Ovipositing females of a short‐lived gall midge take time to assess suboptimal grass seed heads

Abstract The short‐lived adult wheat midge Sitodiplosis mosellana deposits eggs on the seed head of various grasses close to the developing seeds on which larvae feed. The time taken to make egg‐laying decisions is investigated using three types of wheat Triticum aestivum seed heads. Young Roblin, Old Roblin and Young Key differ in their effects on ovipositing females (72%, 22% and 6% of eggs in choice tests, respectively) and effects on feeding larvae (75%, 25% and 5% larval survival, respectively). Within seconds of arriving, the female is able to distinguish Young Roblin from the two lower‐ranked types. However, the lower‐ranked types are not rejected at this time. Instead, all head types are examined before the female eventually flies away. On Young Roblin, probing with the ovipositor is the first behaviour that occurs. Thereafter probing and insertion of the ovipositor occupy most of the female’s time and behavioural transitions tend to be ‘progressive’, signalling a shift from low to high intensity examining. Differences between females visiting Old Roblin and Young Key are significant but take longer to emerge. On both, sitting is the first behaviour but, over the next 5–10 min, the female on Young Key exhibits more sitting, walking and ‘regressive’ transitions than the female on Old Roblin. It is suggested that, when the ovipositing female is short‐lived and incapable of controlled flight in all but essentially windless conditions, her behaviour is designed to thoroughly, rather than rapidly, examine a suboptimal host before abandoning it for the uncertain future of finding a better host.     

Different pathotypes of the sunflower downy mildew pathogen Plasmopara halstedii all contain isometric virions†

Eight pathotypes of Plasmopara halstedii were screened to investigate the occurrence of virions and the potential viral influence on the pathogenicity of the sunflower downy mildew pathogen. In 23 of 26 P. halstedii isolates derived from eight countries in Europe, North America and South America, virions were detected by transmission electron microscopy. By contrast, there were no ultrastructural indications of virus‐like particles in eight other related Oomycetes. The virions of representative P. halstedii isolates were morphologically and biochemically characterized and compared among each other. Regardless of their host's pathotypes, the geographical origin of the isolate and the sensitivity towards the fungicide metalaxyl, the viral characters obtained were uniform. The virions were isometric and measured approximately 37 nm in diameter. One polypeptide of c. 36 kDa and two segments of single‐stranded RNA (3.0 and 1.6 kb) were detected. Both viral RNA segments were detected by capillary electrophoresis in the three remaining P. halstedii isolates where virions were undetectable by transmission electron microscopy. Virus‐specific primers for the 1.6 kb‐segment were synthesized and used to determine and compare a partial sequence of the viral coat protein among virions of different P. halstedii pathotypes. In all tested isolates, fragments of 0.7 kb were amplified which were directly sequenced. Sequence variation was insignificant. As both less aggressive and more aggressive P. halstedii isolates contained virions, the presence or absence of virions could not explain the diverse aggressiveness of the downy mildew pathogen towards sunflower. Moreover, the results indicated that pathogenicity of P. halstedii was not related to variation in morphological or biochemical characters of the virions.     

Degradation of native and exotic riparian plant leaf litter in a floodplain pond

1. A litter‐bag experiment was undertaken in a pond on the margins of a large temperate floodplain in south‐western France to assess the potential influence of the replacement of native by exotic riparian species on organic matter degradation. We determined initial litter chemical composition, breakdown rates and the invertebrate assemblages associated with the litter for five pairs of native dominant and exotic invasive species co‐occurring at different stages along a successional gradient. 2. Litter chemical composition, breakdown rates and abundance and diversity of detritivorous invertebrates were similar for the exotic and native species overall. No overall changes in organic matter degradation can thus be predicted from the replacement of dominant natives by exotic invasives. Breakdown rates were primarily driven by the C/N ratio. 3. One invasive species (Buddleja davidii) showed significantly higher breakdown rates than its native counterpart (Populus nigra), resulting in the disappearance of leaf litter 6 months prior to the next litterfall. In some cases, therefore, invasion by exotic species may result in discontinuity of resource supply for decomposers.     

Bacteriological Changes Occurring During the Culture of Algae in the Liquid Phase of Animal Slurry

The single-celled alga Chlorella vulgaris was cultured in the liquid phase of pig slurry under two different light intensities, and the development of the indigenous bacterial flora was compared qualitatively and quantitatively with that of controls not inoculated with chlorella. Particular attention was given to the occurrence and survival of potential pathogens. Algal growth was found to favour the growth of certain bacteria but had little effect on or was hostile to that of others. The effects of light were mediated by the algae rather than having a direct effect on the bacterial cells. The algae, which were harvested by centrifuging and then freeze-dried, were found to enhance the survival of certain bacteria during this procedure.     

“Spheroplasts” Prepared from Euglena gracilis by Proteolysis*

SYNOPSIS. Protoplast-like, osmotically-sensitive bodies were prepared from Euglena gracilis by treatment with a crude preparation of proteolytic enzymes. These spheroplasts were free of pellicles to varying degree as viewed by phase-contrast microscopy and were more easily ruptured than were untreated controls. The spheroplasts retained D-lactate dehydrogenase and acid phosphatase activity, but with altered permeability characteristics.     

The epidemiology of tomato brown root rot

In the absence of nematodes, three different symptoms of disease, parts of the brown root rot complex (BRR), occurred on tomato roots surviving in soils infested with GSF (= grey sterile fungus) and Colletotrichum atramentarium (Berk. & Br.) Taubenh. In heavily infested soils brown lesions occurred throughout cropping, appearing within a week of planting. Corkiness and black dot, caused by GSF and C. atramentarium respectively, rarely occurred until the third month after planting but towards the end of the season the incidence of black dot sometimes suddenly increased greatly. Observations of crops growing in plots treated with different soil partial sterilants suggested that GSF was more damaging than C. atramentarium. Yield was not related to the incidence of black dot but was inversely proportional to the occurrences of brown lesions and corkiness. The relation with brown lesions was significant within 8 weeks of planting, when most brown lesions gave cultures of GSF, but later more of these lesions gave cultures of C. atramentarium than of GSF. Pathogenicity tests with pure cultures of GSF and C. atramentarium were done on agar media and by artificially infesting partially sterilized soils. Roots of undamaged seedlings on agar media developed 10 mm. brown lesions within 2 weeks of inoculating 10-day-old tomatoes with most GSF cultures isolated from: (1) rotted roots of Lycopersicon esculentum, Solanum capsicastrum, Capsicum annuum var. longum and C. frutescens; (2) browned zones of Lycopersicon hirsutum roots; and (3) apparently healthy roots of Cucumis sativus. After inoculation with C. atramentarium, small (c. 2 mm.) pink lesions developed, whereas none formed using Pyrenochaeta spp. In soil tests the greater root damage done by GSF, including root loss, was reflected in decreased aerial growth and smaller fruit yields; C. atramentarium affected neither. In the second year of soil infestation GSF decreased yields during 6 weeks of picking from 1.96 kg. in the uninoculated controls to 1.02 kg./plant. The pattern of damage done by GSF changed as plants aged. In soil, brown lesions occurred within a few days of planting but corkiness did not appear for 2–3 months, when stem lesions and leaf yellowing often developed simultaneously. A 50% root loss after 21 weeks did not affect fruit yields whereas a 40% loss within 11 weeks of planting was reflected by a 45% yield decrease.