Wen Lu

A Review of Invasive Biology, Prevalence and Management of Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae: Tetrastichinae)

The blue gum chalcid, Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae: Tetrastichinae) is widely distributed in Africa, America, Europe and Asia. Destructive damage of this wasp has been given much attention by many biologists and entomologists. We reviewed the invasive biology, distribution, mode of spread and situation of management of L. invasa in view of the status of rapid spread and serious damage. Research directions and methods of further studies were proposed, including reproductive mode and population expansion, impact factors and the formation mechanism of inducing gall, flight capacity of the wasp, effects of the ecological environment on distribution and occurrence, and control strategies. This information is beneficial to explore the mechanism of invasion and outbreak of invasive species.

Effect of soil moisture on overwintering pupae in Spodoptera exigua (Lepidoptera: Noctuidae)

Spodoptera exigua Hübner (Lepidoptera: Noctuidae), can overwinter as pupae at depths of 0- to 5-cm beneath the ground. In order to understand the effect of soil moisture on the overwintering success of this species, we tested the supercooling point and survival of pupae, and the growth rate of prepupae that were exposed to different temperature and soil moisture in the laboratory. Results showed that supercooling points, body water contents, and survivals after the different pre-treatments were not significantly affected by the soil moisture. The developmental progress of prepupae, survival of prepupae and pupae were negatively correlated with soil moisture as well as the delay of exposure time, which supports the hypothesis that soil moisture acts as a developmental modulator. We presumed that the delay of pupation and lower prepupae survival under higher soil moisture was due to lack of a complete pupal chamber for protection at low temperatures. Low pupal survival was likely attributed to lack of oxygen in the soil, especially under the condition of higher soil moisture. We suggest that using strategies of irrigation and soil tillage during winter may decrease the overwintering population of S. exigua from the perspective of integrated pest management.

Rapid identification of both sexes of Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae: Tetrastichinae) : a morphological perspective

The blue gum chalcid, Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae: Tetrastichinae) is a global pest in Eucalyptus plantations. Diverse modes of reproduction (thelytokous and sexual reproduction) are one of the advantages for rapid colonization and outbreak of this species. Understanding the reproductive strategies is advantageous for us to explore the invasive mechanisms. Importantly, how to identify both sexes of the species in the field is a crucial issue before we explore their reproductive strategies. In the current study, indexes of rapid identification of both sexes in L. invasa were studied. Results showed that the obvious differences in both sexes were the antenna and body size. Male wasps have three anelli and four funicular segments, but in females there are four and three segments, respectively. The longer male antennae, which are covered with longer and finer hairs than in the female, was also observed. Body, expansus, head, and gaster sizes of female wasps were significantly larger than the males. These results are an important base not only for exploring the reproductive strategies, but also for controlling L. invasa.

Parasitoids of the eucalyptus gall wasp Leptocybe invasa (Hymenoptera: Eulophidae) in China

Leptocybe invasa Fisher & La Salle (Hymenoptera, Eulophidae) is an invasive pest in Eucalyptus plantations throughout the world. Potential biological control agents for L. invasa were investigated in the Fujian, Guangdong, Hainan, Guangxi, Jiangxi, and Sichuan provinces of China, where Eucalyptus spp. have been severely damaged by the eucalyptus gall wasp. Three hymenopteran parasitoids of L. invasa were identified: Quadrastichus mendeli Kim & La Salle (Eulophidae), Aprostocetus causalis La Salle & Wu (Eulophidae), and Megastigmus viggianii Narendran & Sureshan (Torymidae); M. viggianii is newly recorded in China. The percentages of parasitization by Q. mendeli, A. causalis, and M. viggianii were 2.96%–19.53%, 2.30%–26.38%, and 24.93%, respectively. The longevity and body length of females were significantly greater than for males in A. causalis and M. viggianii. No males of Q. mendeli were found in China. These parasitoids could be used as biological agents for L. invasa in China.

Cold-hardiness mechanisms in third instar larvae of Spodoptera exigua Hübner (Lepidoptera: Noctuidae)

The mechanisms of cold tolerance in insects which are exposed to suboptimal temperatures for survival include accumulation of cryoprotectants and expression of heat shock protein genes (hsps). Spodoptera exigua is a freeze avoidance insect, which can overwinter as third instar larvae in China. Our previous study illustrated that seasonal and diel fluctuating temperatures enhanced its cold hardening. The survival mechanism for this stage, however, is poorly understood. We hypothesized that the survival mechanisms in third instar larvae of S. exigua were correlated with the induction of cryoprotectants and/or expression of hsps. Results showed that trehalose as the main cryoprotectant was accumulated during the cold acclimation (CA), and the level of accumulation fell off sharply when the CA treatment was discontinued (discontinued cold acclimation, DCA). Furthermore, inducible hsp90 was down-regulated in all treatments compared with the control, and hsp70 was up-regulated under conditions of CA and RCH (rapid cold hardening), which induced 1.23- and 44.00-fold increases compared with the control. Therefore, we considered that both trehalose (as the main cryoprotectant) and hsps (i.e. hsp90 and hsp70) were involved in the regulation of physiological activity of third instar larvae of S. exigua to resist low temperatures during winter. These results are helpful for us to understand the mechanisms underlying this species ecological adaptability and stress tolerance.

Reproductive Biology of Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae)

Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae) is an invasive pest in Eucalyptus plantations around the world. The successful colonization of L. invasa is possibly related to its reproductive biology. The objective of this study was to examine the reproductive biology of L. invasa. In Guangxi Province, the sex ratio (proportion of female, 0.99) of L. invasa was female-dominant throughout the year based on natural and artificial infestation. This result was similar to the ratios observed for other geographic populations in China, including those in Fujian (0.99), Guangdong (0.98), Hainan (0.95), Jiangxi (0.96), and Sichuan (0.99). The offspring sex ratio favored females. A large number of females emerged from the galls produced by females, with few males found. Galls on the petioles and midribs of Eucalyptus plants could be caused by newly emerged females with mature eggs. The lengths of the ovariole, spermatheca, common oviduct, and reproductive glands did not differ among L. invasa females, but their lateral oviducts showed differences from 0 to 42xa0h after emergence, indicating that this insect is proovigenic. These results could explain why L. invasa populations can rapidly increase in invaded areas.

Modeling climate change impacts on overwintering of Spodoptera exigua Hübner in regions of China

Inferential models are usually used to evaluate the effect of winter warming on range expansion of insects. Generally, correlative approaches used to predict changes in the distributions of organisms are based on the assumption that climatic boundaries are fixed. Spodoptera exigua Htibner (Lepidoptera: Noctuidae) overwinters as larvae or pupae in China regions. To understand the climate change impacts on overwintering of this species in regions of China, CLIMEX and Arc-GIS models were used to predict possible changes of distribution based on temperature. The climate change projection clearly indicated that the northern boundary of overwintering for S. exigua will shift northward from current distribution. Thus, the ongoing winter warming is likely to increase the frequency of S. exigua outbreaks.

Effects of the larval host plant on the supercooling capacity and physiological characteristics of beet armyworm pupae, Spodoptera exigua (Lepidoptera: Noctuidae)

The beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), is a serious economic pest worldwide that overwinters as pupae in temperate regions. There is no information on the relationship between the larval host plant and the cold hardiness of S. exigua pupae (pre-overwintering feeding), whereas the acclimation temperature, thermoperiod, photoperiod, and soil moisture level are known to influence cold hardiness. Larvae incubated at 20, 23, and 26°C with a L12:D12 photoperiod were fed one of four host plants (pakchoi, cabbage, shallot and spinach) to determine the effect of larval host plant on energy reserves, supercooling capacity, and cryoprotectant level of pupae. The interaction between temperature and larval host plant was significant in regard to pupal fresh weight (FW), glycogen, lipid, supercooling point (SCP), body water content (BWC), and glycerol levels. Higher FW and glycerol content and lower SCP of pupae that were fed as larvae on pakchoi were observed at the lowest rearing temperature. Moreover, higher glycogen content of pupae from larvae reared on pakchoi was also observed at the highest rearing temperature. Glycerol and glycogen contents of pupae from larvae fed on pakchoi were negatively correlated with temperature. Our results suggest that S. exigua pupae enhance their low temperature tolerance by decreasing their SCP and accumulating cryoprotectant (i.e., glycerol) allowing the animals to tolerate the harsh environmental conditions of winter, provided that the overwintering generation of larvae fed on suitable host plants.

Ultrastructural Observations of Antennal Sensilla in Phauda flammans Walker (Lepidoptera: Zygaenidae)1

Abstractu2003 Ficus microcarpa L. and F. racemosa L. are often extensively damaged by the defoliating pest Phauda flammans Walker (Lepidoptera: Zygaenidae). The structure, morphology, abundance, and distribution of the antennal sensilla of P. flammans were studied using scanning electron microscopy. Six types and eight subtypes of sensilla were observed on the filiform antennae of both sexes, including trichodea (subtypes I and II), basiconica (subtypes I and II), coeloconica, styloconica, chaetica, and Böhm bristles. The most numerous sensilla were the trichodea on the antennae of both sexes. Sexual dimorphism was observed, with the male antennae having more sensilla trichodea than the female antennae. This information is essential for understanding the electrophysiological and behavioral aspects of chemical communication in P. flammans.

Parasitoids of the eucalyptus gall wasp Leptocybe spp.: a global review

The genus Leptocybe Fisher & La Salle (Hymenoptera: Eulophidae) and its type species L. invasa were first described in 2004. Leptocybe spp. are global pests of eucalyptus plantations, and parasitoids play an important role in their control. In this review, we describe the species, distribution, biology, ecology and parasitism levels of Leptocybe spp. parasitoids and the problems associated with biological control programmes against Leptocybe spp. Additionally, prospects for the use of conservation or augmentative biological control programmes against Leptocybe spp. are discussed. Worldwide, approximately 23 species of parasitoids of Leptocybe spp. in 7 families and 10 genera have been found to date. Comparing the parasitism levels of the parasitoids showed that Quadrastichus mendeli, Selitrichodes neseri and several (approximately 4) native Megastigmus spp. could be exploited to manage Leptocybe spp. Available information on the Leptocybe spp. parasitoids is expected to improve our understanding related to ongoing studies of biological control programmes against Leptocybe spp.