Effects Of Host Plant On Growth,Life Table Parameters,and Defense To Pathogens Of The Pea Aphid (Acyrthosiphon Pisum)

The pea aphid,Acyrthosiphon pisum(Hemiptera:Aphididae),is a pest of leguminous crops.It damages crops directly by causing injuries and indirectly by transmitting plant pathogens.The insect has drawn the interest of many researchers globally because of its incomplete immune system but can mount strong immune responses against invaders.Although most defensive responses are known,there is a lack of knowledge of host plants'effects on these responses.Also,the impact of plants on growth rates and life table parameters of the insect have received insufficient attention.Therefore,the major aim of this research was to understand the disparities in growth,life table parameters,and defensive responses of the pea aphid reared on two different host plants.The specific objectives were(1)to investigate the nutritional differences in alfalfa,Medicago sativa,and broad bean,Vicia faba phloem saps and assess their influence on growth rates,body weights,and life table parameters;(2)to determine the variations in immune responses when pea aphids are feeding on the two host plants;(3)to evaluate plants'impact on the alternative defense responses of pea aphid against microbial infections.The results are summarized as follows:1.Potential nutritional variation in the phloem sap of V.faba and M.sativa influences the growth and life table parameters of the A.pisumThe mean concentrations of eight amino acids differed considerably between the two plants:arginine,histidine,lysine,proline,asparagine,glutamine,tyrosine,and phenylalanine.All of these amino acids were significantly higher in V.faba leaf samples than in M.sativa samples.Similarly,the total concentration of all free amino acids was significantly higher in V.faba than in M.sativa.The most abundant amino acids in M.sativa were all non-essential amino acids,i.e.,asparagine(36.03%),glutamic acid(17.37%),and serine(11.92%).Similarly,the most abundant amino acids in V.faba were all non-essential amino acids:asparagine(60%),tyrosine(7.08%),and glutamic acid(3.91%).The total concentration of essential amino acids was significantly higher in V.faba than in M.sativa.The most abundant essential amino acids in M.sativa were threonine(31.28%),leucine(16.1%),and valine(15.72%),while in the V.faba,the most abundant essential amino acids were histidine(48.4%),lysine(14.04%)and valine(9.7%).The contribution of the essential amino acids to the total amount of amino acids was 9.65%for M.sativa and 12.35%for V.faba.The most varied essential amino acids in the two plants were histidine,lysine,and phenylalanine.Fructose,glucose,and sucrose were detected in the phloem sap of both plants.The total concentration of sugars differed significantly between the two plants.Individual sugars in V.faba were all significantly higher than those in M.sativa.Correspondingly,the total amount of all sugars were significantly higher in V.faba than in M.sativa.More importantly,the analysis revealed that both total amino acids and total sugars were significantly higher in V.faba than in M.sativa.Aphids reared on M.sativa(the low-quality plant)had lower growth rates,body weights,intrinsic rate of natural increase,and finite rate of increase than the aphids feeding on V.faba(the higher quality plant).The generation time,pre-reproductive period,and the doubling time were more extended in the aphids reared on M.sativa than in the aphid reared on V.faba.This may indicate that broad beans are more suitable host plants for pea aphids,and the suitability of V.faba may be due to its higher nutritional quality.2.Host plants affect the immune defense of A.pisum in response to pathogenThe survival of aphid after exposure to different pathogens was not affected by the host plants.However,A.pisum reared on M.sativa generally harboured fewer microbial loads than those reared on V.faba.Furthermore,infection of the aphid with Escherichia coli and Staphylococcus aureus induced hydrogen peroxide production(H₂O₂)and significantly increased phenoloxidase(PO)activity in both aphid groups.Interestingly,the aphid group feeding on V.faba showed higher H₂O₂ levels but lower PO activity than those on M.sativa.This could be because the aphids allocate different amounts of resources to each immune response.Also,these two immune responses may have trade-offs between each other.Both in infection and un-infection conditions,A.pisum reared on M.sativa had higher PO activity than their counterparts reared on V.faba,indicating that the host plant influences the activity of this critical enzyme.Aphids from both plant species were able to phagocytize E.coli and S.aureus cells,but there were no significant differences in the ability to engulf bacterial cells.These observations suggest that A.pisum reacts differently to infections on different plants.The host plant moderates interactions between the A.pisum and their pathogens by influencing the aphid's immune responses.3.Host plants affect the alternative defense of A.pisum in response to pathogensRearing pea aphid on different host plants,M.sativa and V.faba,did not change the composition of the secondary symbionts.Aphids raised on V.faba invested more in terminal reproduction in response to bacterial and fungal infections.Aphids raised on M.sativa,on the other hand,responded to Staphylococcus aureus and Beauveria bassiana by producing significantly more winged offspring.This may mean that when food quality or supply is low,aphids will produce winged offspring that can fly away and seek out new plants,while when food quality is high,aphids will respond to infection by increasing reproductive output.This indicates the role of the host plant in these responses.In conclusion,in this study,I found a significant difference in the pea aphid's growth rate,body weights,life table parameters,and defenses when fed on diets with potential variations in the nutritional quality.The most important aspect of the research is how aphids react to pathogens while feeding on diets that differ in nutritional qualities.More specifically,diets influence defensive strategies between terminal reproduction and the production of winged offspring.This information is essential in designing ecological pest management programs for managing aphids with microbial pathogens.