Host Plant Selection Of Spodoptra Exigua (Hübner) And Effects Of Host Plants On Its Development

The beet armyworm, Spodoptera exigua (Hübner), originated in southern Asia and its first record in China ascended to 1892 in Beijing. It was not a serious pest in China before 1980s, but its occurrence has been spreaded and its damage has been magnified since mid to late 1980s. So S. exigua has become a key pest on many kinds of vegetables, especially in recent years. Host plant selection mechanisms of noctuid polyphagous insects are hot spots of the co-evolution between plants and insects. To clarify the relation of interaction between S. exigua and its host plants, experiments were carried out to study the impacts of different host plants on the physiological metabolism of S. exigua, the adult oviposition and larval feeding preferences of S. exigua on different host plants, and the behavioral reaction of S. exigua to different host plants. The main results are as follows:1. Effects of host plants on development and some digestive enzyme activities of S. exiguaBy laboratory feeding experiments and biochemistry detection methods, the nutrition substances, the secondary metabolites of three kinds of host plant (sugar beet, maize and cucumber) were detected, and the effects of these host plants on development and oviposition of beet armyworm Spodoptera exigua (Hübner), total amino acid contents and the digestive enzyme activities in the 5th instar larvae were compared. It showed that comparing with those fed on maize and cucumber, S. exigua individuals fed on sugar beet had shorter larval and pupal periods, higher pupal survival rate, longer adult lifespan and laid significantly more eggs per female, which was 11.4d, 5.6d, 100%, 26.3d, 301.8 eggs respectively. Among the three feeding treatments, the protein contents in the 5th instar S. exigua larvae showed no significant difference, whereas total amino acid contents, amylase and lipase activities differed significantly. According to the host plants, the amylase and lipase activities in S. exigua ranked as maize > sugar beet > cucumber.2. Adult oviposition and larval feeding preferences of S. exigua on three kinds of host plantsThe beet armyworm, Spodoptera exigua (Hübner), has a wide range of host plants. To reveal the relationship between adult oviposition preference and larval feeding preference, seven kinds of plants were selected and tested on S. exigua by laboratory multiple-choice test and field non-choice test. The results showed that in the field non-choice test the amount of S. exigua eggs on different plants ranked as maize > pepper > cotton > cucumber, cowpea, tomato > cabbage. It ranked as maize > pepper, cotton, cucumber, cowpea > tomato and cabbage in the laboratory multiple-choice test. Host plant feeding preference of larvae at different instars differed and changed to some extent with the prolonged observation period. Young larvae at 1st to 2nd instars prefer to feed on cowpea, maize and cucumber, but not on cabbage, tomato, pepper and cotton. Larvae at 4th to 5th instars also showed feeding preference to pepper. Feeding preference of the 5th instar larvae was not so obvious as the young larvae.3. Behavioural reaction of S. exigua to three kinds of host plants and their volatile extractsTo reveal the mechanisms of the oviposition host preference of S. exigua, three kinds of plants, i.e. maize, cucumber and cabbage, were selected according to the amount of oviposition. Behavioural responses of S. exigua females to these plants and their relative volatile extracts were tested by Y-tape olfactometer. The components and contents of volatiles of these plants were compared as by GC-MS. The results showed that females showed strong behavior selection reaction to maize and its volatile extract, followed by cucumber, and the weakest to cabbage. The behavior reaction results were accordant with the oviposition preferences. The GC-MS results showed that the volatile components of maize, cucumber and cabbage differed. The kinds of the volatile components were the least in maize. Alkyl substances existed in all the three plants and the amount was the highest in maize. It indicated alkyl substances might be the effective attractant to S. exigua.4. SummaryFrom the above studies, it showed positive correlation between larval development and amylase and lipase activities in S. exigua feeding on different host plants. Plant volatile, especially alkyl substances, might have important roles in the oviposition host selection of S. exigua. Adult oviposition and larval feeding preferences of S. exigua to different host plants were not consistent.