There was a significant effect on mean larval development time (F3,157 = 7.97,
P<0.01). The WF06-Vip3ASEL population had significantly lower larval development times compared with the WF06-UNSEL population (Table 6.1). There was no significant difference between male and female larval development times within each population (P>0.05).
The mean pupal development time was significantly different (F3,157 = 12.51, P<0.01).
The mean male pupal development time was significantly longer in both the WF06- UNSEL and WF06-Vip3ASEL populations compared with the mean female pupal development time (Table 6.1). There was no significant difference within either male or female pupal development times (P>0.05).
There was a significant effect on mean development time from egg to adult eclosion (F3,157 = 5.59, P<0.01), with WF06-Vip3ASEL female development significantly
faster compared with WF06-Vip3ASEL males, and WF06-UNSEL female and males (Table 6.1). There were no further significant differences between WF06-Vip3ASEL males and WF06-UNSEL females and males (P>0.05).
The proportion of larvae that survived to adult eclosion was significantly different between WF06-Vip3ASEL (72 out of 148) and WF06-UNSEL (89 out of 144) (z1, 28 =
-2.253, P<0.05; Table 6.1).
There were significant effects on pupal weight between the populations (F3, 157 =
15.37, P<0.01; Table 6.1). Mean female and male pupal weights were significantly greater in the WF06-UNSEL population compared with the WF06-Vip3ASEL population (P<0.05). There were no significant differences in mean pupal weight between females and males within each population (P>0.05).
There were no significant differences in the sex ratio for both populations from the expected 50:50 split, with the WF06-UNSEL population having 41 females and 48 males (χ2=0.55, 1 d.f., P>0.05), and the WF06-Vip3ASEL population having 35 females and 37 males (χ2 = 0.056, 1 d.f., P>0.05)
Table 6.1: Mean (± se) development parameters for WF06-UNSEL and WF06- Vip3ASEL populations of Heliothis virescens.
Life history trait Sex WF06-UNSEL1 WF06-Vip3ASEL1
Larval development time (days) female 19.3 ± 0.3 17.1 ± 0.5 male 18.9 ± 0.5 17.9 ± 0.5 Pupal development time (days) female 14.0 ± 0.2 14.3 ± 0.3 male 15.2 ± 0.3 15.3 ± 0.3 Egg to adult development time (days) female 36.3 ± 0.5 34.4 ± 0.7 male 37.1 ± 0.6 36.1 ± 0.7
Pupal weight (mg) female 288 ± 4 264 ± 4
male 290 ± 4 259 ± 5
Adult eclosion (%) - 62 ± 3 49 ± 2
1 Sample size: WF06-UNSEL n (females) = 41, n (males) = 48; WF06-Vip3ASEL n (females)
= 35, n (males) = 37.
6.3.2 Mating success, fecundity and egg viability of WF06-UNSEL and WF06- Vip3ASEL populations and their reciprocal crosses
There was no significant difference in the proportion of pairs that produced eggs between any of the four crosses (χ2=4.47, 3 d.f., P>0.05; Table 6.2). There were significant differences in the proportion of pairs that produced viable progeny. In the WF06-Vip3ASEL and WF06-UNSEL female x WF06-Vip3ASEL male crosses fewer pairs produced viable progeny compared with WF06-UNSEL and WF06-Vip3ASEL female x WF06-UNSEL male crosses (χ2=8.26, 3 d.f., P<0.05; Table 6.2).
There was no significant difference in the mean number of eggs laid excluding pairs that produced no viable offspring between any of the four crosses (P>0.05, 28 d.f.,
n=31; Table 6.2). There was also no significant difference in the mean number of eggs laid that included pairs which produced no viable offspring between any of the four crosses (P>0.05, 63 d.f., n=66).
The mean egg viability of the WF06-Vip3ASEL cross was significantly lower compared with the WF06-UNSEL cross, or the WF06-UNSEL female x WF06- Vip3ASEL male and WF06-Vip3ASEL female x WF06-UNSEL male crosses (P<0.05, 63 d.f., n=66; Table 6.2). There were no other significant differences in mean egg viability (P>0.05, 63 d.f., n=66).
Table 6.2: Mean reproductive parameters for WF06-UNSEL, WF06-Vip3ASEL, WF06-Vip3ASEL female x WF06-UNSEL male and WF06-UNSEL female x WF06- Vip3ASEL male populations of Heliothis virescens.
Life history trait WF06- UNSEL 1 WF06- Vip3ASEL 1 WF06-Vip3ASEL female x WF06- UNSEL male 1 WF06-UNSEL female x WF06- Vip3ASEL male 1 Mating pair success 2
- eggs laid (%) 81 79 100 90
Mating pair success 3
- viable progeny (%) 57 26 59 25
Mean egg no. per
viable pair 2 (± se) 802 ± 157 856 ± 297 939 ± 209 1020 ± 338 Egg viability (%)
(± se) 43 ± 4 10 ± 7 48 ± 6 43 ± 13
1 Number of pairs: WF06-UNSEL = 21; WF06-Vip3ASEL = 19; WF06-Vip3ASEL female x WF06-
UNSEL male = 17; WF06-UNSEL female x WF06-Vip3ASEL male = 20.
2
Number of egg laying pairs: WF06-UNSEL = 17; WF06-Vip3ASEL = 15; WF06-Vip3ASEL female x WF06-UNSEL male = 17; WF06-UNSEL female x WF06-Vip3ASEL male = 18.
3 Number of viable pairs: WF06-UNSEL = 12; WF06-Vip3ASEL = 5; WF06-Vip3ASEL female x
6.4 Discussion
In the present study, the Vip3A resistant population of H. virescens showed faster larval development and faster female development to adult eclosion compared with an unselected population. A Cry1Ac resistant population of P. xylostella (SERD4) has also been reported to show faster larval development time (Sayyed et al., 2003) but other reports on Cry resistant populations have generally shown an increased development time or no effect (Gassmann et al., 2009; Table 6.3). For example, a reduced growth rate was found in a H. virescens population resistant to Cry1Ac and Cry2Aa, although the authors suggested that this cost may have been a result of inbreeding (Gahan et al., 2005). Differences in development time have the potential to lead to non-random mating with resistant adults mating with each other and not susceptible populations. However, the relatively small differences in faster development time for resistant insects observed in the laboratory, in the present study, would probably be mitigated by overlapping generations in the field, particularly as the season progresses (Wu et al., 2002; Bird and Akhurst, 2004). The bias of only using larvae that had moulted during selection to continue the population may also have had an impact on the observed faster development rates of the Vip3A resistant population in comparison to the susceptible population.
The reduced pupal weight for the Vip3A resistant population observed in the present work was similar to findings in other studies (Table 6.3), although an increase in pupal weight was reported in the Cry1Ac resistant SERD4 population of P. xylostella (Sayyed and Wright, 2001b; Sayyed et al., 2003). The decrease in survival to adult eclosion for the Vip3A resistant population in the present study has also been observed in other studies with Bt toxins (Table 6.3), and while survival in some resistant populations was not affected, there appears to have been no reported increase in survival. Larval survival was also found to be reduced in populations of H.
armigera and P. gossypiella (Table 6.3).
Fitness studies that include F1 reciprocal crosses allow the dominance of fitness costs
10
7
Table 6.3: A summary demonstrating the variety of fitness effects correlated with Cry resistance in some lepidopteran pest species. Species
larval
development larval survival
pupal weight survival to adult fecundity egg viability mating success H. virescens
(present study) +ve -ve -ve NE -ve -ve
H. virescens 1 NE / -ve NE NE
H. armigera 2 NE / -ve -ve -ve -ve -ve -ve
P. gossypiella 3 NE -ve NE NE NE / -ve
P. xylostella 4 NE / +ve NE / +ve NE / -ve -ve NE / -ve -ve
P. interpunctella 5 NE / -ve NE / -ve
T. ni 6 -ve -ve
+ve means positive effect on fitness of resistant population compared to susceptible population; -ve means negative effect on fitness of resistant population compared to susceptible population; NE means no effect on fitness of resistant population.
1 (Gould and Anderson, 1991; Gahan et al., 2005) 2
(Akhurst et al., 2003; Bird and Akhurst, 2004; Bird and Akhurst, 2005; Liang et al., 2008; Zhao et al., 2008)
3
(Carrière et al., 2001a; Higginson et al., 2005)
4 (Groeters et al., 1993; Groeters et al., 1994; Sayyed and Wright, 2001b; Sayyed et al., 2003) 5 (Oppert et al., 2000)
cross as well as in the resistant population, would be expected to be most effective for delaying resistance (Gassmann et al., 2009).
In the present study, studies on mating success, fecundity and egg viability revealed variability between the resistant WF06-Vip3ASEL and susceptible WF06-UNSEL populations and their F1 reciprocal crosses. The number of females that successfully
laid eggs was not affected by the cross, indicating that the ability of females to produce eggs was not effected, and this was further demonstrated by no effect on fecundity among the crosses. Successful mating with pairs involving WF06- Vip3ASEL males was reduced as determined by the proportion of pairs producing viable progeny demonstrating a nonrecessive fitness cost. This suggests that resistant males may have reduced virility. However, only the resistant population exhibited reduced egg viability in comparison to WF06-UNSEL and the F1 reciprocal crosses,
indicating that expression of reduced egg viability requires mating between resistant males and females, a recessive fitness cost. It may be more likely, therefore, that mating frequency was reduced or mating did not occur with some resistant males.
Fitness costs on fecundity and egg viability have been found in other studies (Table 6.3), for example, with Cry1Ac resistant H. armigera (Liang et al., 2008), and Btk resistant P. xylostella (Groeters et al., 1994). Fitness effects on resistant males have also been reported. Male mating frequency was reduced in a Btk resistant population of P. xylostella population (Groeters et al., 1993). While Zhao et al. (2008) found that the incidence of successful mating was reduced in a Cry1Ac resistant population of H. armigera, where differences between F1 crosses suggested that resistant males
reduce the incidence of mating paternity (egg viability) rather than mating frequency. Reduced male fertility was also observed in another Cry1Ac resistant population of H.
armigera and in F1 crosses (Bird and Akhurst, 2005). Higginson et al. (2005) found
no effect on mating frequency or egg viability in the absence of competition for several Cry1Ac resistant populations of P. gossypiella. However, competition studies with susceptible males resulted in reduced egg viability with resistant males that had mated first, caused perhaps by reduced sperm precedence.
in the present study to maintain large populations of selected and unselected populations to ensure that differences in fitness of populations are a result of Vip3A resistance. However, the nature of establishing a laboratory population will no doubt result in the genetic background of such a population differing over time with that of a wild population, therefore, measuring the fitness of a population is very difficult and it is important to understand the limitations of drawn conclusions (Reed and Bryant 2004; section 6.1).
Fitness studies on Vip3A resistant H. virescens could be extended to include other parameters investigated in Cry toxin resistant insect populations, such as emergence of overwintering populations (Carrière et al., 2001b; Bird and Akhurst, 2004; Carrière
et al., 2007), the effect of host plants on larval development and survival (Raymond et
al., 2005; Bird and Akhurst, 2007) and the effect of pathogens (Raymond et al.,
2007a).
Fitness costs expressed in the present study revealed a variety of effects on the Vip3A resistant WF06-Vip3ASEL population. The reduced mating success observed in resistant males may help to limit an increase in the frequency of the resistant allele (Zhao et al., 2008), and with the possible paternal influence on Vip3A resistance (Chapter 5), contribute to delays in the evolution of resistance in the field with the involvement of current management strategies involving the use of refuges (Gassmann et al., 2009).