Electrically recorded feeding behaviour of cassava mealybug on host and non-host plants

The feeding behaviour of cassava mealybug (Phenacoccus manihoti) was analysed by the electrical penetration graph technique (EPG, DC-system) and by light microscopy. This study confirmed a typical phloem-feeding behaviour with an aphid-like predominance of extracellular pathways of stylets. Similarities of EPGs from mealybugs with those of aphids and whiteflies allowed adoption of standard pattern labelling. The main differences with aphid EPGs consisted of fewer but longer intracellular punctures (pd duration of 20 svs 5–7 s for aphids), longer times to the first phloem-ingestion period and a lower motility of stylets within the phloem searching process. Comparison of the feeding behaviour on 6 plant types, including two occasional hosts (Talinum and poinsettia), one cassava hybrid and three common cassava cultivars (Manihot esculenta), allowed differentiation of penetration profiles. Intracellular punctures were shorter on occasional hosts, on which phloem finding and subsequent ingestion were markedly delayed although a great variability within plants persisted. Chemical analysis of some secondary plant substances revealed that none of the plants tested contained detectable amounts of alkaloids, and that cyanides were restricted to true hosts (cassavas andManihot hybrid). Levels of total flavonoids did not differ between hosts and non-hosts, in contrast to those of phenolic acids. Preliminary correlations with EPG paterns are discussed.

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Résumé Le comportement alimentaire de la cochenille du manioc (Phenacoccus manihoti) a été étudié par électrographie de pénétration (EPG, analyse en courant continu) et par microscopie optique. Cette étude a permis de confirmer le caractère typiquement phloémophage de l'alimentation de cet Homoptère, présentant une large prédominance du trajet extracellulaire des stylets, comme cela est également observé chez les pucerons. Les similitudes des tracés EPG effectués sur cochenille et sur pucerons ou aleurodes ont permis d'adopter la même typologie du signal. Les principales différences avec les pucerons concernent le nombre moins important et la durée plus longue des ponctions intracellulaires (20 s en moyenne contre 5 à 7 s chez les pucerons), l'allongement du temps d'accès au phloème et une moins grande mobilité des stylets lors de la phase de recherche du phloème. La comparaison du comportement alimentaire sur 6 plantes, parmi lesquelles deux h?tes occasionnels (Talinum et poinsettia), un hybride et trois variétés courantes de manioc (Manihot esculenta), a permis de différencier plusieurs profils de pénétration. Les ponctions intracellulaires se sont révélées plus courtes sur h?tes occasionnels, qui induisent également, malgré la variabilité observée, un allongement net de la période de localisation du phloème et donc un retard à l'ingestion. Aucune des plantes testées ne contient d'alcalo?des, et les composés cyanés se restreignent au genreManihot, h?te naturel de la cochenille. Les flavono?des totaux ne permettent pas de distinguer plantes h?tes et non-h?tes, à la différence des acides phénoliques.

     

Mating disruption method against the vine mealybug,Planococcus ficus: effect of sequential treatment on infested vines

The vine mealybug, Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae), is a major pest of vineyards. Here, we tested the efficacy of the mating disruption method against the pest when applied during one or two successive years in high and low infestation levels. Following 1 year of treatment, at low initial infestation levels a shutdown of pheromone traps was observed, along with a significant reduction in infested vines. With initially high infestation levels, a gradual reduction in infested vines was observed, with a trap shutdown seen only after the second year of pheromone application. We discuss the implications of the male mating disruption method for this pest in which the wingless females are aggregated with limited movement among vines, offering multiple mating opportunities for the flying male.     

Evaluation of the synthetic sex pheromone of the obscure mealybug,Pseudococcus viburni,as an attractant to conspecific males,and to females of the parasitoid Acerophagus maculipennis

The obscure mealybug, Pseudococcus viburni (Signoret) (Hemiptera: Pseudococcidae), is a cosmopolitan pest. In New Zealand, recently introduced management tools include the host‐specific parasitoid Acerophagus maculipennis (Mercet) (Hymenoptera: Encyrtidae) established in 2001, and pheromone‐baited monitoring traps available since 2005. Red delta traps baited with rubber septum lures impregnated with 4.0 μg of the mealybug synthetic sex pheromone, placed in apple orchards in Hawke's Bay and Nelson, trapped both male P. viburni and female A. maculipennis. Two generations of both species per year were discernible, but numbers were low in spring and parasitoids were not trapped during winter (June to September). Male P. viburni catches reached a plateau at a pheromone dose of ca. 1.0 μg per lure but numbers of A. maculipennis per trap increased up to 100 μg per lure, the maximum dose tested. A mathematical model showed that the lures had a half‐life of about 7.4 days and were most attractive to P. viburni with a dose of 0.19 μg, and that the trap effectiveness decreased rapidly once the release rate dropped below the optimum. The model also predicted that the initial pheromone dose should be increased from 0.19 to 5.41 μg per lure as the desired period of deployment increased from 0 to 9 weeks. A dose of 4.0 μg had an initial relative effectiveness of about 55%, reached peak effectiveness after about 5 weeks, and fell to 55% relative effectiveness again after about 8.3 weeks. We conclude that an initial pheromone load of 4.0 μg is appropriate for practical monitoring of P. viburni during the New Zealand summer. Future applications of the sex pheromone for managing the pest and parasitoid are discussed.     

Effect of native natural enemies on the population dynamics of the grape mealybug,Pseudococcus maritimus (Hom.: Pseudococcidae), in apple and pear orchards

The impact of native natural enemies on populations of the grape mealybug,Pseudococcus maritimus (Ehrhorn) in apple and pear orchards was assessed using a combination of techniques, including exclusion cages, limb-banding, and visual inspection of shoots and fruits. The complex of native natural enemies (which included two encyrtid parasitoids, (Pseudaphycus websteri Timberlake andMayridia species), a coccinellid beetle (Hyperaspis lateralis Mulsant), and a chamaemyiid fly (Leucopis verticalis Malloch), provided reasonably good control in orchards that had not been treated with insecticides for one to two years. However, surveys indicated that most of these species were absent from orchards regularly sprayed with pesticides.     

Comparison of performance on different host plants between the B biotype and a non-B biotype of Bemisia tabaci from Zhejiang, China

The capacity of the B biotype of the whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), to invade has often been linked to its presumably wider host range than the non‐B indigenous biotypes. However, there are few experimental studies of the relative performance of the B biotype and non‐B biotypes on different host‐plant species. Here, we compared the performance of the B biotype and an indigenous non‐B biotype (China‐ZHJ‐1) of B. tabaci from Zhejiang, China on five commonly cultivated host plants, each from a different family: cotton, tobacco, cabbage, squash, and kidney bean. We also examined the effect of rearing host plants on the performance of the B biotype. Overall, the performance of the B biotype on the five species of plants was much better than that of the indigenous non‐B population. On tobacco, cabbage, and kidney bean, no individuals of ZHJ‐1 completed development to adulthood, whereas the B biotype developed successfully from egg to adult on all three plants. On squash, the B biotype survived better, developed to adulthood earlier and had a higher fecundity than ZHJ‐1. The two biotypes performed more equally on cotton, but even on this plant the B biotype female adults lived nearly twice as long as that of ZHJ‐1 and may have realized a higher life‐time fecundity. The B biotype also showed a substantial capacity to acclimatize to alternative host plants for improved survival and reproduction, on both highly suitable and marginally suitable host plants. We conclude that the host range of the B biotype of B. tabaci may be much wider than those of some indigenous biotypes, and this advantage of the B biotype over the non‐B biotypes may assist in its invasion and displacement of some indigenous biotypes in the field.     

Oviposition behaviour, influence of experience on host size selection, and niche overlap of the solitary Leptomastix epona and the gregarious Pseudaphycus flavidulus, two endoparasitoids of the mealybug Pseudococcus viburni

Oviposition behaviour and host size selection of the solitary parasitoid Leptomastix epona(Walker) and the gregarious Pseudaphycus flavidulus(Brèthes) [both Hymenoptera: Encyrtidae] were examined on five size classes of the mealybug Pseudococcus viburni(Signoret) [Hemiptera: Pseudococcidae]. The host size classes mostly consisted of one stage (first, second, third instar nymph, young adult and preovipositing adult) and were presented together to wasps of either parasitoid species. Both parasitoid species locate the host by drumming the surface of the patch with the antennae. Leptomastix eponaseems to use mainly the antennae to examine the host but P. flavidulusmay accept or reject a host for oviposition after antennation or insertion of the ovipositor. Leptomastix eponaattempts oviposition in all the host stages from second instar nymphs but P. flavidulusincludes first instar. Both parasitoid species select mainly larger hosts (>1 mm, third instar nymphs) to oviposit but P. flavidulusis able to parasitize more second instar nymphs compared to L. epona. Female wasps of L. eponamay host feed on small mealybugs (second and third instar nymphs) that they do not use for oviposition. Oviposition experience of either parasitoid species for 24 hours does not influence host size selection on patches with hosts of similar mixed sizes. Oviposition decisions are independent of the host sizes of the preceding ovipositions. Implications about stability of a single parasitoid – host system and the success of biological control of the mealybug were discussed in respect of the developmental refugia of the two parasitoid species. Niche overlap of the two parasitoid species was discussed with a view to giving an insight into a single or multiple introduction.     

Performance of native grape phylloxera on host plants within and among terrestrial islands in Arizona, USA

In Arizona, USA, the canyon grape, Vitis arizonica Englemann, and grape phylloxera (Daktulosphaira vitifoliae Fitch, Homoptera, Phylloxeridae) are distributed among mountain ranges that are surrounded by expanses of desert lacking Vitis habitat, thus forming a system of terrestrial islands. Both herbivore and host populations may have diverged genetically among mountain ranges under the influence of restricted gene flow and variable selection among sites. Herbivore adaptation to local hosts would be expected to ensue, with the potential to promote divergence, both in traits under selection and by further reducing the probability of interisland colonization. To test the hypothesis that phylloxera are adapted to local hosts, demographic components of fitness of field-collected native grape phylloxera were measured in the greenhouse on vines of V. arizonica that were categorized as either natal, neighboring, and or isolated hosts. There was no evidence for greater adaptation to natal or neighboring hosts but there were significant interactions between herbivore and host treatments in one experiment. There was genetic variation for gall formation among six clones tested. Though a failure to detect local adaptation could have resulted from low statistical power, benign experimental conditions, or inadequate genetic variation, the divergence of isolated grape populations is suggested to have been insufficient to promote local adaptation in grape phylloxera at the level of isolated mountain ranges. It is further suggested that, within populations, adaptation to individual host plants could be forestalled by selection for ’general purpose genotypes’ through wind-borne displacement of colonizers into the unpredictable environment of a heterogeneous array of hosts. In addition, short-term extinction/colonization dynamics could promote gene flow such that time is insufficient for adaptive mutations or gene combination to arise. Received: 26 December 1998 / Accepted: 24 May 1999     

全球气候变暖对“植物-害虫-天敌”互作系统的影响

全球气候变化是近来人类关注的焦点问题,其最显著的特征是气候变暖。因为昆虫具有生活周期短、繁殖率高等特点,所以,气候变暖对昆虫的发育、繁殖和存活会产生强烈的直接影响。气候变暖促使一些昆虫提前春天的物候现象,向高纬度或高海拔地区迁移。然而,昆虫在自然界并非孤立地存在,它们与寄主植物和自然天敌相互联系、相互作用,并在长期的进化过程中逐渐适应特定区域的气候条件。因此,全球气候变暖对"植物-害虫-天敌"的种间关系必然产生直接或间接的影响,导致不同昆虫之间以及昆虫与其相关营养层的物种之间的相互关系在气候变化下呈现出时间上的异步性和空间上的错位,从而影响植物的适应性和抗虫性、害虫的发生规律和危害程度以及天敌的种群消长和控害效能。昆虫除了可以通过休眠或滞育的方式在时间上避开高温的影响外,还可以通过迁飞或移动的方式在空间上避开高温的影响,在这种迁移和扩散不同步的情况下可能使害虫食性和取食植物的害虫及其天敌的种类发生变化,从而改变生物群落的组成与结构,影响生态系统的服务和功能。     

Effects of tomato spotted wilt virus isolates, host plants, and temperature on survival, size, and development time of Frankliniella occidentalis

Tomato spotted wilt virus (TSWV) replicates in both its plant hosts and its thrips vectors. Replication of TSWV within thrips suggests the potential for pathological effects that could affect the fitness of its vectors directly, whereas infection of the plant may alter its suitability as a host for thrips development. This study was undertaken to examine the influence of TSWV isolate, host plant, and temperature on potential direct and host-mediated effects of virus infection of the thrips and the plant on Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), an important vector of TSWV. Neonate F. occidentalis were reared to adult eclosion on excised foliage of Datura stramonium (L.) (Solanaceae) or Emilia sonchifolia (L.) (Compositae) infected with either the CFL or RG2 isolate of TSWV, or not infected. Effects of the TSWV isolates and host plants on thrips were measured at 18.3, 23.9, and 29.4 °C. Results demonstrate significantly improved survival and a small but significant decrease in development time of F. occidentalis on TSWV-infected plants. These effects resulted from the combined influence of the direct effects of the virus on infected thrips and plant-mediated effects resulting from virus infection of the thrips’ host plant. Our results extend previous findings and help to explain inconsistencies among previously published reports by demonstrating that the manifestation and magnitude of effects of TSWV on F. occidentalis are dependent on host plant, virus isolate, and temperature.     

Effects of tomato spotted wilt virus (TSWV) isolates, host plants, and temperature on survival, size, and development time of Frankliniella fusca

The effects of different isolates of the tomato spotted wilt tospovirus (TSWV), host plants, and temperatures on Frankliniella fusca (Hinds) (Thysanoptera: Thripidae), the most important vector of TSWV in North Carolina, were measured in the laboratory. Thrips were reared at either 18.3, 23.9, or 29.4 °C until adult eclosion on excised leaves of Datura stramonium L. or Emilia sonchifolia (L.). Plants were either infected with the TSWV isolates CFL or RG2, or left uninfected (control). The results revealed a positive relationship between larval survival and temperature, regardless of host plant or TSWV isolate. Both survival to adult and percentage transmission of TSWV by F. fusca were significantly affected by the interaction between host plant and TSWV isolate. The consequence of this interaction was that the cohort‐based percentage transmission from infected E. sonchifolia plants for CFL was 1.3‐fold greater than that of RG2, whereas the percentage transmission from infected D. stramonium plants for RG2 was twice that of CFL. Both host plant and TSWV isolates showed significant effects on thrips development time to adult and head capsule width of adult thrips, as well as on the incidence of thrips infection with TSWV. The infection status of these thrips was determined by ELISA for the NSs viral protein. Infected thrips reared on infected host foliage took longer to develop to adult and were smaller than non‐infected thrips which had also been reared on infected host foliage, demonstrating a direct effect of the TSWV on thrips. However, non‐infected thrips reared on non‐infected leaves took longer to develop than non‐infected thrips reared on infected leaves, suggesting an effect of the plant tissue on thrips. In addition, adult thrips reared on TSWV‐infected D. stramonium at 29.4 °C developed smaller head capsules than thrips developing on infected foliage at lower temperatures and on non‐infected leaves of D. stramonium or E. sonchifolia. Both TSWV isolates and host plants differentially affected females more than males. In conclusion, both the infection of thrips by TSWV and TSWV‐mediated changes in host plant quality were found to have significant biological effects on F. fusca.     

Field studies on the transmission of Schistosoma mansoni and on the bionomics of its intermediate host, Biomphalaria glabrata, on St. Lucia, West Indies

Schistosoma mansoni is transmitted by the snail Biomphalaria glabrata on the West Indian island of St. Lucia. A 2$\text{1}\text{2}$-year study of natural snail populations is reported together with simultaneous observations on the transmission of the parasite. Populations in static habitats (pools, marshes and lateral banana drains) were maximal during the rainy part of the year (June–December) but declined during dry spells between January and May. Low infection rates (< 1 %) were observed sporadically in these populations. Higher, more persistent infection rates (5–15%) were obtained in flowing habitat populations (streams and main banana drains) which, however, only became established in periods of low rainfall. The importance of seasonal transmission through flowing habitat colonies was borne out by mouse exposures, supplementary snail studies and human incidence data which between them also gave an insight into the dynamics of transmission. These findings have been used to design a mollusciciding programme as one of several methods of controlling transmission currently being evaluated on St. Lucia.     

Adaptation of the fungal parasite Zygorhizidium planktonicum during 200 generations of growth on homogeneous and heterogeneous populations of its host, the diatom Asterionella formosa

We followed adaptation of the chytrid parasite Zygorhizidium planktonicum during 200 generations of growth on its host, the freshwater diatom Asterionella formosa, in a serial passage experiment. Evolution of parasite fitness was assessed both on a homogenous and heterogeneous host population, consisting of respectively a single new and ten different new host strains. These 10 host strains were genetically different and also varied in their initial susceptibility to the parasite. Parasite fitness increased significantly and rapidly on the new, genetically homogenous host population, but remained unaltered during 200 generations of growth on the heterogeneous host population. Enhanced parasite fitness was the result of faster and more efficient transmission, resulting in higher values of R0 (number of secondary infections). Consequently, parasites that evolved within the uniclonal host population infected significantly more of these hosts than did their ancestors. We thus provide experimental evidence for the widely held view that host genetic diversity restricts evolution of parasites and moderates their harmful effects. Genetically uniform host populations are not only at increased risk from fungal epidemics because they all share the same susceptibility, but also because new parasite strains are able to adapt quickly to new host environments and to improve their fitness.