| Wheat is one of the major cereal crops worldwide and plays a significant role in food security of human.Currently,China is a leading country for wheat production over the world.Aphids are considered as serious insect pests of the wheat crop and most extensively spread around the globe.The aphids can cause both direct and/or indirect damages to the wheat crop.Natural enemies are well known for their ability to control aphid species in a wide range of cropping systems.Synthetic insecticides are one of the most important tools used for the management of aphids in the wheat fields.Nevertheless,insecticides are harmful to the environment and disrupt the biological services.Competition is one of the essential forces responsible for structuring biological communities and driven by environmental disturbances such as biotic and abiotic factors.Insecticides and natural enemies are considered as a good example for the anthropic abiotic factors and natural biotic factors.Environmental disturbances caused by insecticides and natural enemies have received very limited attention and have seldom been examined it.Therefore,more understanding the interactions in complex agroecosystems is necessary for enhancing pest management programs.The aim of the study is to answer the central question;how population structure and competition among cereal aphids' Rhopalosiphum padi L.and Sitobion avenae Fab(Hemiptera:Aphididae)can be influenced by natural(e.g.natural enemies mainly predators and parasitoids)and anthropic(e.g.pesticides particularly commonly used imidacloprid insecticide)as well their interactions?To answer this question,R.padi and S.avenae were exposed to intra and interspecific competitions on insecticide-free and exposure wheat seedlings under laboratory conditions.Moreover,two consecutive field seasons 2014/2015 and 2015/2016 using exclusion cage,restriction cage,and open field cage were conducted.The results of this study can be summarized as follows:1.The instantaneous rate of population growth(rt)of two species of wheat aphids,R.padi and S.avenae without insecticide treatment with the low initial population density is significantly higher than in high initial population density during the experimental periods.Whereas,the rt in experimental units which were treated with imidacloprid insecticide were changed.2.The interspecific competition experiments between R.padi and S.avenae under laboratory and field conditions,clearly demonstrated that the occurrence of either aphid species negatively influenced the population density and the reproductive rate of the other.Moreover,there was strong interspecific competition between R.padi and S.avenae under natural colonization;although the presence of insecticide on wheat plants significant suppressed the interspecific competitions.Furthermore,the results also revealed that the R.padi consistently is a strong competitor than S.avenae without insecticide;whereas S.avenae become the stronger in after application of insecticide.In addition,the results of laboratory experiments showed that the uneven interspecific competition revealed that there is a stronger interspecific competition than intraspecific competition between R.padi and S.avenae with and without insecticide.3.The present study concluded that the insecticide mediated a shift in aphid species dominance and competition outcome indicating that insecticides are relevant mediators of species interaction,potentially influencing community composition and raising management concerns as a potential cause of secondary pest outbreaks.This study provided more understanding about competitions of wheat aphids in the presence of insecticides;which can be useful in insect pest management.This finding suggested that imidacloprid insecticide can play an important role to switch the intra and interspecific competitions of R.padi and S.avenae.4.The competition among cereal aphids affected by the natural enemies,S.avenae was more attacked by the parasitoids than R.padi.Both aphid species showed similar the response to the predation.In the present study it is observed that the predators are a response to aphid density rather than species of aphid;therefore.high density of predators recorded with a high density of aphids either S.avenae or R.padi with or without insecticide.The presence of predators in the natural habitat has more effect on the supper competitor species which it can create a balance between competitors.5.The combined effects of imidacloprid and natural enemies contributed to the best suppression of aphid competition despite that the density of natural enemies was negatively affected by imidacloprid treatment.We suggest that the natural enemies and imidacloprid create a balance in the competitions among cereal of aphids.In conclusion,there were strong effects of the application of insecticide,natural enemies as well as their combined impact on the competitions of cereal aphids in the wheat field.Under natural colonization R.padi is a strong competitor than S.avenae;whereas the insecticide-induced shift in the dominance and competitive potential from R.padi to S.avenae.The population growth rates of R.padi and S.avenae strongly declined by the application of imidacloprid,nevertheless,the parasitism rate was also significantly affected.Therefore,imidacloprid insecticide is still effective in suppressing cereal aphid populations in winter wheat fields of North China throughout the growing seasons;however.the negative impact on the biological control services needs to be considered with caution.Interestingly,this is the first report focused on competitions among cereal aphids under stress of insecticide under laboratory and field conditions.Therefore,this study added a new value to the basic science particularly in fields of entomology,ecology,toxicology and environmental sciences. |
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