Bt Research 2015, Vol.6, No.5, 1-10
2
2012). The maintenance of natural enemies helping to
control pest populations in
Bt
crop areas is another
possibility for delaying the evolution of resistance
(Wolfenbarger et al., 2008; Desneus et al., 2010).
The maintenance of natural enemies is critical to
prevent phytophagous insect populations from reaching
levels that are capable of causing economic damage
(Berti and Ciociola, 2002). Studies show that parasitoids
belonging to the order Hymenoptera can promote the
natural control of pests (mainly from Lepidoptera and
Hemiptera) in rice and maize cultivation areas (Martins
et al., 2004).
These parasitoids are very sensitive to changes in their
hosts after the ingestion of toxins, as they usually
complete their development in a single host. When
susceptible hosts are treated with
Bt
toxins, there is a
greater possibility of both predators and parasitoids
being affected because generalists often feed on
different prey (Salama et al., 1983). Godfray (1994)
suggests that the quality of the host may affect the
survival, size and development time of the parasitoid.
If the host is not able to survive, the parasitoid will
also not complete its development (Godfray, 1994).
One of the most destructive and economically important
pest insects found in Brazil is
S. frugiperda
, which is a
polyphagous species native in tropical regions and is
widely distributed because of its wide range of host
plants species (Carvalho et al., 2013). Among the
parasitoids of
S. frugiperda
,
C. flavicincta
is a major
parasitoid of the larvae of this species (Lucchini and
Almeida, 1980). These micro-hymenopterans are
approximately 15 mm in size and perform oviposition
inside the
S. frugiperda
larvae during their early
instars, where the
C. flavicincta
larvae complete their
entire cycle feeding on the inner content of the host,
reaching the pupal stage at the end of the cycle and
replacing the
S. frugiperda
adult with an adult
parasitoid (Cruz et al., 1995).
In this context, the consequences of the effects of
Bt
plants on parasitoids have received attention, and
studies have revealed both positive and negative
impacts (Johnson and Gould, 1992; Baur et al., 2003;
Sanders et al., 2007; Dhillon and Sharma, 2010).
Although important, there are few studies on the
interactions between
Bt
plants and parasitoids of
insect pests (Rasmann and Turlings, 2008; Lundgren et
al., 2009; Bruck, 2010; Tian et al., 2013; Liu et al.,
2014; Kumar et al., 2014).
The identification of the effects of a control agent, in
this case
Bt
plants, on parasitoids can lead to the
development of control practices that aim to emphasize
the strengths of each of these agents. This study aimed
to investigate the interactions of the
Bt
proteins (from
strains of
B. thuringiensis
, rice and
Bt
corn) on the
parasitoid
C. flavicincta
and the armyworm
S. frugiperda
.
1 Results
1.1 Mortality bioassays of
Spodoptera frugiperda
parasitized by
Campoletis flavicincta
and exposed
to genetically modified plants of
Bt
rice (Cry1B) or
Bt
maize (Cry1Ab).
The results of these bioassays are shown in Figures 1
and 2.
The mortality observed in all treatments at ten
days after the installation of the bioassay differed
significantly from the control (F= 5,0. df= 3,8 p< 0,05).
The mortality recorded in larvae fed with
Bt
rice
(Cry1B) for ten days, was 84.3%, while that in larvae
that were only parasitized by
C. flavicincta
was 52%.
However, when the larvae were both parasitized and
fed with
Bt
rice, mortality reached 95.5%. The results
observed in the last two treatments also differed
significantly from those in larvae exposed only to the
parasite (F= 22.3. df= 3,8 p= 0.000). The mortality of
the larvae that were both exposed to parasitism and fed
with
Bt
rice was also higher than that observed in the
larvae that were either fed
Bt
rice or parasitized.
In the assays for assessing the mortality of larvae that
were both exposed to parasitism and fed with
Bt
rice
(Cry1B) for three days after the installation of the
bioassay (Figure 2B), all of the treatments were
significantly effective compared with the control (F=
48,8. df= 3,8 p= 0.000). The highest mortality was
observed on plants where the larvae were simultaneously
exposed to parasitism and fed with
Bt
rice (T2, 73.3%),
followed by the treatment with only
Bt
rice plants (T3,
63.3%), and that where larvae were only exposed to
parasitism (T3, 61%). The efficiency of
Bt
decreased
compared with the previous assay (Figure 2A).
In these experiments, which aimed to evaluate the
mortality of larvae exposed to parasitism and fed with
Bt
maize plants (Cry1Ab), treatment T4 (
Bt
maize and
C. flavicincta
) was the most efficient (F=53,0, gl= 3,8,
p= 0.000, Figure 3A), causing 90% mortality of the
