Leaf osmotic potential decrease: a possible cause of mortality of greenhouse whitefly eggs

Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) white eggs were detached from the leaf and exposed to a range of osmotic concentrations (between −0.4 and −2.2 MPa) in controlled conditions. Eggs were able to hatch in the low and medium solute concentrations. In the higher concentrations, eclosion was delayed and significant mortality due to egg desiccation was observed. These results indicate leaf osmotic potential is a plant characteristic that affects greenhouse whitefly survivorship. Therefore leaf osmotic potential should be considered when evaluating greenhouse whitefly resistance.     

Realized heritability of resistance to dicrotophos in greenhouse whitefly

Realized heritability (h ²) of resistance to dicrotophos in greenhouse whitefly,Trialeurodes vaporariorum Westwood, was estimated from a laboratory selection experiment. Five generations of selection increased the LC₅₀ approximately 13-fold. Estimatedh ² of resistance to dicrotophos was 0.40 when calculated with the method of Tabashnik (1992) and 0.35 with the method of Tanaka & Noppun (1989). These results suggest that 35 to 40% of the total phenotypic variation in resistance was caused by additive genetic variation. For thirteen previously reported estimates ofh ² of insecticide resistance in other insect pests, the mean was 0.29. The relatively highh ² of dicrotophos resistance forT. vaporariorum is consistent with rapid resistance development in field populations.     

Flight behaviour of Encarsia formosa in response to plant and host stimuli

The flight behaviour of Encarsia formosa Gahan (Hymenoptera: Aphelinidae), a parasitoid of Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) has been studied in response to the following natural components: bean plant (Phaseolus vulgaris L.) infested with T. vaporariorum (PHC, plant host complex), host damaged bean plant from which whiteflies were removed (HDP, host damaged plant), T. vaporariorum third instar larvae (H, host), uninfested bean plant (P, plant) and T. vaporariorum empty pupal cases (EPC, empty pupal cases). A combination of chemical and visual stimuli is necessary in host location from a distance by E. formosa and seems to deeply affect the dispersal of this parasitoid. The presence of host (PHC, H) or of its parts (EPC) led to a substantial increase of oriented flights suggesting the occurrence of host derived chemicals. At a closer range, a combination of visual and chemical stimuli deriving from all the components of the plant host complex elicited the transition flying-landing and the following landing on the source.     

Host preference and suitability of some selected crops for two biotypes of Bemisia tabaci in Ghana

The economic importance of Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) is mainly due to its wide host range, variable kinds of damage, and great intraspecific variation. The delineation of two African biotypes of this pest has been carried by molecular, ecological, and host‐affiliation approaches, with largely consistent results. However, an understanding of its intricate host–pest interaction is necessary as a basis of its sustainable integrated control. This study investigated the host preference and suitability of cassava and okra biotypes of B. tabaci, based on multiple‐choice landing and oviposition preference assays and stage‐specific survival on eight common whitefly hosts. The cassava biotype significantly preferred cassava, Manihot esculenta, for landing and oviposition, but did not oviposit on okra, Abelmoschus esculentus. The okra biotype preferred okra, oviposited on eggplant, Solanum melongena, tomato, Lycopersicon esculentum, garden egg, Solanum integrifolium, and cowpea, Vigna unguiculata, but did not oviposit on cassava. The okra biotype developed on all hosts except cassava, but only survived marginally on cabbage, Brassica oleracea, and pepper, Capsicum annum var. grosum, while the cassava biotype did not develop on okra, cabbage, or pepper. Thus the observed host acceptance of the two biotypes is wider than earlier reported by host transfer experiments and molecular genetic surveys. Mortality was highest in the first instar nymphal stage, during which total mortality occurred on non‐hosts. Development time was slightly longer on marginal hosts than on the preferred hosts. Cowpea, garden egg, and tomato are additional common hosts of the two biotypes, whose role as reservoir hosts and biotype interbreeding grounds should be investigated further.     

Defense against Pieris rapae in cabbage plants induced by Bemisia tabaci biotype B

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) and Pieris rapae L. (Lepidoptera: Pieridae) are serious pests of vegetables, that can occur simultaneously on a single cabbage plant [Brassica oleracea var. capitata L. (Brassicaceae)]. We determined whether pre‐feeding or infestation by B. tabaci on cabbage could induce physiological and biochemical responses of the plant against P. rapae. Developmental time, length, and weight of immature P. rapae, and defense‐related plant compounds (SOD, superoxide dismutase; POD, peroxidase; CAT, catalase; APX, ascorbate peroxidase) were measured. Development of P. rapae larvae was 21% slower on B. tabaci‐pre‐infested plants than on plants without B. tabaci infestation. When feeding on B. tabaci‐pre‐infested plants, 22% of P. rapae larvae pupated as compared with 83% on B. tabaci‐free plants. Weights of P. rapae from first to fourth instars that fed on B. tabaci‐pre‐infested plants were also reduced, whereas those of fifth instars and pupae were not. Similarly, body length of P. rapae from first to fourth instars was affected by B. tabaci pre‐infestation, whereas that of the fifth instars was not. Peroxidase and APX activities of the B. tabaci‐pre‐infested plants increased more than SOD and CAT. Peroxidase and SOD activities of B. tabaci and P. rapae co‐infested plants increased as compared with those of P. rapae‐infested plants; however, CAT and APX activities were not different between B. tabaci‐ and P. rapae‐infested plants. These results showed that B. tabaci infestation had a negative effect on P. rapae when they occurred simultaneously on the same host plant. The implications of the induced plant changes on the herbivore are discussed.     

Pre‐copulation intervals,copulation frequencies,and initial progeny sex ratios in two biotypes of whitefly,Bemisia tabaci

The whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) is a species complex, and its systematic classification requires controlled crossing experiments among its genetic groups. Accurate information on pre‐copulation intervals, copulation frequencies, and initial frequency of egg fertilization of newly emerged adults is critical for designing procedures for collecting the virgin adults necessary for these experiments. In the literature, considerable variation is reported between B. tabaci populations, with respect to the length of the pre‐copulation interval and the initial frequency of egg fertilization. Here, we used a video‐recording method to observe continuously the copulation behaviour of the Mediterranean/Asia Minor/Africa (B biotype) and the Asia II (ZHJ1 biotype) groups of B. tabaci. We also recorded the initial frequency of egg fertilization, as determined by the sex of the progeny. When adults were caged in female–male pairs on leaves of cotton plants, the earliest copulation events occurred 2–6 h after emergence; at 12 h after emergence 56–84% of the females had copulated at least once, and nearly all (92–100%) had copulated at least once by 36 h after emergence. Both females and males copulated repeatedly. Approximately 80 and 20% of copulation events occurred during the photophase and scotophase, respectively. By 72 h post‐emergence, the females of the B and ZHJ1 biotypes had copulated on average 6.1 and 3.9 times, respectively. When adults were caged in groups on plants 1–13 h after emergence, 30–35% of the eggs deposited during this period were fertilized, and approximately 90% of females were fertilized by the end of the 13 h. Although timing of copulation differed in detail between the two genetic groups, the results demonstrate that B. tabaci adults can start to copulate as early as 2–6 h post‐emergence and the majority of females can become fertilized on the day that they emerge.     

Post-release survey to assess impact and potential host range expansion by Amitus hesperidum and Encarsia perplexa, two parasitoids introduced for the biological control of the citrus blackfly, Aleurocanthus woglumi in Dominica

Four years after the release of two exotic parasitoids, Amitus hesperidum Silvestri (Hymenoptera: Platygasteridae) and Encarsia perplexa Huang and Polaszek (Hymenoptera: Aphelinidae) for the classical biological control of the citrus blackfly (CBF), Aleurocanthus woglumi Ashby (Hemiptera: Aleyrodidae) in Dominica, a survey was conducted to assess establishment as well as potential nontarget effects especially on Aleyrodidae and other related taxa. CBF populations were low to non-existent in 50 of 51 field sites examined. At the site where CBF was encountered, both E. perplexa and A. hesperidum were present and CBF populations were declining. The two parasitoids were not among the several species collected on nontarget Aleryodidae and Hemiptera. It is concluded that E. perplexa and A. hesperidum have kept CBF populations under effective biological control in Dominica and there is no evidence of any nontarget effects on other Aleyrodidae or their natural enemies. Handling Editor: Dirk Babendreier.     

Pathway phase waveform characteristics correlated with length and rate of stylet advancement and partial stylet withdrawal in AC electrical penetration graphs of adult whiteflies

A technique was developed for measuring the length of stylet insertion during adult whitefly probing. The distance that the labium shortens during a probe was shown to be equal to the length of stylets that were inserted into the plant tissue. The length of labial shortening then was measured in high-magnification video recordings of adult female silverleaf whitefly, Bemisia argentifolii, in conjunction with recording electrical penetration graphs (EPGs – AC method). Using a split-screen device, video images of the whitefly's labium during a probe and the EPG waveforms produced during the probe were recorded simultaneously on the same video tape. On playback, changes in labial length could be measured during specific EPG waveforms to determine the length of stylet insertion that occurred during the waveforms. The focus of the study was on two characteristics of the pathway phase sawtooth waveform: the frequency of voltage peaks and the increase in voltage level that occurs over time during sawtooth waveforms. The rate of stylet penetration was significantly and positively correlated with frequency of sawtooth waveform voltage peaks (r ²=0.33) and the length of stylet penetration was significantly and positively correlated (second-order polynomial) with the relative difference in voltage level between the beginning and end of the sawtooth waveform (r ²=0.43). Stylet advancement did not appear to occur during the few low-flat waveforms (unknown behavioral correlation) and high-flat waveforms (phloem phase) that were observed. Voltage drops occur sporadically during sawtooth waveforms, and these were associated with partial stylet withdrawal (indicated when the labium increased in length, but the probe was not terminated) with an accuracy of 99%.     

Mating compatibility, life-history traits, and RAPD-PCR variation in Bemisia tabaci associated with the cassava mosaic disease pandemic in East Africa

The pandemic of a severe form of cassava mosaic virus disease (CMVD) in East Africa is associated with abnormally high numbers of its whitefly vector, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). To determine whether a novel B. tabaci biotype was associated with the CMVD pandemic, reproductive compatibility, fecundity, nymphal development, and random amplified polymorphic DNA (RAPD) variability were examined in, and between, B. tabaci colonies collected from within the CMVD pandemic and non-pandemic zone in Uganda. In a series of reciprocal crosses carried out over two generations among the six CMVD pandemic and four non-pandemic zone cassava B. tabaci colonies, there was no evidence of mating incompatibility. All the crosses produced both female and male progeny in the F₁ and F₂ generations, which in a haplo-diploid species such as B. tabaci indicates successful mating. There also were no significant differences between the sex ratios for the pooled data of experimental crosses, between individuals from two different colonies and control crosses between individuals from the same colony. Only one instance of mating incompatibility occurred in a control cross between cassava B. tabaci from Uganda and cottonB. tabaci from India. Measures of fecundity of the pandemic and non-pandemic zone B. tabaci on four cassava varieties showed no significant differences in their fecundity, nymphal development or numbers surviving to adult eclosion. Cluster analysis of 26 RAPD bands using six 10-mer primers was concordant with the mating results, grouping the pandemic and non-pandemic zone colonies into a single large group, also including a B. tabaci colony collected from cassava in Tanzania. These results suggest that it is unlikely that the severe CMVD pandemic in East Africa is associated with a novel and reproductively isolated B. tabaci biotype.     

Resistance to Bemisia tabaci biotype B of Solanum pimpinellifolium is associated with higher densities of type IV glandular trichomes and acylsugar accumulation

Advances in tomato breeding for pest resistance have been achieved via gene introgression from wild Solanum (section Lycopersicon) species (Solanaceae). Ninety‐nine F₃ families derived from an interspecific cross using as parental lines Solanum lycopersicum L. ‘LAM‐148' (susceptible standard) and Solanum pimpinellifolium L. ‘TO‐937‐15’ (multiple pest resistance accession with type IV glandular trichomes and acylsugar accumulation) were evaluated for their resistance against the whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) biotype B in free‐choice and no‐choice tests for oviposition and adult colonization. The parental lines and eight F₃ families with contrasting levels of resistance against the whitefly were selected and investigated in additional assays, which included the estimation of trichome densities and foliar acylsugar levels. The F₃ families BTR‐302 and BTR‐331 exhibited low amounts of eggs of whitefly and transgressive segregation for type IV glandular trichome density with values greater than that of TO‐937‐15 plants. However, the tested families did not surpass the total foliar acylsugar content found in TO‐937‐15. BTR‐331 exhibited low colonization in the free‐choice test and it was the least preferred F₃ family in the no‐choice test. The higher resistance levels of BTR‐331 were associated with a positive combination of higher type IV trichome density and higher acylsugar levels. Some F₃ families displayed reduced fruit set due to the presence of flowers with style exertion of the antheridial‐cone. Fruit weight at harvest stage of the selected families (from 4.9 to 14.5 g) was lower than that of LAM‐148 (139.5 g) but higher than that of TO‐937‐15 plants (1.3 g). Therefore, although difficult to reach due to the simultaneous segregation of many polygenic traits, the combination of high B. tabaci resistance levels with superior horticultural traits is feasible. These results confirm TO‐937‐15 as a source of biotype B resistance. From the breeding standpoint, the genetic similarity between S. lycopersicum and S. pimpinellifolium would allow a more efficient resistance introgression by facilitating recombination and minimizing the potentially undesirable linkage drag associated with this trait.