Development of Encarsia bimaculata (Heraty and Polaszek) (Hymenoptera: Aphelinidae) in Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) nymphs

Encarsia bimaculata was recently described from India as a potentially useful parasitoid of Bemisia tabaci. Its developmental biology was studied in the laboratory at 25–30 °C and 70–75% RH. Results showed that E. bimaculata is a solitary, arrhenotokous, heteronomous, autoparasitoid. Mated females laid eggs internally in B. tabaci nymphs that developed as primary parasitoids. Males developed as hyperparasitoids, either in females of their own species or in other primary aphelinid parasitoids. Superparasitism was common under cage conditions. Both sexes have an egg, three larval instars, prepupal, and pupal stages. Development from egg to adult took 12.70 ± 2.10 days for females and 14.48 ± 2.60 days for males. Individual B. tabaci nymphs were examined for E. bimaculata parasitization using three isozymes: esterase, malate dehydrogenase, and xanthine dehydrogenase. All three isozymes showed differential banding patterns that identified E. bimaculata parasitized or unparasitized B. tabaci nymphs.     

Effect of parasitism on flight behavior of the soybean aphid, Aphis glycines

Many aphid species possess wingless (apterous) and winged (alate) stages, both of which can harbor parasitoids at various developmental stages. Alates can either be parasitized directly or can bear parasitoids eggs or larvae resulting from prior parasitism of alatoid nymphs. Winged aphids bearing parasitoid eggs or young larvae eventually still engage in long-distance flights, thereby facilitating parasitoid dispersal. This may have a number of important implications for biological control of aphids by parasitoids. In this study, we determined the effect of parasitism by Aphelinus varipes (Hymenoptera: Aphelinidae) on wing development and flight of the soybean aphid, Aphis glycines (Hemiptera: Aphididae). We also quantified the influence of aphid flight distance on subsequent A. varipes development. Parasitism by A. varipes was allowed at different A. glycines developmental stages (i.e., alatoid 3rd and 4th-instar nymphs, alates) and subsequent aphid flight was measured using a computer-monitored flight mill. Only 35% of aphids parasitized as L3 alatoid nymphs produced normal winged adults compared to 100% of L4 alatoids. Flight performance of aphids parasitized as 4th-instar alatoid nymphs 24 or 48 h prior to testing was similar to that of un-parasitized alates of identical age, but declined sharply for alates that had been parasitized as 4th-instar alatoid nymphs 72 and 96 h prior to testing. Flight performance of aphids parasitized as alate adults for 24 h was not significantly different from un-parasitized alates of comparable ages. Flight distance did not affect parasitoid larval or pupal development times, or the percent mummification of parasitized aphids. Our results have implications for natural biological control of A. glycines in Asia and classical biological control of the soybean aphid in North America.     

Resistance and susceptibility of Boronia megastigma cultivars to infestations by the psyllid Ctenarytaina thysanura

Studies were conducted on 23 boronia cultivars to determine the mechanisms of resistance and susceptibility to the psyllid, Ctenarytaina thysanura Ferris and Klyver. No antibiosis effects were detected in any of the boronia cultivars investigated. No cultivar was tolerant to C. thysanura attack. However, in both free-choice and no-choice tests in the glasshouse, ovipositional antixenosis (non-preference) was detected in cultivar HC27, whereas cultivars HC4 and HC142 were most preferred for oviposition. Measurement of terminal shoot hardness of cultivars revealed variations in relative hardness. Given a choice between boronia cultivars of the same size and colour but of differing hardness of the terminal shoots, C. thysanura laid more eggs on those cultivars with softer terminal shoots (HC4 and HC142) and laid fewest eggs on harder terminal shoots (HC27). This may explain differences in the psyllid population on different cultivars in the field. Fewer eggs were laid on terminal shoots exceeding a hardness rating of 80 g/mm terminal shoot thickness.     

Parasitoid attack and its consequences to the development of the galling psyllid Baccharopelma dracunculifoliae

This study was aimed to determine the parasitism rates of the galling psyllid Baccharopelma dracunculifoliae (Hemiptera: Psyllidae), and the effects of parasitoid attack on gall growth dynamics. Also, life history traits of the most common parasitoid, Psyllaephagus baccharidis (Hymenoptera: Encyrtidae), are described. A cohort of galls was followed throughout its development, and a sample of galls was collected monthly from April to August 1999. Furthermore, a caging experiment was performed to determine the periods of parasitoid attack. Parasitism rates reached 45.2% at the end of gall development, and the caging experiment suggested that most of the attack occurred at the beginning of gall development. P. baccharidis was responsible for 93% of the observed parasitism. This koinobiont parasitoid attacked galls early during their development and probably stimulated nymph feeding, causing a higher growth of parasitised galls. This alteration in gall growth dynamics may have indirect, harmful consequences to the host plant, since larger galls usually are stronger nutrient sinks for the plant. Healthy nymphs of B. dracunculifoliae growing in galls with parasitised nymphs had a larger size than healthy nymphs in unparasitised clutches, regardless of nymph number. This may have unexpected beneficial consequences to the galling psyllid, since the resultant larger adults may have increased survivorship and reproductive success. These results contrast with the common view that parasitoids are always beneficial to the host plant in three-trophic systems, and koinobiont parasitoids may constitute exceptions due to their feeding behaviour.

Zusammenfassung

Diese Studie zielte darauf ab, die Parasitierungsraten der gallbildenden Psyllide Baccharopelma dracunculifoliae (Hemiptera: Psyllidae) und die Effekte des parasitoiden Befalls auf die Wachstumsdynamik der Gallen zu bestimmen. Außerdem wird die Lebensweise des häufigsten Parasitoiden Psyllaephagus baccharidis (Hymenoptera: Ecyrtidae) beschrieben. Eine Kohorte von Gallen wurde durch ihre Entwicklung verfolgt und eine Probe von Gallen monatlich von April bis August 1999 gesammelt. Darüber hinaus wurde ein Käfigexperiment durchgeführt, um die Zeitspannen des parasitoiden Befalls zu bestimmen. Die Parasitierungsraten erreichten 45.2 % am Ende der Gallentwicklung und das Käfigexperiment läßt vermuten, dass der Großteil des Befalls zu Beginn erfolgte. P. baccharidis war für 93 % der beobachteten Parasitierung verantwortlich. Dieser koinobionte Parasitoid befiel die Gallen früh in ihrer Entwicklung und stimulierte vermutlich den Fraß der Nymphe und verursachte damit ein höheres Wachstum der parasitierten Gallen. Diese Veränderung in der Wachstumsdynamik der Gallen könnte indirekte, schädliche Auswirkungen auf die Wirtspflanze haben, da größere Gallen normalerweise größere Stickstoffsenken für die Pflanze sind. Gesunde Nymphen von B. dracunculifoliae, die in Gallen mit parasitierten Nymphen wuchsen, waren unabhängig von der Nymphenzahl größer als gesunde Nymphen in nicht parasitierten Gelegen. Das könnte unerwartete positive Konsequenzen für die gallbildende Psyllide haben, da die resultierenden größeren Adulten eine erhöhte Überlebenschance und einen erhöhten reproduktiven Erfolg haben könnten. Diese Ergebnisse stehen im Widerspruch zu der allgemeinen Meinung, dass Parasitoide in tritrophischen Systemen immer positiv auf den Wirt wirken. Konobionte Parasitoide könnten aufgrund ihres Fraßverhaltens eine Ausnahme bilden.     

Thermal requirements for the embryonic development of Periplaneta americana (L.) (Dictyoptera: Blattidae) with potential application in mass-rearing of egg parasitoids

The embryonic development of oothecae of Periplaneta americana was evaluated under four different constant temperatures (5, 10, 15, 20, 25, 30, and 35 °C) and also at different exposure times at <5 °C. Their suitability as hosts after the treatment for the parasitoids Evania appendigaster and Aprostocetus hagenowii was also assessed. Temperatures of 5, 10, 15, and 35 °C adversely affected the development of the cockroaches, and exposure times to <5 °C longer than 5 days sufficed to kill all the embryos in the oothecae. The lower thermal threshold for complete development of P. americana was estimated to be 6.8 °C, with a required total amount of 900.9 degree-days. Cold-killed oothecae were still fit for the development of parasitoids. Parasitism rates of A. hagenowii were higher than those of E. appendigaster, although with lower emergence rates. Our results can be useful in aiding mass-rearing of these parasitoids for biological control programmes of P. americana, and may help forecast the time of emergence of nymphs of American cockroaches in infested areas.     

Influence of pesticide treatments on the dynamics of whiteflies and associated parasitoids in snap bean fields

To determine the influence of pesticidetreatments on the population dynamics of thewhiteflies Trialeurodes vaporariorum(Westwood) and Bemisia tabaci Gennadius (Homoptera: Aleyrodidae)and their naturally occurring parasitoids, weperformed field experiments on insecticidesprayed and unsprayed fields during a croppingseason of snap bean in Pradera, Valle delCauca, Colombia. Substantially largerpopulations of whitefly nymphs occurred in theunsprayed field than in the sprayed field. Parasitoids were more frequent in unsprayedthan in sprayed fields with Encarsianigricephala Dozier being more prevalent thanAmitus fuscipennis MacGown & Nebeker. Insprayed fields the nymphs parasitized by E. nigricephala exceeded the unparasitizedwhitefly nymphs at the end of the croppingseason. Our results suggest that while undercurrent agricultural practices whiteflies onsnap beans cannot be exclusively controlled bynaturally occurring parasitoids, parasitoidsmay be integrated with chemical control inorder to reduce crop damage.     

Evaluation of the local population of Eretmocerus mundus (Hymenoptera: Aphelinidae) for biological control of Bemisia tabaci biotype B (Hemiptera: Aleyrodidae) in greenhouse peppers in Argentina

Bemisia tabaci biotype B is a key pest in pepper crops in Argentina. The parasitoid Eretmocerus mundus is frequently found parasitizing this whitefly in greenhouses without pesticide applications. The present studies were carried out with the objective of evaluating control obtained with different rate and number of parasitoid releases under experimental conditions. Release rate: cages with pepper pots were positioned in an experimental greenhouse and randomly assigned to the release rate treatments (0, 1 and 3 pairs of E. mundus/plant/week with a total of three introductions). Number of releases: similar cages were assigned to the number of parasitoid introduction treatments (0, 1, 2 and 3) with the best release rate obtained in the previous trial. In both assays whitefly (adults and nymphs) and parasitoid (parasitized nymphs) population sizes in each cage were monitored weekly for a period of 10 weeks. Results suggested that the introduction of 2 E. mundus/plant/week was enough to suppress host population compared to control treatment (peaks of 7.75 adults and 58.75 nymphs/cage and 643.75 adults and 1598 nymphs/cage, respectively) (p < 0.05), with 85% of parasitism. E. mundus had to be introduced three times to achieve the best pest control (peaks of 1.17 adults and 20.33 nymphs/cage vs. 55.67 adults and 75 nymphs/cage in control treatment) with 84% of parasitism (p < 0.05). These results were then validated in a pepper crop under experimental greenhouse conditions. Whitefly population was lower in those greenhouses where E. mundus was released compared to control greenhouses (0.15 adults and 0.71 nymphs/4 leaves and 0.73 adults and 1.64 nymphs/4 leaves, respectively), with a peak of 54% of parasitism (p < 0.05). We concluded that good suppression of B. tabaci could be achieved using E. mundus under spring conditions in Argentina.     

The effect of host size on the sex ratio of Syngaster lepidus, a parasitoid of Eucalyptus longhorned borers (Phoracantha spp.)

The solitary larval ectoparasitoid, Syngaster lepidus Brullé, parasitizes the cryptic larvae of two wood-boring beetles, Phoracantha recurva Newman and Phoracantha semipunctata F. The objective of this study was to determine how the female parasitoids allocated the sex of progeny when presented with larval hosts of uniform size classes. Host size was directly correlated with age of the Phoracantha larval hosts. Groups of Phoracantha larvae of a single age class (2-, 3-, 4-, or 5-week-old) were exposed to parasitoids, and sex ratios of the resulting parasitoid progeny from each host age class were determined. A significant relationship was observed among the sizes of P. recurva and P. semipunctata hosts and the sex ratio of emerging parasitoids. Parasitized 2-week-old beetle larvae of both Phoracantha spp. produced only male S. lepidus progeny, whereas older larval hosts produced increasing proportions of female parasitoids (up to 80% females from 5-week-old hosts). Two-week-old Phoracantha larvae of both species produced fewer parasitoids than host larvae 3–5-week-old. The size of parasitoid progeny consistently increased with host larval age (size), and female parasitoids were larger than males across all host size classes. Male S. lepidus developed in approximately 25 days from 2-week-old hosts, and 19–21 days in 3–5-week-old hosts. Female S. lepidus developed in 22–25 days, with developmental time increasing with host size.     

Field studies on colour preferences of Ctenarytaina thysanura in Tasmanian boronia farms

Field tests on attraction of Ctenarytaina thysanura (Hemiptera: Psyllidae) adults to different coloured 30×30 cm sticky traps revealed a preference for yellow. Among the enamel colours tested, more psyllids were captured on yellow traps followed by green, then blue and least on red, cyan and magenta. Dilution of yellow enamel with 50% white (1Y: 1W) and 75% white (1Y: 3W) to produce yellow-white hues resulted in a significant decrease in psyllid capture indicating that the psyllids response to yellow was one of positive attraction and could suggest true colour discrimination. Reflectance spectra of painted surfaces of the enamel colours and also yellow to white hues indicated that psyllid capture rates were directly related to the proportion of light reflected in the 500–560 nm region. The biological basis of the observed C. thysanura response may be that yellow is the most intensely reflective colour in the general part of the spectrum for leaves which reflect most light in the 500–600 nm (peak 550 nm) range.     

Establishment of <Emphasis Type="Italic">Psyllaephagus parvus</Emphasis> and <Emphasis Type="Italic">P. perplexans</Emphasis> as serendipitous biological control agents of Eucalyptus psyllids in southern California

The free-living lemon gum psyllid, Cryptoneossa triangula Taylor, and the lerp-forming spotted gum psyllid, Eucalyptolyma maideni Froggatt (Hemiptera: Psyllidae) are invasive pests of eucalypts in California. In 2007, Psyllaephagus parvus Riek (Hymenoptera: Encyrtidae) was discovered parasitizing spotted gum psyllids and Psyllaephagus perplexans Cockerell was collected from lemon gum psyllids. While neither parasitoid species was purposely introduced, presence of the parasitoids was significantly associated with reduced intensity and duration of population peaks for both psyllid species. Spring peaks were reduced more than fall peaks. We estimated minimum rates of parasitism from the ratio of mummies to live nymphs. Higher parasitism was recorded in coastal than inland locations during the spring, while parasitism was similar for coastal and inland populations in the fall. Logistic regression models suggest parasitoids were the determining factor of psyllid population densities, although physical parameters, such as irrigation, may affect psyllid or parasitoid populations.     

Introduction and establishment of parasitoids for the biological control of the apple ermine moth, Yponomeuta malinellus (Lepidoptera: Yponomeutidae), in the Pacific Northwest

Four parasitoids were imported from five countries in Eurasia and released in northwestern Washington, US, to control the apple ermine moth (AEM), Yponomeuta malinellus Zeller, which colonized the Northwest around 1981. From 1988 to 1991, 95,474 individuals of Ageniaspis fuscicollis (Dalman) from France, China, Korea, and Russia were released in Washington. Parasitism of AEM increased 4- to 5-fold over that produced by preexisting natural enemies between 1989 and 1994 at 22 monitored sites. Subsequently, the wasp dispersed up to 20 km from release sites. A. fuscicollis also parasitized the cherry ermine moth, Yponomeuta padellus (L.), which was discovered in the Pacific Northwest in 1993. A total of 1813 individuals of Herpestomus brunnicornis (Gravenhorst) from France, Korea, and Japan were released in 1989–1991, and 26 wasps were recovered in 1994–1995. From 1989 to 1991, 2647 Diadegma armillata (Gravenhorst) individuals from France were released. D. armillata was recovered at one site in 1991 two months following release, but no other recoveries have been made. A total of 8274 Eurystheae scutellaris(Robineau-Desvoidy) individuals were released in 1989 to 1991. However, this tachinid has not been recovered. A consistent decline of AEM populations occurred in 1989–1995, including at sites where A. fuscicollis was not recovered, suggesting other factors also contributed to this pest’s decline. Now well established in western Washington, A. fuscicollis may help suppress future outbreaks of Y. malinellus and its congener, Y. padellus.     

Suppression ofOstrinia nubilalis byTrichogramma nubilale in sweet corn

We conducted inundative release experiments withTrichogramma nubilale (Hymenoptera: Trichogrammatidae) to suppressOstrinia nubilalis (Lepidoptera: Pyralidae) in sweet corn (Zea mays): two experiment duringO. nubilalis first generation and three experiments during second generation. Five measurements of ear and stalk damage were used to assess.O. nubilalis control in treated and untreated plots within each experimental field. In one experiment during second generation, naturalO. nubilalis populations were sufficiently high to demonstrate that the parasitoids (three releases totaling 4.4 million parasitoids per ha) parasitized an estimated 57.4% of the placedO. nubilalis egg masses and reduced the mean number ofO. nubilalis larvae per ear by 97.4% the number of tunnels per stalk by 92.9%, and the number of larvae per stalk by 94.3% in the release plot. Ear damage in this experiment was suppressed to meet acceptable standards for use in cut-corn commercial processing. Larval mortality was apparently density independent, which implies that density-dependent larval loss would not compensate for egg parasitism byT. nubilale.     

Evaluation of the Entomopathogenic Fungi Beauveria bassiana and Paecilomyces fumosoroseus for Microbial Control of the Silverleaf Whitefly, Bemisia argentifolii

Collaborative research was conducted at the USDA-ARS Subtropical Agricultural Research Center in southern Texas to assess the microbial control potential of Beauveria bassiana and Paecilomyces fumosoroseus against Bemisia whiteflies. Laboratory assays demonstrated the capacity of both pathogens to infect Bemisia argentifolii nymphs on excised hibiscus leaves incubated at relative humidities as low as 25% at 23 ± 2°C (ca. 35% infection by B. bassiana and P. fumosoroseus resulted from applications of 0.6–1.4 × 10³ conidia/mm² of leaf surface). In small-scale field trials using portable air-assist sprayers, applications at a high rate of 5 × 10¹³ conidia in 180 liters water/ha produced conidial densities of ca. 1–2.5 × 10³ conidia/mm² on the lower surfaces of cucurbit leaves. Multiple applications of one isolate of P. fumosoroseus and four isolates of B. bassiana made at this rate at 4- to 5-day intervals provided >90% control of large (third- and fourth-instar) nymphs on cucumbers and cantaloupe melons. The same rate applied at 7-day intervals also provided >90% control in zucchini squash, and a one-fourth rate (1.25 × 10¹³ conidia/ha) applied at 4- to 5-day intervals reduced numbers of large nymphs by >85% in cantaloupe melons. In contrast to the high efficacy of the fungal applications against nymphs, effects against adult whiteflies were minimal. The results indicated that both B. bassiana and P. fumosoroseus have strong potential for microbial control of nymphal whiteflies infesting cucurbit crops.     

Effect of herbivory by Teleonemia scrupulosa on the performance of Longitarsus bethae on their shared host, Lantana camara

Herbivory by insects may change the characteristics of nutrients and secondary plant chemicals of the foliage, thereby altering the acceptability and suitability of the plant for oviposition, feeding and development for subsequent herbivores. In the current study, the effect of herbivory by the sap-sucking lace bug, Teleonemia scrupulosa Stäl (Heteroptera: Tingidae), on the suitability of Lantana camara L. (Verbenaceae) for the root-feeding flea beetle, Longitarsus bethae Savini & Escalona (Chrysomelidae: Alticinae), was investigated under laboratory conditions. Preference of adult L. bethae was not influenced by the intensity of feeding damage caused by T. scrupulosa adults. However, high densities of T. scrupulosa nymphs and their feeding damage caused L. bethae adults to emigrate and colonize less infested or uninfested plants. Oviposition by L. bethae was significantly reduced at high densities of T. scrupulosa nymphs. While low infestation of T. scrupulosa had no effect the survival of L. bethae, moderate and high infestations caused significant reduction in percentage survival of L. bethae. The number of T. scrupulosa nymphs was negatively correlated with the percentage survival of L. bethae. Neither the duration of development nor the body size of L. bethae was influenced by the intensity of T. scrupulosa infestation. Overall, undamaged or slightly damaged plants that allowed better survival of L. bethae were often chosen as oviposition sites in preference to those that were highly infested, and on which survival was poor. Although the present study indicates the likelihood of inter-specific competition between L. bethae larvae and T. scrupulosa, this is likely to be mitigated by female flea beetles choosing to oviposit on less infested or uninfested plants in the field.     

Larvicidal activity of lectins onLucilia cuprina: mechanism of action

Foraging behaviour and host-instar preference of young and old females of the solitary aphid parasitoid,Lysiphlebus cardui Marshall (Hymenoptera: Aphidiidae), were studied in the laboratory. The analysis of interactions between parasitoids and different stages ofAphis fabae cirsiiacanthoidis Scop. (Homoptera: Aphididae) revealed that encounter rates between aphids and parasitoid females and defence reactions of the aphids influenced the degree to which a particular aphid age class is parasitized. Encounter rates between hosts and parasitoid females depended on the foraging pattern of the parasitoid, which varied with age. In mixed aphid colonies patch residence time increased with parasitoid age. Furthermore, younger parasitoids (≦1 day old) laid more eggs into second and third instars, while older parasitoids (≧4 days old) did not show distinct host instar preferences. It is suggested that the oviposition behaviour ofL. cardui is influenced by the physiological state, i.e. the age of the wasp.     

Effects of different Beauveria bassiana isolates on field populations of Lygus lineolaris in pigweed (Amaranthus spp.)

The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), is a pest of various fruit, vegetable, fiber, and seed crops; including cotton. Lygus spp. populations often build on alternate host plants before moving to cotton, and in the midsouthern U.S. wild host plants, such as pigweed (Amaranthus spp.), play a major role in L. lineolaris population development. Three isolates of the entomopathogenic fungus Beauveria bassiana (Balsamo) were evaluated for L. lineolaris control in redroot pigweed (Amaranthus retroflexus L.): one from L. lineolaris in Mississippi (TPB3); one from Lygus hesperus (Knight) in California (WTPB2); and one commercial isolate from Mycotrol^® (GHA). Fungal applications resulted in moderate to high mycosis in adults (33 to 80%) and moderate mycosis in nymphs (36 to 53%) that were collected from field plots at 2 days post-treatment and incubated under laboratory conditions. Although TPB3 was previously found to be more pathogenic in laboratory bioassays, there was not a consistent separation of this isolate from the other two isolates in field trials. Where differences in adult mycosis or mortality were observed, TPB3 was the most pathogenic. However, in one field trial 7 day mortality for nymphs treated with GHA was higher than those treated with TPB3 or WTPB2. Infection rates at 2, 7, and 14 days post-treatment from caged and non-caged adults suggested that movement of adults among plots occurred, which could have masked some treatment effects. Fungal treatments did not significantly reduce populations relative to controls. This may have been caused by delayed mortality rates under field conditions and/or difficulties with estimating population change under field conditions characteristic of wild host plant populations (e.g., heterogeneous populations, adult movement, and small plot size). Further work evaluating time–dose–mortality over dynamic temperatures, spring and fall field trials on this and other wild hosts, and improved methods for estimating populations on wild hosts are needed.     

Host-recognition kairomone from Sogatella furcifera for the parasitoid Anagrus nilaparvatae

A kairomone produced by the rice white-backed planthopper Sogatella furcifera (Horvath) for the mymarid egg parasitoid Anagrus nilaparvatae Pang et Wang was investigated. Eggs, female and male adults, nymphs, exuvia, honeydew, nymph-damaged plants and plants with S. furcifera eggs all elicited searching behavior in the parasitoid. Eggs, female adults and plants with eggs were the most attractive, while exuvia and honeydew evoked the weakest responses. The active compound(s) from S. furcifera female adults, nymphs and plants with eggs could be effectively extracted with methanol, acetone, n-hexane, ethyl ether and dichloromethane. Isolation of the active chemical(s) in three solvent extracts, the acetone and n-hexane extracts of S. furcifera female adults, and the acetone extract of S. furcifera nymphs, was accomplished by thin-layer chromatography. Only one fraction was active and had the same chemical properties in the three solvent extracts. Infra-red analysis of this fraction revealed a kind of ester with saturated and unsaturated fatty acids, which was previously found to be active in the same compound as the extracts from the nymphs and female adults of the rice brown planthopper, Nilaparvata lugens (Stål), and identified as palm oil. The results are discussed in relation to host location by A. nilaparvatae.     

Long PCR Is a Sensitive Method for Detecting Liberobacter asiaticum in Parasitoids Undergoing Risk Assessment in Quarantine

The accidental importation of plant pathogens in or on the bodies of parasitoids imported as natural enemies has been raised as a potential risk of classical biological control projects involving insects that serve as vectors of plant diseases. During quarantine evaluation of two parasitoids, Tamarixia radiata Waterston and Diaphorencyrtus aligarhensis (Shafee, Alam and Agarwal), imported for classical biological control of Asian citrus psylla Diaphorina citri (Kuwayama) in Florida, we were asked to determine whether these parasitoids were free of the causal agent of Asian greening disease, the bacterium Liberobacter asiaticum. Preliminary tests with allele-specific polymerase chain reaction (Standard PCR) suggested that the assays were prone to false negatives. Another PCR protocol, Long PCR, previously was shown to be more reliable than Standard PCR for screening of insects for another bacterium (Wolbachia). The sensitivity of Long and Standard PCR protocols was compared by use of plasmid DNA containing two DNA fragments from the greening disease agent or plasmid mixed with DNA extracted from host plants, psyllids, or parasitoids. Results indicated that inhibitors of the PCR were present in both plant and insect DNA, making the Standard PCR relatively insensitive and allowing high levels of false negatives. Long PCR, which incorporates a second DNA polymerase with proof-reading activity, yielded consistent results and was orders of magnitude more sensitive than the Standard PCR. As few as 100 copies of plasmid mixed with either plant or insect DNA consistently could be detected. Long PCR assays conducted on pooled and individual T. radiata and D. aligarhensis, their psyllid hosts, or their host plants over a period of 6 months failed to produce any positives, indicating that release of these two parasitoids should elicit little concern that greening bacteria would be introduced accidentally into Florida through this classical biological control program.     

The role of natural enemy guilds in Aphis glycines suppression

Generalist natural enemy guilds are increasingly recognized as important sources of mortality for invasive agricultural pests. However, the net contribution of different species to pest suppression is conditioned by their biology and interspecific interactions. The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is widely attacked by generalist predators, but the relative impacts of different natural enemy guilds remains poorly understood. Moreover, low levels of A. glycines parasitism suggest that resident parasitoids may be limited through intraguild predation. During 2004 and 2005, we conducted field experiments to test the impact of different guilds of natural enemies on A. glycines. We contrasted aphid abundance on field cages with ambient levels of small predators (primarily Orius insidiosus) and parasitoids (primarily Braconidae), sham cages and open controls exposed to large predators (primarily coccinellids), and cages excluding all natural enemies. We observed strong aphid suppression (86- to 36-fold reduction) in treatments exposed to coccinellids, but only minor reduction due to small predators and parasitoids, with aphids reaching rapidly economic injury levels when coccinellids were excluded. Three species of resident parasitoids were found attacking A. glycines at very low levels (<1% parasitism), with no evidence that intraguild predation by coccinellids attenuated parasitoid impacts. At the plant level, coccinellid impacts resulted in a trophic cascade that restored soybean biomass and yield, whereas small natural enemies provided only minor protection against yield loss. Our results indicate that within the assemblage of A. glycines natural enemies in Michigan, coccinellids are critical to maintain aphids below economic injury levels.     

Life history ofAphis gossypii on cucumber: influence of temperature, host plant and parasitism

Life table data forAphis gossypii Glover (Homoptera: Aphididae), an important pest in glasshouse cucumber crops, were studied at 20, 25 and 30°C on two cucumber cultivars (Cucumis sativus L.) in controlled climate cabinets. The development time on the cucumber cv. ‘Sporu’ ranged from 4.8 days at 20°C to 3.2 days at 30°C. Immature mortality was approximately 20% and did not differ between temperatures. Most mortality occurred during the first instar. Reproduction periods did not differ among temperatures, but at 25 and 30°C more nymphs were produced (65.9 and 69.8 nymphs/♀, respectively) than at 20°C (59,9 nymphs/♀) because of a higher daily reproduction. Intrinsic rate of increase was greatest at 25°C (r _m =0.556 day⁻¹). At 20 and 30°C the intrinsic rate of increase was 0.426 and 0.510, respectively. On cv. ‘Aramon’, the development time ofA. gossypii was approximately 20% longer at all temperatures. Immature mortality did not differ between the two cultivars. The intrinsic rate of increase on cv. ‘Aramon’ was 15% smaller than on cv. ‘Sporu’. The use of cucumber cultivars partially resistant to aphids is discussed in relation to biological control of cotton aphid in glasshouses. Development time and immature mortality on leaves of the middle and upper leaf layer of glasshouse grown cucumber plants (cv. ‘Aramon’) were comparable to development in the controlled climate cabinets. On the lower leaves immature mortality was much higher (approximately 82%) than on leaves of the middle (24.0%) and upper leaf layer (24.5%). Reproduction was less on the lower leaf layer (45.9, 70.5 and 70.1 nymphs/♀ on leaves of the lower, middle and upper leaf layer, respectively). Aphids, successfully parasitized byAphidius colemani Viereck (Hymenoptera: Braconidae) only reproduced when they were parasitized after the third instar. Fecundity was 0.1 to 0.9 and 10.5 to 13.3 nymphs/♀ for aphids parasitized in the fourth instar or as adults, respectively. Reproduction of aphids that were stung but survived the attack was lower than for aphids not stung. Average longevity of these aphids was equal to the longevity of aphids not stung byA. colemani.