R. Samara

Comparison of different European strains of Trichogramma aurosum (Hymenoptera: Trichogrammatidae) using fertility life tables

Abstract Life table parameters were assessed for seven strains of Trichogramma aurosum Sugonjaev and Sorokina (Hymenoptera: Trichogrammatidae) collected in different European countries, in order to compare their performance when reared on eggs of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) as a potential factitious host for mass-rearing. The average number of progeny per female, cumulative fertility and emergence rate did not differ significantly, whereas female longevity and sex ratio significantly differed between the seven parasitoid strains. The Danish strain survived the longest (6.05 days) and the Dutch strain survived the shortest (2.75 days). Progeny was always female-biased with varying proportions (57.7–96.7%). Survival rates started to decrease after 3 days for some of the strains studied. The mean cohort generation duration (T c) was 11.40, 10.15, 10.62, 10.63, 9.28, 9.70 and 11.30 days for the Austrian, Luxemburgian, Belgian, French, Dutch, Danish and German strains, respectively. Population doubling time (D t) was 4.50, 7.96, 3.56, 5.30, 5.23, 7.36 and 3.30 days, respectively. Daily intrinsic rate of increase (r m) and finite rate of increase (exp. r m) ranged between 0.087 and 0.210 and 1.091–1.233, respectively. The German strain might be a potential candidate for mass rearing and releases against the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), due to its high net reproduction rate (R 0=10.65 female), a high intrinsic rate of natural increase (r m=0.210), a high finite rate of increase (exp. r m=1.23), and a short population doubling time (D t=3.3 days). The relevance of intra- and interstrain variability as well as the usefulness of fertility life tables for pre-introductory research is discussed.

Genetic divergence of Trichogramma aurosum Sugonjaev and Sorokina (Hymenoptera: Trichogrammatidae) individuals based on ITS2 and AFLP analysis

Taxonomy and phylogeny of members of the genus Trichogramma is often critical because of the fact that proper species discrimination can only be achieved by male morphology. Cryptic species, particularly when only females are available (in case of parthenogenetic species or strains), are common in this genus with consequences for practical purposes like biocontrol, unless males can be obtained after antibiotic treatment. The internally transcribed spacer 2 region of the ribosomal DNA was used to assess the identity of Trichogramma aurosum Sugonjaev and Sorokina individuals collected on eggs of Nematus tibialis Newman (Hymenoptera: Tenthredinidae) from different locations in Middle Europe. Amplified products were identical in length (ca. 450bp), sequences showed a high percent similarity (>96%), and no cryptic species could be detected in the samples. In contrast, a comparison with T. aurosum populations from the USA showed values between 86% and 90%. Additional studies are needed to clarify the relationship between US and European populations. Furthermore, amplified fragment length polymorphism (AFLP) analysis was conducted with T. aurosum wasps collected at 25 different European locations. One hundred and twenty‐three AFLP fragments could be detected using three different AFLP primer combinations of which 98% were polymorphic in more than one individual. An analysis of genetic distances based on the obtained AFLP markers indicated the existence of some genetic variability between the European T. aurosum individuals and allowed a grouping according to their geographic origin. This study represents the first successful application of the AFLP marker technique to such tiny insects as Trichogramma species.

Comparative biology and life tables of Trichogramma aurosum on Cydia pomonella at constant temperatures

The influence of constant temperatures on biological parameters of German strains of Trichogramma aurosum Sugonjaev & Sorokina (Hymenoptera: Trichogrammatidae) was evaluated in the laboratory on eggs of Cydia pomonella L. (Lepidoptera: Tortricidae). Development time and longevity of all strains were decreased as temperature increased. Development time of the strains differed significantly only when exposed to 15°, 20°, and 25°C. Cumulative fertility and longevity differed significantly at 15° and 20°C. Realized fertility differed significantly at all constant temperatures. Emergence rates of all strains were less than 65% and were decreased even further as temperature increased. Female-biased sex ratio ranged from 65% to 100% at all constant temperatures. The low temperature threshold for T. aurosum was 10°C and the mean number of degree-days at 15°, 20°, 25° and 30°C was 175, 183, 173 and 185, respectively. The Bavarian strain tolerated high temperatures and had the highest parasitization capability, while the Hessian strain had the lowest parasitization at all temperatures. Fertility life table analysis revealed a major effect of temperature on the population growth parameters. Net reproductive rate was highest at intermediate constant temperatures in all strains, with the highest rate recorded for the Bavarian strains at all constant temperatures. Mean cohort generation time, and population doubling time decreased as temperature increased. The daily intrinsic rate of increase and finite rate of increase were positively correlated with temperature. The relevance of our results is discussed in the context of climatic adaptation, intraspecific variability and biological control.

Development of a detached leaf procedure to evaluate susceptibility to Plum pox virus infection by the green peach aphid (Myzus persicae (Sulzer)) in peach

Abstract A method was developed to evaluate susceptibility of peach leaves to Plum pox virus (PPV) infection by aphids. We examined whether virus multiplication could be detected in aphid-inoculated detached leaves and if transmission efficiency of PPV by green peach aphids to detached leaves was comparable with that of peach seedlings. Results demonstrated that transmission efficiencies of viruliferous aphids transferred to detached peach leaves subsequently maintained on an agar layer for 3 weeks was not significantly different from that for intact seedlings. Overlaying infected PPV plum or peach leaf segments on the healthy peach leaves with subsequent application of aphids to the infected leaf pieces provided a comparable transmission efficiency. Reduced handling of the aphids using this method minimized the possibility of damaging the aphids and facilitated higher throughput testing. Comparable infection rates were obtained for detached leaves using either 50 or 25 viruliferous aphids per leaf. Residual PPV was not detected by direct quantitative reverse transcriptase polymerase chain reaction assay (DqRT-PCR) on non-host plants probed by viruliferous aphids. The effect of short-term storage temperatures pre- or post-inoculation did not significantly alter the susceptibility of peach leaves to PPV infection or the transmission rate. Application of the leaf overlay method to evaluate seasonal changes in susceptibility of peach leaves in the field is the subject of an ongoing study.

Horticultural mineral oil influences Plum pox virus transmission by Myzus persicae

The residual activity of horticultural mineral oil (HMO) on the ability of green peach aphids, Myzus persicae (Sulzer), (GPA) to transmit Plum pox virus (PPV) to peach was measured by infection rates of detached leaves from plants sprayed with either HMO or water as a control that were inoculated using transfer of 25 viruliferous aphids per leaf at 0, 2, 4, 7, 9, 11 and 14 days after treatment (DAT). Persistent effects of HMO residue on the probing and feeding behaviours of GPA were also monitored with the electrical penetration graph (EPG) system. For glasshouse‐grown peach seedlings, the residual activity of HMO reduced PPV infection rates by more than 58% for up to 4 DAT following an initial reduction of approximately 81%. EPG recordings of GPA feeding behaviour showed that HMO significantly delayed first feeding probes and first intracellular punctures by more than 50 min without changing the ensuing stylet penetration behaviour. Applying HMO reduced virus infection rates for up to a week depending on the environmental conditions. EPG monitoring of aphid probing showed that HMO reduced the mean duration and mean number of potential drop (PD) phase feeding occurrences, compared with the water control. A reduction in the PD that has been shown to be related to the transmission of non‐persistently transmitted viruses may partly explain the reduction in PPV infection rates.

Plant growth regulator-mediated anti-herbivore responses of cabbage (Brassica oleracea) against cabbage looper Trichoplusia ni Hübner (Lepidoptera: Noctuidae)

Plant elicitors can be biological or chemical-derived stimulators of jasmonic acid (JA) or salicylic acid (SA) pathways shown to prime the defenses in many crops. Examples of chemical elicitors of the JA and SA pathways include methyl-jasmonate and 1,2,3-benzothiadiazole-7-carbothioate (BTH or the commercial plant activator Actigard 50WG, respectively). The use of specific elicitors has been observed to affect the normal interaction between JA and SA pathways causing one to be upregulated and the other to be suppressed, often, but not always, at the expense of the plants herbivore or pathogen defenses. The objective of this study was to determine whether insects feeding on Brassica crops might be negatively affected by SA inducible defenses combined with an inhibitor of detoxification and anti-oxidant enzymes that regulate the insect response to the plants defenses. The relative growth rate of cabbage looper Trichoplusia ni Hübner (Lepidoptera: Noctuidae) fed induced cabbage Brassica oleraceae leaves with the inhibitor, quercetin, was significantly less than those fed control cabbage with and without the inhibitor. The reduced growth was related to the reduction of glutathione S-transferases (GSTs) by the combination of quercetin and increased levels of indole glucosinolates in the cabbage treated with BTH at 2.6× the recommended application rate. These findings may offer a novel combination of elicitor and synergist that can provide protection from plant disease and herbivores in cabbage and other Brassica crops.

Impact of Plum pox virus (PPV-D) infection on peach tree growth, productivity and bud cold hardiness

Abstract In 2000, the Dideron (D) strain of Plum pox virus (PPV) was detected in commercial peach and nectarine orchards in the Niagara region of Ontario where most of Canada’s stone fruit crops are produced. As part of a disease management research programme, peach (Prunus persica L. Batsch) trees in a commercial orchard at Niagara-on-the-Lake were assayed for PPV annually for 3 years. The orchard consisted of two blocks of the cultivars ‘Allstar’ and ‘Brighton’, of which 4 of 288 and 5 of 252 trees, respectively, were infected with PPV-D. The growth and health of these PPV-infected and non-infected trees were evaluated based on the annual growth rates, vigour (chlorophyll content) and bud winter hardiness. Comparative fruit quantity and quality index values were based on total yield and marketable yield per tree, fruit size and weight, fruit pH, titratable acidity, total soluble solids, flesh firmness, and fruit skin colour. Results from these preliminary studies showed that trees infected with this mild Ontario isolate of PPV produced slightly more fruit of smaller size that ripened earlier than non-infected trees. However, yield efficiencies based on weight of fruit relative to the trunk cross-sectional area did not differ statistically. Screenhouse studies on three graft-inoculated fresh market peach cultivars (‘Babygold’, ‘Catherina’, and ‘Garnet Beauty’) similarly did not demonstrate any differences in growth or fruit production in the second and third year post inoculation, but fruit on infected trees matured somewhat earlier.

Influence of horticultural oil (Superior-70) on Plum pox virus (PPV) detection in treated and untreated PPV-infected peach (Prunus persicae L.) leaves

Abstract The effect of foliar application of horticultural oil on the detection of Plum pox virus (PPV) in infected peach leaves was examined. No significant differences in virus detection using ELISA or DRT-qPCR were found between matching detached half leaves treated either with oil or water immediately after oil application, and up to 3 weeks post application. Similarly, in vitro amendments of oil to dilutions of PPV-infected leaf macerates did not affect DRT-qPCR detection of virus, and only reduced detection of virus by ELISA at the lowest virus dilutions containing 5 µl mL−1 of oil. Application of horticultural oil by growers to reduce aphid transmission of PPV should have no impact on virus detection in regulatory surveys associated with monitoring the quarantine zone.