| The diamondback moth, Plutella xylostella (L.), is one of the most destructive pests of cruciferous crops with a cosmopolitan distribution. Taibai County, Shaanxi Province (34°28.204N,108°07.448E), is a cruciferous vegetable production base in a high plateau basin; the minimum temperature is below-20℃in the winter. In this isolated environment, we conducted serials of experiments both in the laboratory and under field conditions. I conducted three experients:1. General biology. We studied the oviposition preference, development, survival, longevity, fecundity and the life history parameters of P. xylostella were investigated on16cruciferous species in China. The widely planted cruciferous crops studied include Chinese cabbage, canola, cauliflower, Chinese radish, pakchoi, kohlrabi, cabbage, and broccoli and eight wild cruciferous species, Rorippa indica, Cardamine hirsuta, Descurainia sophia, Capsella bursa-pastoris, Cardamine leucantha, Orychophragmus violaceus, Thlaspi arvense, and Cardamine macrophylla. In the ovipostion preference test, the oviposition preference index (OPI) was less than1on all the crop hosts, which ranged from0.28(kohlrabi) to0.94(canola). For the wild plants, the OPI was higher than1on the R. indica (1.40), C. macrophylla (1.32), C. bursa-pastoris (1.11), while the minimum OPI on wild plant was0.61(D. sophia). In cultivated crop group, the development time from eggs to adult emergence was shortest on Chinese cabbage (15.5days) followed by canola (15.7days) and the longest on broccoli (22.3days). While, among the wild species, thelongest period was20.8days (C. macrophylla) and the shortest was15.8days(R.indica). P. xylostella reared oncanola (rm=0.2753) and C. hirsute (rm=0.2402) had the highest intrinsic rate of increase in the crop group and wild group respectively. The results from this study will be useful for interpretation of the performance and population dynamics of P. xylostella on wild hosts and cultivated cruciferous vegetables.2. Variation of different populations in selected georgraphic regions. To determine the relations of local and regional populations, we performed a mitochondrial COI gene analysis of eight P. xylostella populations from different locations in or around the Qinling Mountains and two other populations, one from Beijing and the other from Guangdong. The mtDNA divergences among the10populations were high, and32haplotypes were detected in149adults. The mean haplotype divergence was1.7%(range0.04-4.1%). Haplotype diversity in the10populations varied from0.571(AK) to0.885(HZ); and the nucleotide diversity varied from0.00286(AK) to0.0117(HZ). The results also did not show significant correlation between genetic and geographical distance. Also, the effective number of migrants between populations (Nm) ranged from1.43to infinite, suggesting that population exchange and gene flow among the P. xylostella populations Ooccurred. However, principal component analysis (PCA) showed that the TB and TC populations were differentiated from other populations, indicating possible across-mountain barrier to migration and gene flow.3. Cold tolerence and ovwerwinter. To determine the ability of cold tolerance of P. xylostella and whether the P. xylostella could overwinter in Taibai County. A series of tests were conducted both in laboratory and field. The supercooling point and freezing point of fourth-instar larvae collected in early winter field were-11.26℃and-9.97℃; while those in the pupae were-21.06℃and-18.96℃. When the fourth-instar larvae and pupae were transferred to0,-1,-3,-5,-7,-9,-11,-13,-15,-17, and-20℃treatment for3h, exposure to zero temperature for3h showed zero mortality. However, all the larvae or pupae were killed following exposure to-20℃for3h. When the insects were maintained at0℃for one month,60.4%of larvae and79.5%of pupae survived; but all larvae and pupae died when they were chilled at0℃for two months. In the field, the fourth-instar and pupae were buried in the soil5cm below the surface, after one month, approximate30～40%of larvae pupated, and the rest died; and10～20%pupae became adults. All the larvae and pupae died after two months in the soil. Among all the tests and surveys in the field, we collected four direct evidences to prove P. xystella could successfully overwinter in Taibai County. First, from April2012to March2014, the adults in field were checked using pheromone trap. Except February and March in2013and March2014, the adults were trapped in every month, including in the coldest months, December and January. And, we also collected a mated female moth in January2014, and it deposited58valid eggs. P. xylostella larvae were also found in the coldest months in cabbage and Chinese cabbage stubble heads. We found as many as50larvae in a Chinese head. The lowest temperature inside Chinese cabbage heads was2℃higher than the lowest air temperature in Tabibai County in the winters of2013-2014. Third, the fourth instar larvae placed in the cages in the fields could survive through the winter and developed to adults in bothe years. In each cage, an average of0.8and6adults were collected in the winters of2012-2013, and2013-2014respectively. In addition, we found that some P. xylostella could overwinter in the greenhouses in the winter. From November2012to April2013, we collected0.6-1.6adults in each pheromone trap. Therefore, we concluded that the data reported under laboratory condition in previous researches did not reflecte those in the natural conditions. Our results clearly revealed that P. xylostella could successfully overwinter in Taibai County, Shaanxi Provionce, where the sub-zero temperature lasts over four months and the extreme temperature could be well below-20℃. Hwever, we did not know how much the overwintering populations that contribute to the initial population on spring cruciferous vegetables are not known, which should be further studied in the future. |
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