture,
Science, and Education, Port Antonio, Portland, Jamaica for help
with the field work, and Terrence Thomas of the Environmental
Foundation of Jamaica from which this project received substantial
financial support.
Approved for publication by the Director, Louisiana
Agricultural Experiment Station as Manuscript number: 97-17-0029.
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No. larvae per plant
Overall cultivar means ± SEM2
Agree 50 WP®
Untreated
Tropicana
0.30 ± 0.21
1.55 ± 0.39
0.93 ± 0.31
Early Jersey
0.88 ± 0.25
4.23 ± 0.33
2.55 ± 0.66
KY Cross
2.23 ± 0.70
4.95 ± 0.40
3.59 ± 0.64
Overall treatment means ± SEM1
1.13 ± 0.34
3.58 ± 0.48
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