Abstract: Current pest control methods rely on a pesticide dominated paradigm and there is need to adopt a more ecological approach based on renewable technologies such as host plant resistance and natural biological control, which are available even to resource poor farmers. Resistant cultivars complement natural enemy action in lowering pest infestation while intrinsic rate of increase of pest species on resistant varieties is lower. An experiment whose aim, was to test whether the African white rice stem borer (Maliarpha separatella), can be managed by use of resistant rice varieties in combination with entomopathogenic nematodes was set up at Kenya Agricultural Research Institute, Mwea. The study was arranged as a 4x4 factorial design and each treatment replicated three times. First factor was rice cultivars at four levels, resistant cultivar (M27615), second resistant cultivar (M27628), the highly susceptible but tolerant cultivar (M27608) and commercial check Basmati 370 variety, which were planted in 1x1m experimental plots. Second factor was application of EPN (Hetrerorhabitidis indica) as a suspension in distilled water to the cultivars at four different times after they were transplanted (no application, 3 weeks after transplant date (WAT), 5 WAT and 6 WAT. Results showed that M. separatella infestation was lowest on cultivar M27615 where H. indica was applied at 3 WAT, while cultivar M27608 had the highest yield despite high number of white heads and stem tunneling indicative of high levels of M. separatella infestation. The findings showed that host-plant resistance and EPNs can be integrated to manage M. separatella infestation (250 words).
INTRODUCTION
The ability of insect pests to respond to mortality factors by evolving into resistant biotypes makes it difficult for pest control operators to rely on a single control strategy in the long term. Integrated Pest Management (IPM) entails use of combination of host plant resistance, cultural methods and biological agents. It advocates minimal use of insecticides especially with known action thresholds. Litsinger et al. (2009) indicated that rice hoppers Nilaparvata lugens bred slowly on rice cultivars which were recognized as very resistant and there was no yield loss even with infestations of 30 hoppers per plant. Koppenhofer and Kaya (1998) described a strong synergistic effect on mortality of two scarab species, Cyclocephala hirta LeConte and C. pasadenae with combinations of imidacloprid insecticide and entomopathogenic nematodes. It has been reported that inoculative release which involve releasing small numbers of natural enemies into a crop cycle in combination with resistant varieties is able to protect the crop from pest infestations (Litsinger et al., 2011). Gassmann et al. (2008) found that improved resistance management for Bacillus thuringiensis crops was achieved with IPM strategies that incorporated entomopathogenic nematodes in non-Bt refuges. Reports indicate that host plant resistance influences the susceptibility of insect pests to pathogens including entomopathogenic nematodes (Cory and Hoover, 2006). The use of resistant rice cultivars in combination with a biological control agent may be a useful tool for integrated pest management programs to reduce pesticide input. This study was conducted to evaluate a combination of resistant rice cultivars and entomopathogenic nematodes for efficacy against M. separatella in flooded rice cultivation.
MATERIALS AND METHODS
Study site and experimental plots lay out: The experiment was set up at KARI-Mwea field testing site (Kirogo) (037.22760E, 0039.00S, Altitude 1150 a.s.l). The study was arranged as a 4x4 factorial design and each treatment replicated three times. First factor was rice cultivars at four levels, resistant cultivar (M27615), second resistant cultivar (M27628), the highly susceptible but tolerant cultivar (M27608 and a commercial check Basmati 370 variety. The experimental field (30x30 m) was divided into 72 plots of 1x1 m with 0.5 m between the plots. The cultivars were planted in the field in a randomized block design at a spacing of 20x20 cm.
Entomopathogenic nematodes application: Entomopathogenic nematode (Hetrerorhabitidis indica) which was the second factor was applied to rice cultivars at the rate of 6.7x104 Infective Juveniles (IJs) per liter of distilled at 3,5 and 6 weeks after transplant date (WAT). Heterorhabitidis indica rate of application in Infective Juveniles (IJs) per square meter was calculated as:
| (1) |
Where:
| N | = | No. of IJs required |
| h | = | No. of hills per plot (25 at 20x20 cm spacing) in 1M2 plot |
| p | = | Estimated Heterorhabitidis indica IJs concentration |
| t_i | = | Estimated number of M. separatella infested tillers |
| k | = | No. of experimental plots to be applied with nematodes |
which gave; N is 25 (157*17) or 66725 IJs per square meter.
Before EPNs application soil samples from the experimental plots were baited with Galleria mellonela L to determine whether there were any EPNS in the plots before application of the treatments. Application was by spraying the mixture of H. indica suspension in cool sterile water with a 15 L knapsack sprayer. Application was done at 5.30PM, just before sunset as EPNs are rapidly inactivated by ultraviolet light (Greenwood and Rebek, 2008). Control plots were sprayed with distilled water without EPNs.
Sampling and data collection: Data was collected on M. separatella infestation (number of egg batches, white heads, tunnelled tillers and larvae/pupae) M. separatella damage (numbers of non-productive panicles, Empty panicles, unfilled grains and 1000 grains weight) and yield (grain weight, dry matter weight and harvest index). At harvest, all stems were dissected and number with tunnels, number of larvae and pupae recorded. The number of productive panicles, filled grains, empty grains and 1000 grain weight from 10 randomly selected tillers was also recorded. The infestation was expressed as number of egg batches, larvae and% white heads calculated by the formula (Shafique et al., 2000):
| (2) |
Data analysis: Analysis of Variance (ANOVA) was conducted to determine the differences in pest infestation, damage levels and yield and yield components between the different treatments. Least significant differences (p<0.05) were used to the separate means when found significant. The statistical analysis was performed with GENSTAT (2009) version 12 statistical software.
RESULTS
Highest number of egg batches was on Basmati 370 (20.27) and the lowest on M27615 (15.40) where no H. indica has been applied. Analysis of variance indicated that cultivar had significant influence on the number of egg batches (p<0.001), while effect of time of EPN application was not significant (p> 0.05) and there was no interaction (p>0.05). Basmati 370 cultivar had significantly high number of egg batches than the other cultivars (p<0.001) (Table 1).
Percent white heads and tunneled tillers were low in all cultivars where Heterorhabtidis indica was applied at 3 WAT. The lowest number of whiteheads was on M27615. The highest number of percent white heads and percent tunneled tillers was in Basmati 370 where there was no application of H. indica. Analysis of variance showed that cultivar and time of H. indica application did not have any significant influence on percent white heads and percent tunneled tillers (p>0.05) (Table 2).
The heaviest grains were in cultivar M27608 (23.0) where H. indica was applied at 3 WAT. It also had the highest percentage of empty grains (27%) in absence of H. indica application. The lowest 1000 grains weight (19.44) was in Basmati 370 cultivar where H. indica was applied at 6 WAT. The results indicated that cultivar M27615 had 21.89% empty grains in the treatment where there was no H. indica application (Table 3).
| Table 1: | Effect of cultivar and time of H. indica application on number of egg batches at KARI-Mwea field station in 2010 short rains season |
| p: Probability, SE: Standard error, CV: Coefficient of variation | |
Grain yield and dry weight of straw in the different cultivars were influenced by H. indica application at different periods. Harvest index was generally low in all the cultivars. However, it was highest in cultivar M27615 where H. indica was applied at 3 WAT and lowest in cultivar M27608 where there was no H. indica application (Table 4).
| Table 2: | Influence of cultivar and time of H. indica application on percent white heads and percent tunneled tillers at KARI-Mwea field station in 2010 short rain season |
| p: Probability, SE: Standard error, CV: Coefficient of variation | |
| Table 3: | Effect of cultivar and application time of H. indica on percent empty grains and 1000 grain weight at KARI Mwea field station in 2010 short rains season |
| p: Probability, SE: Standard error, CV: Coefficient of variation | |
| Table 4: | Effect of cultivar and application time of H. indica on weight of grain, straw and harvest index KARI Mwea field station in 2010 short rains season |
| p: Probability, SE: Standard error, CV: Coefficient of variation | |
DISCUSSION
White heads and stem tunneling data indicated that there was consistently low M. separatella damage in plots where Heterorhabtidis indica was applied at three Weeks after Transplanting (WAT). High levels of damage occurred where treatment was applied at 5 and 6 WAT. These results suggest that late application of the entomopathogenic nematodes after 5 WAT may have been too late to reverse the damage already inflicted by M. separatella. The subsequent reduction in grain yield and increase in dry straw weight is consistent with similar findings, where the pest infestations at early stages of plant growth stage leads to production of many nodal tillers which may have few productive panicles (Nwilene et al., 2011). It was found in this study that although there were low levels of infestation on resistant cultivars where Heterorhabitidis indica was applied none of the treatments completely protected the rice crop. This finding is consistent with the reported lack of complete resistance only partial resistance in most rice genotypes to stem borers (Shafique et al., 2000) and contradicts the results in chapter 7 where there were no white heads in M27615. The cultivar M27615 in this study indicated partial resistance while M27608 indicated tolerance. Host plant resistance to stem borers in rice is reported to be highly variable (Litsinger et al., 2009). The effectiveness of combination of resistant cultivars and EPNs in control of M. separatella is supported by results of Thomas (1999) which showed that host plant resistance and natural enemies were able to reduce aphid populations and that the effectiveness of individual mechanisms in suppressing pest population growth rate depended on pest life history. He also showed that biological control and host plant resistance can be compatible and can combine additively or synergistically to improve pest control. It has been argued that partial resistance, like host-plant tolerance, is potentially important for host plants for it allows retention of natural enemies. In contrast, a very high level of resistance could be detrimental because it could lead to extinction of natural enemies similar to effects of insecticides leading to ecological imbalances which is contrary to the tenets of Integrated Pest Management (Sandler, 2010).
The results of low levels of infestation on resistant cultivars where Heterorhabitidis indica was applied is supported by results of Gassmann et al. (2008) which indicated that presence of entomopathogenic nematodes in refugia increased fitness cost of Bacillus thuringiensis resistance, indicating that their presence may slow pest adaptation to B. thuringiensis crops. They suggested that nematodes presence in refuges may delay resistance by pests to B. thuringiensis crops. Host plants can influence susceptibility of insects to pathogens (Cory and Hoover, 2006) including entomopathogenic nematodes. As such, it would be important to know the fitness and fecundity of adult moths of M. separatella emerging from cultivars where antibiosis and antixenosis is high. Information on fitness and fecundity of entomopathogenic nematodes infecting the larvae of M. separatella feeding on resistant rice varieties will also be important. These studies including the effects of these cultivars on Trichogramma sp. which is an important M. separatella egg parasitoid would shed light on the compatibility of H. indica and resistant rice cultivars in the management of the pest. While this study has demonstrated that an integration of entomopathogenic nematodes and resistant or tolerant cultivars can reduce M. separatella infestation, it also suggests that their efficacy can be improved by modifying cultural practices to allow for their integration in flooded rice cultivation.
CONCLUSION
Combination of resistant cultivars and EPNs was effective in controlling M. separatella. Maliarpha separatella infestation was lowest on cultivar M27615 where Heterorhabtidis indica was applied at three weeks after transplant date and M27608 had the highest yield despite high M. separatella infestation. This showed that host plant resistance and natural enemies are able to reduce M. separatella infestations and that biological control and host plant resistance can be compatible and can combine additively or synergistically to improve pest control.
ACKNOWLEDGMENT
The authors are grateful to Director Kenya Agricultural Research Institute and Chief executive officer/Secretary National Council for Science and Technology for funding the study.