| Xinjiang has the largest cotton plantation in China.The cotton aphid Aphis gossypii(Heteroptera:Aphididae)is a serious threat to cotton growth,owing to its short generation time,overlapping generations,and fast development in resistance to chemical pesticides.Natural enemies are diverse and abudnant in nature,including generalist predatory insects such as ladybird beetles(Coleoptera:Coccinellidae)and lacewings(Neuroptera:Chrysopidae).Numerical empirical studies and pest control practices show that protection and enhancement of natural enemies can be an effective approach to suppress of A.gossypii infestation in cotton field.Intraguild predation is common in aphidophagous predators.Theory predicts that intraguild predation between higher trophic organisms can interfere with suppression of lower trophic herbivore population.While this prediction is supported by some empirical studies,it is not by others.This PhD thesis research program focuses on intraguild predation between 4 dominant generalist aphidophagous predatory insects,including two lacewings Chrysopa pallens and Ch.carnea(Neoroptera:Chrysopidae),and two ladybird beetles Coccinella septempunctata and Hippodamia variegate(Coleoptera:Coccinellidae).Experiments were made in greenhouse and survey in the cotton field in northern Xinjiang.The main results and conclusions are summarized as followed.1.Intraguild predation between ladybird beetles The generalist predatory ladybirds H.variegate and C.septempunctata are of major importance for biological control of aphids on agricultural crops.To investigate intraguild predation(IGP)intensity in relation to life stages of involving protagnists,experiments with 17 combination treatments were conducted in lab to evaluate IGP between H.variegate and C.septempunctata.A pair of stages with the equal number of insects was released onto a potted cotton plant without the presence of aphids and surviving insects were counted 24 hours later.The results showed that in pairings between different stages of the two predatory insects,later stages significantly survived more than earlier stages.Adult predation of earlier life stages increased in a quadratical form.In pairings between same stages of the two predatory insects,H.variegata adults survived less than their counterparts of C.septempunctata.The results of this study suggested that in the absence of extraguild prey(cotton aphids),the intraguild predation between H.variegata and C.septempunctata depends on life stages when different life stages are encountered,but on special biology when same life stages get into confrontation.2.Intraguild predation between ladybird beetle and lacewing predators The generalist predatory ladybird H.variegate,C.septempunctata and the lacewing Ch.pallens are notable natural enemies of aphids on agricultural crops.This study was to investigate intraguild predation effects between ladybirds and lacewings at varying stages.Two predators were paired with either same or different life stages,which were released onto a potted cotton plant without the presence of aphids Surviving insects were counted 24hours later.The results showed that in pairings between different stages of the two predatory insects,later stages significantly survived more than earlier stages.Adult predation of earlier life stages increased in a quadratic form.In pairings between H.variegata and Ch.pallens,H.variegata adults survived more than their counterparts of Ch.pallens when both were at the same stage.In pairings between C.septempunctata and Ch.pallens,C.septempunctata larva won more contests if both predators were at higher instars,but Ch.pallens won more if both were at lower instars.When C.septempunctata and Ch.pallens were at different life stages—between adults and larvae,the larvae won more contests when they were at higher instars,but C.septempunctata was the winner when the larvae involved were at lower instars.The results of this study suggested that in the absence of extraguild prey(cotton aphids),the intraguild predation intensity between a ladybird and a lacewing depends on life stages when different life stages are encountered,but on species biology when same life stages get into confrontation.3.Effects of Cocinellina septempuntata egg food on developmental performances of Chrysopa pallens Intraguild predation is widespread in predatory insect natural enemies of aphids.To examine defensive effects of eggs of the 7-spot ladybird beetle C.septempuntata from arid environments in Xinjiang,we fed Chrysopa pallens from either neonate or the 4th instar larvae on with aphids(control),the 7-spot ladybird eggs,or aphids + 7-spot ladybird eggs,and measured the lacewing survival to adult emergence.The results showed a significant interaction effect between foot type and feeding stage on the survival rate,which performed similar patterns across food treatments between feeding stages,higher when fed with aphids and aphids + 7-spot ladybird eggs but lower when fed with only 7-spot ladybird eggs.But the survival rate was lower when the lacewing larvae were fed from neonate than from the 4th instar.The results of this study suggest that C.septempuntata eggs in the arid environments are protective against predation by lacewings.4.Effects of ladybird and lacewing combination on aphid density Intra-guild predation is common among predator community of aphids.Quantifying effects of intra-guild predation on aphid population can provide data for evaluating and conserving natural enemies.C.septempunctata L.and Ch.pallens(Rambur)are two major predators of A.gossypii in cotton fields in north Xinjiang.To investigate how intra-guild predation between them affected aphid abundances,we made experiments using potted cotton plants in greenhouse to observe aphid population density under different paired-off treatments between the two predators at varying stages.The results of this study showed that while the pairs with Ch.pallens larvae resulted in an increasing aphid density,those with Ch.pallens adults lead to a decreasing aphid density.The results of this study suggest that intra-guild predation between Ch.pallens and C.septempunctata can have contrast effects on aphid population depending on predator life stages involved.5.Numerical responses of main predatory insects to Aphis gossypii density in cotton field in North Xinjiang To understand the numerical responses of main predators of aphids in cotton fields in north Xinjiang,we undertook a two-year survey of A.gossypii and its four predatory insects,two ladybird beetles,C.septempunctata and H.verigata,and two lacewings,Ch.carnea and Ch.septempunctata,in cotton fields in the Wujiaqu region of northern Xinjiang.The survey showed differential numerical responses to cotton aphids and varying seasonal patterns among the four predatory insects.C.septempunctata increased strongly in abundance in response to aphid abundance;H.verigata and Ch.carnea increased gradually but Ch.septempunctata did not change in abundance,in response to aphid densities.The four predatory insects showed similar seasonal trends in abundance,all peaking at the end of July.C.septempunctata and Ch.carnea were more abundant than H.verigata and Ch.septempunctata across the entire cotton-growing season.The results of this study indicate that C.septempunctata and Ch.carnea are the dominant natural enemies of A.gossypii in cotton fields in the Wujiaqu region of northern Xinjiang.Our study provides new data for conservation of insect natural enemies of aphids and for cotton aphid control practices. |
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