Development of Microplitis bicoloratus on Spodoptera litura and implications for biological control

Microplitis bicoloratus Chen (Hymenoptera:Braconidae:Microgastrinae), a new species of Microplitis Förster from China, is a solitary endoparasitoid of the larvae of the cotton leafworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). This parasitoid is the first to be successfully reared and evaluated in the laboratory as a potential agent for the biological control of S. litura in China. Oviposition, immature development, and the effects of parasitism on the development of S. litura were studied. In long-term oviposition trials, females laid eggs on S. litura larvae for up to 10 days; oviposition was heavily skewed toward the first few days, with approximately one third of the eggs laid on day 1 and over 50% laid by day 3. This rapid oviposition rate increases the potential for biological suppression of host populations because the likelihood of mortality for the parasites from exposure to detrimental environmental factors or generalist predators increases with time. Immature development of the parasitoid in its host only required 7 days: eggs hatched within 24 h, the first instar larva required 2 days, the second instar larva needed 3 days, and the third instar larvae exited the host and pupated in 1 day, at 27±1°C, 60–80% relative humidity and a 12:12-h (long day) photoperiod. The development of the parasitized hosts was disrupted. When the parasitoid larvae finished development, the body weights of host larvae were significantly reduced regardless of which host instar was parasitized. Our results suggest that M. bicoloratus has considerable potential as a biological control agent for S. litura.     

Cross mating studies among five fruit fly parasitoid populations: potential biological control implications for tephritid pests

The reproductive compatibility between four different species/populations of the tephritid parasitoid Psyttalia (Walker) species from Kenya and individuals of the morphologically identical Psyttalia concolor (Szépligeti) (Hymenoptera: Braconidae) from a laboratory culture in Italy used in augmentative biological control of olive fly, Bactrocera oleae (Gmelin) (Diptera: Tephritidae) was assessed through cross mating tests using single-pair and group mating methods. Reciprocal crosses among the species resulted in the production of viable offsprings up to the second generation. In spite of the successful production of viable offspring in the laboratory, Psyttalia species are known to have specific host fruit and/or host fly preferences and populations/species may be isolated in one way or the other. However, it is not known whether these populations/species interbreed in the field. We discuss the ability of these parasitoids to interbreed and the potential effects of that on their use as biological control agents, especially in environments where other closely related species are present or in situations where multiple parasitoid introductions are intended.     

Evolution in invasive plants: implications for biological control

Evidence is increasing that invasive plants can undergo rapid adaptive evolution during the process of range expansion. Here, we argue that evolutionary change during invasions will also affect plant-antagonist interactions and, thus, will have important implications for biological control programmes targeted at invasive plants. We explore how altered selection in the new range might influence the evolution of plant defence (resistance and tolerance) and life history. The degree to which such evolutionary processes might affect biological control efficacy is largely unexplored. We hope that, by testing the hypotheses that we propose here, a closer link can be established between biological control and evolutionary biology, to the benefit of both disciplines.     

Diversity of agromyzidae and associated hymenopteran parasitoid species in the afrotropical region: implications for biological control

Agromyzidae (Diptera) is a family with many species of economic importance on agricultural plants. However, many species are attacked by hymenopteran parasitoids which are known to be habitat rather than species specific. In the Afrotropical region, information about agromyzid and parasitoid diversity in different habitats is scattered in literature. Our aim was to assemble this dispersed information and discuss future needs of biological control efforts against invasive agromyzids in the Afrotropics. From published information, 581 agromyzid records comprising 302 species from 20 genera were recorded from 48 plant families. Parasitoids (90 records) were associated only with 20 agromyzid species belonging to 10 different genera. The comparatively low parasitoid diversity in the Aftrotropical region is, however, most likely a result of poor sampling effort and lack of taxonomic expertise for parasitoid species in this area. More research on native parasitoids and associated Agromyzidae is therefore required before embarking on extensive biological control programmes.     

Zoophytophagous pentatomids feeding on plants and implications for biological control

Zoophytophagous insects can feed on a variety of prey, plants and plant products. By studying the interactions between predatory hemipterans and plants harbouring the prey of these insects, scientists have started to establish two potential outcomes: (1) positive effects like the enhancement of their life history characteristics by acquiring plant contents; and (2) negative effects mediated by plant resistance to herbivores or prey ingesting secondary plant metabolites. Despite this research, there is a lack of information about the feeding sites of predatory hemipterans on their host plants, what they ingest from plants, and whether they cause damage to their host plants. The results presented here indicate that the xylem is one of the feeding sites of predatory hemipterans on plants. The dissection of predators that fed on plants with marked vessels and testing insects for the presence of Cry protein constitutively expressed in the cytoplasm of plant cells revealed that bugs are not able to acquire cytoplasm contents from the plant cell. In addition, we demonstrate that systemic insecticide circulating inside plants from soil applications contaminates these predators. Our results are discussed in the context of zoophytophagous feeding behaviour exhibited by predatory hemipterans and the use of systemic insecticides for the conservation of natural enemies. This interaction contradicts the concept of ecological selectivity obtained for natural enemies through the placement of systemic insecticide in the soil as a selective method of deploying chemical control and predatory hemipteran conservation within the integrated pest management framework.     

Host discrimination of a larval parasitoid: the quick movement of Microplitis demolitor

Many parasitoids discriminate previously parasitised hosts from unparasitised ones to avoid mortality of offspring. Parasitoids that parasitise aggressive hosts such as lepidopteran larvae are known to attack hosts very quickly to avoid being attacked. However, little is known about host discrimination of such quick attacking parasitoids. We investigated host discrimination of Microplitis demolitor (Wilkinson) (Hymenoptera: Braconidae) a quick attacking parasitoid of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Results showed that ratios of female wasps that rejected the hosts after antennal examination did not differ between parasitised and unparasitised hosts, indicating that M. demolitor did not discriminate hosts by antennal examination. However, 95% of females that inserted ovipositor into unparasitised hosts actually laid eggs, whereas it was only 31% for parasitised hosts, indicating that females discriminated hosts by oviposition insertion. Analyzing video recordings revealed that the ovipositor exploration of the host took 0.3 s. Female wasps that had experienced high-host density of unparasitised hosts readily rejected parasitised hosts, while wasps with experience of low host availability of parasitised hosts tended to accept parasitised hosts. This suggests that host discrimination behaviour of M. demolitor is affected by previous experience of different host availability and host quality.

     

Time measurement and the control of flowering in plants

Many plants are adapted to flower at particular times of year, to ensure optimal pollination and seed maturation. In these plants flowering is controlled by environmental signals that reflect the changing seasons, particularly daylength and temperature. The response to daylength varies, so that plants isolated at higher latitudes tend to flower in response to long daylengths of spring and summer, while plants from lower latitudes avoid the extreme heat of summer by responding to short days. Such responses require a mechanism for measuring time, and the circadian clock that regulates daily rhythms in behaviour also acts as the timer in the measurement of daylength. Plants from high latitudes often also show an extreme response to temperature called vernalisation in which flowering is repressed until the plant is exposed to winter temperatures for an extended time. Genetic approaches in Arabidopsis have identified a number of genes that control vernalisation and daylength responses. These genes are described and models presented for how daylength might regulate flowering by controlling their expression by the circadian clock. BioEssays 22:38-47, 2000.     

Effects of Bt transgenic Chinese cabbage on the herbivore Mamestra brassicae (Lepidoptera: Noctuidae) and its parasitoid Microplitis mediator (Hymenoptera: Braconidae)

We investigated the effects of a diamondback moth-resistant Chinese cabbage (Brassica campestris subsp napus variety pekinensis Makino), expressing the insecticidal protein CrylA(c) toxin derived from Bacillus thuringiensis, on the nontarget herbivore Mamestra brassicae (L.) (Lepidoptera: Noctuidae) and its parasitoid wasp Microplitis mediator (Haliday) (Hymenoptera: Braconidae). A decreased survival rate at neonate stage was observed in M. brassicae when reared on Bt cabbage, although overall development was not significantly affected. According to enzyme-linked immunosorbent assay test using CrylA(c) antibody, the Cry toxin was only detected in the alimentary canal, not in the hemolymph or remaining body parts of M. brassicae, indicating that the ingested Cry toxin is neither distributed inside the body nor transferred through the trophic level. As expected, no Cry toxin was found in the larvae and cocoons of M. mediator. In addition, no significant changes were observed in the parasitization rate, larval period, pupal period, cocoon weight, or adult emergence rate when M. mediator wasps were reared on the M. brassicae larvae fed with transgenic Chinese cabbage. In summary, no direct or indirect adverse effects of transgenic Chinese cabbage on the two nontarget insect species were observed, suggestive of low risk in herbivore-parasitoid food chain.     

蜜粉源植物对天敌昆虫的作用及其在生物防治中的应用

寄生性和捕食性天敌昆虫成虫普遍存在通过取食蜜粉源植物补充营养的行为,这可不同程度地促进天敌昆虫性成熟、延长其寿命、提高其生殖力或寄生率,以及搜寻寄主效率和子代雌性比率,从而显著提高天敌昆虫在生物防治中的控害能力和效果。蜜粉源植物花的结构及植物对天敌昆虫产生的嗅觉、视觉信号和花蜜花粉对天敌昆虫产生的味觉信号又显著影响天敌昆虫选择蜜粉源植物的行为和结果。但是,蜜粉源植物也可成为害虫的补充营养植物,从而提高害虫的为害能力。因此,需深入研究不同蜜粉源植物对天敌昆虫及害虫的作用,趋利避害,才可能应用蜜粉源植物成功调控天敌与害虫的益害比,实现害虫的可持续控制。     

Patterns of parasitoid host utilization and development across a range of temperatures: implications for biological control of an invasive forest pest

Although climate change frequently has been linked to observed shifts in the distributions or phenologies of species, little is known about the potential effects of varying temperatures on parasitoids and their relationships with hosts. Using the egg parasitoid Oobius agrili (Hymenoptera: Encyrtidae) we examined host utilization patterns of this species across a range of temperatures (20–35 °C) to explore how changing climate could affect the interaction with its host—the emerald ash borer (EAB) (Coleoptera: Buprestidae), a serious invasive forest pest that has killed tens of millions of ash (Fraxinus spp.) trees in North America. Results from our study showed that the window of host susceptibility to O. agrili parasitism declined significantly from 14.8 to 2.6 days in an inverse second-order relationship with increasing exposure temperatures from 20 to 35 °C. In contrast, parasitoid host attack rate changed in a bell-shaped second-order relationship—i.e., increased with temperatures from 20 to 25 °C, but decreased at about the same rate when temperatures increased from 30 to 35 °C. This range of temperatures also significantly affected the development and mortality of immature parasitoids with 35 °C resulting in 100 % mortality. There was little mortality (0–4.5 %) and no significant differences in the percentage (20.9–34.9 %) of immature O. agrili that entered diapause (as mature larvae) at 20, 25, and 30 °C. However, there were significant differences in the time event of adult wasp emergence within this temperature range. The median time for 50 % of immature O. agrili emerging as adults at 20, 25, and 30 °C were 38, 18, and 17 days after parental wasp oviposition, respectively. Together these findings indicate that the non-linear and unequal temperature effects on these host utilization parameters are likely to result in differential host parasitism rates, and thus could reduce the efficacy of this parasitoid in suppressing host populations due to climate change (global warming and extreme heat).     

Flight responses in a wind tunnel of the parasitoid Microplitis croceipes to three spring and three summer host plants of Heliothis spp.

Flight responses of the parasitoid, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae) to two phenological stages of spring host plants (crimson clover, Trifolium incarnatum L., hairy vetch, Vicia villosa Corbiere, and cutleaf geranium, Geranium dissectum L.) and summer host plants (cotton, Gossypium hirsutum L., sorghum, Sorghum bicolor L., and soybean Glycine max (L.) Merr.) of Heliothis virescens (F.), were measured in a dual choice situation in a wind tunnel. Preflowering clover was more attractive than preflowering geranium, followed by preflowering vetch. Clover was preferred over geranium in flowering stages as well. No within species differences were detected for the two phenological stages. Preflowering and flowering cotton and soybean were preferred over preflowering and flowering sorghum. The findings could be important in pre-determining release sites for M. croceipes in the field.     

Breeding systems in flowering plants and the control of variability

Plants have three basic means of reproduction, by outcrossing, by selfing, and asexually. In most plant populations, at least two and often all three of these options are everpresent, so that individuals adopt mixed mating strategies at evolutionarily stable strategy (ESS) threshholds. Because mating systems are genetically controlled and affect genotype structure, they are liable to feedback. Productive habitats with a large standing crop are more likely to favour outcrossing, while unproductive habitats may favour asexuality or selfing, so that mating systems may change through seral development, even within the same species. Outcrossing tends to break up linkage disequilibria, but may also favour the creation of adaptive linkage groups. Mechanisms whereby male sexual selection, small population size and selfing can influence the genetic structure of populations are examined.     

Three viruses found in the braconid parasitoid Microplitis croceipes and their implications in biological control programs

Productivity and longevity decreased in a laboratory colony of the parasitoid wasp Microplitis croceipes (Cresson) (Hymenoptera: Braconidae). Using light microscopy, it was determined that the colony was free of microsporidia. However, samples of the colony examined for pathogens by electron microscopy revealed three types of viruses: a nonpathogenic polydnavirus which is produced by all female wasps; a nonoccluded baculovirus which is pathogenic to late-stage pupae and adults; and a picorna-like virus which is present in larvae, pupae, and adults. The nonoccluded baculovirus was eliminated from the laboratory colony of M. croceipes by selection of progeny from wasps which had oviposited within 2 to 3 days after emergence from the cocoons and which had lived for at least 14 days post-emergence. Upon death, the wasps were examined by negative stain electron microscopy and only progeny from baculovirus-free wasps were retained. Parasitoid colonies should be systematically examined for pathogenic viruses that may reduce their productivity and efficacy as biological control agents. In addition, exotic parasitoids and predators should be evaluated for viruses and other pathogens while in quarantine.     

Assessing the potential of candidate DNA barcodes for identifying non‐flowering seed plants

In plants, matK and rbcL have been selected as core barcodes by the Consortium for the Barcode of Life (CBOL) Plant Working Group (PWG), and ITS/ITS2 and psbA‐trnH were suggested as supplementary loci. Yet, research on DNA barcoding of non‐flowering seed plants has been less extensive, and the evaluation of DNA barcodes in this division has been limited thus far. Here, we evaluated seven markers (psbA‐trnH, matK, rbcL, rpoB, rpoC1, ITS and ITS2) from non‐flowering seed plants. The usefulness of each region was assessed using four criteria: the success rate of PCR amplification, the differential intra‐ and inter‐specific divergences, the DNA barcoding gap and the ability to discriminate species. Among the seven loci tested, ITS2 produced the best results in the barcoding of non‐flowering seed plants. In addition, we compared the abilities of the five most‐recommended markers (psbA‐trnH, matK, rbcL, ITS and ITS2) to identify additional species using a large database of gymnosperms from GenBank. ITS2 remained effective for species identification in a wide range of non‐flowering seed plants: for the 1531 samples from 608 species of 80 diverse genera, ITS2 correctly authenticated 66% of them at the species level. In conclusion, the ITS2 region can serve as a useful barcode to discriminate non‐flowering seed plants, and this study will contribute valuable information for the barcoding of plant species.     

Starch-accumulating (S-type) sieve-element plastids in Hydatellaceae: implications for plastid evolution in flowering plants

TEM investigation of sieve-element plastids in three species of Trithuria, the sole genus of the small aquatic family Hydatellaceae, show that P-type plastids are absent from this genus and only starch-accumulating (S-type) sieve-element plastids are present. This discovery is consistent with the recent transfer of Hydatellaceae from the highly derived monocot order Poales (grasses and their allies) to the early-divergent angiosperm order Nymphaeales (waterlilies) based on molecular phylogenetic data. Species of Poales consistently possess P2-subtype plastids, in common with other monocots, but only S-type plastids are present in Nymphaeales. The results confirm that Hydatellaceae do not belong in monocots. Optimisation of the two major types of sieve-element plastid onto a recent phylogeny of early-divergent angiosperms confirms that S-type is the primitive form and indicates that P-type sieve-element plastids have evolved more than once in angiosperms.