Effects Of Cropland Use Changes On Population Abundance Of Helicoverpa Armigera(hübner):A Landscape Perspective

Cropland use changes usually have significant effects on the occurrence of agricultural pests population in fields,through changing living conditions,food resources and enemy-mediated biocontrol ability for pests.In last decades,cropping structures in China suffered drastic and accelerated changes.Given this new situation,research on ecological mechanisms involved in the changing pest status,mediated by crop structure in the landscape,become one of the key theme in agricultural pests management.The cotton bollworm,Helicoverpa armigera(Hübner),is an important polyphagous pest,whose population density experienced increase and decrease in north and northwest(Xinjiang)of China,respectively,under the adjustment of cropping structure recently.However,drivers for these dynamics have rarely been investigated from the large scale.In this study,a meta-analysis was performed to reveal effects of landscape pattern on the occurrence of polyphagous pests from worldwide scale.And ecological fundamental mechanisms for cropland use changes effects on the population dynamic of cotton bollworm were detected in landscape scale,in north and northwest of China.The main results were summarized as follow.1.We conducted a meta-analysis of 42 landscape level studies to detect the effects of landscape pattern on the population density of polyphagous pests.We found that non-crop habitats within an agriculture landscape displayed no significant effect on pests,crop management intensity within the landscape has a significant effect on pests.The production of intensively managed crops(i.e.pesticide spray frequently,Bt crops)had a negative effect on insect density,while the normal management crops increased insects density significantly.2.The research in cotton production area in northern China,in 2015-2017,found that population abundance of cotton bollworm adults in cotton fields was negatively correlate with cotton area in a landscape,and positively correlated with other crops(i.e.corn,peanuts and vegetables)production,in July and August.The ratio of cotton account for all host crops was also correlate with cotton bollworm abundance negatively.In northern China,the cotton species are transgenic,the adaption of Bt cotton suppressed cotton bollworm population effectively.However,the drastic decrease of cotton area in this region results the recovered population directly.3.The research in non-cotton production district in northern China,in 2015-2019,found that the density of overwinter generation cotton bollworm adults was positively correlate with the proportion of preceding summer corn,was correlate with semi-natural habitats negatively.For the first generation,the adults population was correlate with wheat and other crops areas positively,and also deceased with the increased proportion of semi-natural habitats area.For the second and third generations,corn proportion showed positive effects on the population,and the semi-natural area also had a negative effect.In this region,summer maize acted as the main overwinter sources for cotton bollworm,the first generation was develop on wheat and some other spring crops,the second and third generation main come from corn field probably.The increased area of wheat and corn in northern China provide sufficient food resources for cotton bollworm,accelerate the damage of cotton bollworm in multi-crops.4.The research in south Xinjiang,in 2017-2019 found that,in the district with diversified land use,adults density of the first and second generation cotton bollworm was negatively correlate with cotton planting,and this negative relationship was more obvious for the second generation.The proportion of other host crops(i.e.wheat,vegetables and corm)correlated with the density positively,and the ratio of cotton account for all host crops showed a negative effect on the population.However,the abundance of cotton bollworm was positive with cotton area in the low diversified land use district.In south Xinjiang,the increasing area of Bt cotton suppressed the population growth of cotton bollworm,was the main reason for their lower damage.