Study On Interaction Between Beet Armyworm, Spodoptera Exigua (H.) And Chenopodiaceous Plants

The beet armyworm, Spodoptera exigua (Hübner), a key agricultural pest around the world, is highly resistant to most insecticides and has been causing serious infestations due to its wide range of hosts. Studies on the interactions between the beet armyworm and its hosts are helpful to developing new methods against the pest. Based on aspects of behavior, biology, and phytochemistry, we studied the interactions between Chenopodiaceous plants and the beet armyworm.1. Oviposition and feeding preference of the beet armyworm on Chenopodiaceous plantsLaboratory experiments showed that females preferred chenopod vegetables (Beta vulgaris L. var.cicla Koch., Spinach oleracea L, Salsola komarovii Iljin) to chenopod weeds (Chenopodium album L, C. serotinum L. and Suaeda salsa (L.) Pall.) for oviposition, but had a similar preference among chenopod vegetables or chenopod weeds. Females preferred chenopod vegetables to crucifers (Chinese cabbage and radish) for oviposition, but had a similar preference between chenopod weeds and the two crucifers. Females significantly preferred chenopodiaceous plants to common cabbage for oviposition. Feeding preference tests indicated that larvae preferred crucifers (common cabbage, Chinese cabbage and radish) to S. salsa, but larvae had a similar feeding preference between other chenopod plants (B. vulgaris var. cicla, S. oleracea, C. Album, C. serotinum) and the crucifers.2. Effect of adult experience on behavior of oviposition in the beet armywormBehavioral experiments showed adult experience affected subsequent oviposition habit and oviposition preference. Newly-emergent adults, that have experienced either C. serotinum or Poa annua (non-host), decreased the proportion of eggs laid on cage walls and increased the proportion of eggs laid on plants. In the oviposition preference experiment between chenopod and crucifer, adults that experienced the two crucifers increased their preference for radish and Chinese cabbage, and had a similar preference between the beet and the 2 crucifers. However, adults still showed a significant oviposition preference to chenopod although their preference to common cabbage increased after experiencing common cabbage. During no-choice oviposition experiment, newly-emerged adults oviposited on non-host plants after experience. The longer the adult experienced the plant, the more eggs they laid on the non-host plants. However, fecundity on non-host plant was significantly lower than that on host plants.3. Biology of the beet armyworm on Chenopodiaceous plantsThe performance of a laboratory population of the beet armyworm was observed on five species of chenopod plants (B. vulgaris var. cicla, S. oleracea, C. album, C. serotinum and S. salsa) and the performance of a field population of the beet armyworm was observed on C. album and C. serotinum, respectively. The laboratory population performed better on chenopodiaceous vegetables than on chenopodiaceous weeds. The insect showed the highest fitness on S. oleracea, but lowest fitness on S. salsa among the five chenopodiaceous plants. The field population seemed to show a higher fitness on the two weeds (C. album and C. serotinum) than the laboratory population, but the differences between the two populations were not significant.4. Trapping effect of chenopod weedsTrapping effect of C. serotinum on oviposition of the beet armyworm was studied when the weed was planted with common cabbage and Chinese cabbage respectively in screen houses. The proportion of plants with eggs and the mean number of egg masses per plant laid by S. exigua was significantly higher on C. serotinum than on common cabbage, indicating that C. serotinum had a significant trapping effect on the beet armyworm in common cabbage fields. In test with Chinese cabbage, C. serotinum plants also showed some trapping effect for oviposition by the beet armyworm .5. Extraction and identification volatiles of chenopodiaceous plantsSDE, SPME, and dynamic pumping absorption methods were utilized to extract volatiles from five Chenopodiaceous plants (B. vulgaris var. cicla, S. oleracea, C. album, C. serotinum and 5. salsa). Dynamic pumping tenax trap was used to extract volatiles from healthy and mechanically damaged plants. SPME(100μm PDMS) was used to extract volatiles from the plants that were either healthy, mechanically damaged, pest infested, or treated by high temperature. Composition of all the volatiles was analyzed and identified with Agilent GC (6890N)-MS (5973N). Compositions of volatiles of the five plant species extracted by SDE were basically similar, and there were 16 volatiles in common (specific odors in Chenopodiaceae). These 16 volatiles belong to aldehyde, ketone, alcohol, hydrocarbon, hydroxybenzene, and acid etc., but most of them belong to aldehyde and ketone. Benzeneacetaldehyde, 3-Buten-2-one, 4-(2,6,6-trimethyl-1- cyclohexen-1-yl)-, (E), n-Hexadecanoic acid, Phytol, then 2-Hexenal,(E)-, Benzaldehyde, 2-Methoxy-4-vinylphenol, 2-Pentadecanone, 6,10,14-trimethyl-, 5,9-Undecadien-2-one,6,10-dimethyl-, (E)-, had relatively higher percentages in the composition. Volatiles obtained by SPME were similar to those obtained by dynamic pumping tenax trap, but differed somewhat from those obtained by SDE. The volatiles extracted by SPME and dynamic pumping tenax belong to terpenoid, ketone, and ester etc, and there were chemicals in common in the volatiles extracted by the two methods. We concluded that different extracting methods resulted in different chemical profiles, which were diverse in varieties and quantities by different treatments. Plants increased the quantity of volatiles released when encountered light, high temperature, mechanical damage, and pest infestation. We speculated that Benzeneacetaldehyde, Benzaldehyde, Nonanal, (E)- 2-Hexenal, Caryophyllene,α-Caryophyllene, cis-3-hexenyl pentanoate, (Z)-3-Hexen-ol acetate, which were extracted by each of the three methods, are potentially the active elements in the volatiles. These and other chemicals constituted the particular chemical fingerprint of Chenopodiaceae, which were strongly attractive to the beet armyworm.This paper provides fundamental information and theoretical evidence for exploring Chenopodiaceae resources to develop new methods in beet armyworm IPM programs.We summarize the important novel findings as follows:(1) This study represents the first report of attraction of chenopodiaceous plants to the beet armyworm, which elucidated the feasibility to use C. serotium as a trap crop. Furthermore, fitness of the beet armyworms on chenopodiaceous weeds are lower than that on chenopodiaceous vegetables,(2) For the first time, the learning behavior of the beet armyworm was observed. Adults change either their oviposition habit or preference after experience, and females can oviposit on non-host plants after they have experienced the non-host plants.(3) For the first time, various methods were utilized to extract chenopodiaceous plant volatiles under different conditions. Volatiles were identified with GC-MC method.