Effects Of Pea Aphid Pre-infestation On The Defense Response Of Fava Beans Against Conspecific And Allospecific Aphids

Plants are often attacked simultaneously or sequentially by a variety of insect herbivores,and the attack of insect herbivores will interrupt the defense system of the plant and change the metabolism,thereby affect the fitness of subsequent insect herbivores.Most studies have shown that the attack of insect herbivores induce plant defenses,however,this theory does not explain the phenomenon of aphid aggregation:even if the aphid population is quite large on a plant,the aphids rarely move to intact plants nearby.We tested a hypothesis that whether aphids induce plant defenses depends on the time of infestation:short-term infestation induces plant defense,and long-term infestation induces plant susceptibility.In this paper,the pea aphid（Acyrthosiphon pisum）,Malvaceae-specialized（MA）and cucurbit-specialized（CU）biotype of Aphis gossypii and fava bean were used as the research system to test the above hypotheses.Firstly,the fitness changes of pea aphids and two non-host aphids（MA biotype and CU biotype）on fava beans that were pre-infested by pea aphid for different duration were studied,and then the hydrogen peroxide content and total sugar content in fava beans were studied.In this paper,we attempted to reveal the relationship between aphid infestation duration and induced plant defense,and provide reference for aphid control,and the results are as follows:1.The fitness of pea aphids decreased on fava bean plants that were pre-infested for 1 d and increased on fava beans that were pre-infested for 5 d by pea aphidsThe fitness of pea aphids on pre-infested fava beans was evaluated by selection experiments,life tables and feeding behaviors.It was found that pea aphids preferred significantly 5-d pre-infested fava beans to healthy plants and significantly negatively preferred 1-d pre-infested fava beans to healthy fava beans.Compared with healthy fava beans,the internal growth rate（r_m）and net growth rate（R₀）of pea aphid were significantly reduced in the 1-d pre-infested fava beans,and increased in the 5-d pre-infested fava beans.The EPG data showed that the total duration of E1+E2 wave（reflecting saliva secretion and phloem feeding）of pea aphids on 5-d pre-infested fava beans were 6.44%longer than that on healthy fava beans.2.The fitness of two non-host aphids decreased on fava bean plants that were pre-infested for 1 d and increased on fava beans that were pre-infested for 5 d by pea aphidsTo verify the relationship between infestation duration and induced resistance or induced susceptibility,the fitness of two non-host aphids（MA biotype and CU biotype）on pre-infested fava beans was determined.It was found that MA biotype aphids preferred significantly 5-d pre-infested fava beans to healthy plants and negatively preferred 1-d pre-infested fava beans to healthy fava beans.Compared with healthy fava beans,the internal growth rate（r_m）and net growth rate（R₀）of MA biotype aphids were significantly reduced in the 1-d pre-infested fava beans.Compared with healthy fava beans and pre-infested fava beans,the time for the first E1 and E2 waves of MA biotype aphids on the 1-d pre-infested fava beans was significantly increased,and the average total duration of E1+E2 waves of the 1-d pre-infested fava beans were significantly reduced compared with healthy fava beans.Compared with healthy fava beans,CU biotype aphids tended significantly to pre-infested fava beans,and the internal growth rate（r_m）and net growth rate（R₀）were significantly reduced in the 1-d pre-infested fava beans,and the net growth rate（R₀）was significantly higher in the 5-d pre-infested fava beans.The EPG data showed that compared with healthy fava beans,the average total duration of E1+E2（phloem secretion of water-soluble saliva and phloem to obtain nutrients）waves on pre-infested fava beans was shortened by 37.32%.The fitness of MA biotype aphids was reduced on the 1-d pre-infested fava beans and the 5-d pre-infested fava beans,but the fitness was increased on the 5-d pre-infested fava beans than on the 1-d pre-infested fava beans.Compared with healthy fava beans,the fitness of CU biotype aphids on the 1-d pre-infested fava beans was reduced,and the fitness of the 5-d pre-infested fava beans was increased.The above results showed that the fitness of the two non-host aphids decreased on the 1-d pre-infested fava beans and increased on the5-d pre-infested fava beans.3.The hydrogen peroxide of fava beans that were pre-infested for 1 d and total sugar of fava beans that were pre-infested for 5 d were increased,while the hydrogen peroxide that were pre-infested for 5 d and total sugar that were pre-infested for 1 d were decreasedIn this study,hydrogen peroxide and reducing total sugar were determined in healthy fava beans,1-d pre-infested fava beans,and 5-d pre-infested fava beans.The hydrogen peroxide in 1-d pre-infested fava beans increased significantly,and the hydrogen peroxide gradually decreased with the extension of the pre-infestation time.Compared with healthy fava beans,the hydrogen peroxide of fava beans pre-infested for 1 days increased by 90.54%,and the hydrogen peroxide of fava beans pre-infested for 5 days decreased by 10.06%.Hydrogen peroxide is a stress defense substance,and its trend shows that when plants are subjected to short-term aggression,hydrogen peroxide rises to resist aphid feeding,while hydrogen peroxide decreases during long-term invasion,becoming conducive to aphid feeding.Compared with healthy fava beans,the total sugar of 1-d pre-infested fava beans decreased by 8.25%,and the sugar of 5-d pre-infested fava beans increased by 0.31%.The above results show that the content of endogenous hydrogen peroxide in fava beans increases first and then decreases with the increase of pre-infestation time,and the content of total reducing total sugars shows a trend of first decreasing and then rising,which explains the reasons why the fitness of latecomers increased on long-term pre-infested fava beans and decreased on short-term pre-infested fava beans.In summary,the pre-infestation of pea aphids for different duration had effects on the fitness of pea aphids and two non-host aphids to fava beans.Short-term pre-infestation of pea aphids induces resistance and enhances plant resistance to later herbivore insects.Long-term pre-infestation induces plant susceptibility and enhance the fitness of later herbivore insects.Therefore,aphids and their offspring had fitness profits on plants that were pre-infested by conspecific or allospecific aphids,thus answered why aphids tend to aggregate on plants.To maximize crop yields,we recommend to control aphids before aphids induce plant susceptibility.