The Nontarget Effect Of Agasicles Hygrophila,A Biological Control Agent Of The Invasive Plant Alternanthera Philoxeroides,on A Native Plant Alternanthera Sessilis

Classical biological control,based on the introduction and release of the natural enemy of invasive species(i.e.parasitoids,predator or pathogens,etc),could be a highly cost-efficient and effective method to control the invasive species.However,some biocontrol agents not only fed on their target invasive plants,but also adversely affect the non-target native species directly or indirectly and threaten the local ecosystem after releasing,thus leading to non-target effects.It is a great challenge for biological control participants to select the safe and efficient biocontrol agent and improve the risk assessment of potential agents.The host choice and utilization of insects could be affected by a variety of biological and non-biological environmental factors,such as climate,soil nutrition and water,host plant quality and availability as well as natural enemies.However,the testes of host range and preference of biocontrol agents are usually carried out in the laboratory or in the greenhouse.Field surveys can assist to accurately predict the host choice and utilization of biocontrol agents in nature,that is,the ecological host range of insects.In this study,we conducted feld survey,common garden and field experiments to test whether the non-target species,Alternanthera sessilis,could support the individual growth and sustain the population of the Agasicles hygrophila,a biocontrol agent introduced for controlling the invasive plants,Alternanthera philoxeroides,and assess the effects of climate,host plant and natural enemies on the non-target effects of A.hygrophila.We found that:(1)Agasicles hygrophila could complete its life cycle from egg,larva,pupa to adult in Alternanthera sessilis in the field.There were significantly positive correlations between the population size of A.hygrophila and the diameter,plant height and biomass of A.sessilis,among which the diameter and plant height were the most important factors that influence the population size.There were significantly positive correlations between the population size and water content,total nitrogen content,pH,longitude and latitude,among which the water content and longitude were the most important factors that influence the population size.(2)Common garden experiment was conducted to test the effects of soil nutrients,water content and plant density on the individual traits of A.hygrophila.It was found that environmental factors could not only affect the plant traits,but also the growth,development and survival of the beetle.Soil nutrient was the key factor that determined the survival of A.hygrophila,while soil nutrient,water and plant density interacted to affect the growth and development of A.hygrophila.From sand to soil treatment,the number of pupae and adult increased and the developmental time decreased.The developmental time decreased with reducing water content.(3)Agasicles hygrophila could sustain its populations on A.sessilis,but only beetles at the Guilin site could successfully overwinter on A.sessilis,beetles at Wuhan and Kaifeng all died in the winter.Different climate zones may directly,through affecting overwinter,or indirectly,through affecting the phenologies of plant,influence the development and population maintenance of A.hygrophila.(4)Pupation into the stem of A.sessilis is the key factor that influence the non-target host use of A.hygrophila.There were higher number of boring pupae and eclosion adults,lower mortality rate of larvae and larger beetle size on A.sessilis with larger diameter stems.In the laboratory environment,the beetle could complete its development with the pupation outside the stem.However,the number and weight of out-stem adult decreased and developmental time increased with the reduced water content in the environment.The results of our out-door cage experiment on the effects of potential natural enemies also show that A.hygrophila pupae need the protection from stem to exclude natural enemies.These results fully demonstrate that the native plant,A.sessilis is the ecological host of A.hygrophila.In this study,we examined the effects of various biotic and abiotic factors that influence the non-target host use of biocontrol agent from the perspective of natural environment.Our results have important implications for future research on the non-target effect of biological control.Considering both the biotic and abiotic factors that influence host choice and utilization of biocontrol agents could improve the risk assessment in the future.