Spatial and temporal patterns of predation by ants on eggs of Cactoblastis cactorum

ABSTRACT.

• 1 Predation of Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) eggs by ants was studied for a summer and a winter generation on prickly pear Opuntia ficus-indica (L.) Miller plants in the field.
• 2 Egg Predation between plants was spatially density-dependent in the winter but not in the summer generation.
• 3 To test for the presence of delayed density-dependent mortality within generations, percentage egg Predation during 5-day periods was plotted against the number of eggs available in each period. Data points joined in a time sequence moved in an anticlockwise direction in the summer generation, suggesting a delayed density-dependent response by the ants, whereas in the winter generation the points moved in a clockwise direction.
• 4 The degree of coincidence of the C. cactorum eggs with the peak larval production periods of the ants is suggested as the main reason for the temporal changes in egg Predation.

     

Cold-induced purpling of Pinus contorta seedlings depends on previous daylength treatment

The characteristic cold-induced foliar accumulation of anthocyanin (‘purpling’) seen in many species of pine seedlings in the autumn can be contingent upon photoperiod received months previously. In nursery-grown seedlings of Pinus contorta Dougl. var. latifolia Engelm., a 4-week exposure during August of 10-h days followed by return to ambient photoperiod diminished the frequency of purpling following the November onset of cold weather (21% compared to 71% of the untreated controls). Anthocyanin accumulation (primarily cyanidin 3-glucoside) was controlled differently than general phenolic metabolism, and involved the integration of two temporally separated environmental signals.     

Top-down and bottom-up influences on populations of a stream detritivore

1. Knowledge of the influence of predatory fish in detritus‐based stream food webs is poor. We tested whether larval abundance of the New Zealand leaf‐shredding caddisfly, Zelandopsyche ingens (family Oeconesidae), was affected by the presence of predatory brown trout, Salmo trutta and the abundance of their primary detrital resource (Nothofagus leaves). 2. The density of Z. ingens and the biomass of leaves were determined in seven fishless streams and four trout streams in the Cass region, central South Island, on four occasions spanning 5 years. 3. Physicochemical conditions were similar in trout and fishless streams, but ancova indicated that Z. ingens numbers were positively related to leaf biomass and that caddisfly numbers were significantly greater in fishless streams than trout streams for any given biomass of leaf. The cases of trout stream larvae were also heavier per unit length than those in fishless streams. 4. Our results provide evidence for both top‐down and bottom‐up influences on a detritus‐based stream food web. Although stream detritivores may benefit from a habitat that provides both food and a degree of protection from predators, top‐down effects of predators on detritivore population abundance were still important. Thus, detrital resource availability may determine maximum attainable population size, whereas predation is likely to reduce the population to a level below that.     

Variability in mesophyll conductance between barley genotypes,and effects on transpiration efficiency and carbon isotope discrimination

Leaf internal, or mesophyll, conductance to CO₂ (g_m ) is a significant and variable limitation of photosynthesis that also affects leaf transpiration efficiency (TE). Genotypic variation in g_m and the effect of g_m on TE were assessed in six barley genotypes (four Hordeum vulgare and two H. bulbosum). Significant variation in g_m was found between genotypes, and was correlated with photosynthetic rate. The genotype with the highest g_m also had the highest TE and the lowest carbon isotope discrimination as recorded in leaf tissue (Δ_p). These results suggest g_m has unexplored potential to provide TE improvement within crop breeding programmes.     

Seasonal,demographic and density‐related patterns of contact between Tasmanian devils (Sarcophilus harrisii): Implications for transmission of devil facial tumour disease

Abstract Devil facial tumour disease (DFTD), is an emerging infectious cancer thought to be spread by biting. It is causing ongoing, severe population decline of the Tasmanian devil (Sarcophilus harrisii), the largest surviving marsupial carnivore and there are concerns that DFTD may lead to extinction of the devil. Whether extinction is likely depends on contact rates and their relationship to host density. We investigated contact rates using two different datasets. The first consisted of field observations of contact and biting behaviour around prey carcasses and, the second was a 3‐year longitudinal series of injuries in a marked devil population. During feeding interactions at carcasses, contact rates were significantly positively associated with population density and subadults delivered more bites than adult males and females. Injuries from the marked devil population did not differ between adult males and females. In two of the three years, penetrating biting (resulting in injury) increased markedly during the mating season and was more frequent in adults than in subadults. Among injured devils with wounds penetrating the dermal layer, adults were more frequently bitten in the head (the location of primary tumours) in the mating season than in other seasons, and had more head bites than subadults. Our results suggest that the mating season may be the key period for disease transmission. If most penetrating bites occur during mating interactions, DFTD transmission is likely to be frequency dependent, which means that there would be no threshold host density for disease persistence, and disease‐induced extinction is possible.     

Analysis of potential sources of variation in tissue culture derived celery plants

Somaclonal variation derived from tissue culture is a potential source of variation that can be used in crop improvement programmes. The characteristics of this variation are first shown in the regenerant generation and their heritability is then confirmed by examination of the progeny. There would be savings of time, space and labour if this variation could be detected in vitro using easily assessed visual cues. The aim of this study was to relate variation in the source of explant and the morphology of the newly initiated callus to the characteristics of the regenerant plant, of which the most important was resistance to leaf spot disease caused by Septoria apiicola. Associations were investigated by isolating four stem explants from each of 564 surface sterile seedlings, var. Celebrity, on a callus initiation medium (MS medium, 30 g litre‘¹sucrose, 0.5 mg litre’¹2,4-D, 0.6 mg litre‘¹kinetin) and assessing the morphology and colour of the callus. After this initial culture (8 wk), each callus was transferred to a regeneration medium (MS medium, 30 g litre“¹sucrose). Plantlets were regenerated from many of the callus cultures and these were transferred to the glasshouse. When all of the surviving regenerant plants (276) were mature, leaf shape, amount and composition of the essential oils and resistance to late blight were assessed. Statistical analysis revealed that the character of the newly initiated callus (width, height, colour, organogenesis) showed poor correlation with all aspects of the regenerated plant measured. However, it was shown that increased variation resulted from different seedlings more than from plants derived from within seedlings or within callus.     

The Nitrogen-Nutrition of Hippophae rhamnoides L.

It is shown that Hippophaë plants, inoculated at the six-leafstage with a suspension of crushed nodules, begin to developnodules in about two weeks and soon afterwards become able togrow satisfactorily in culture solution free of combined nitrogen,up to 79 mg. nitrogen per plant having been accumulated during7 months' active growth. It is concluded that the nodules fixatmospheric nitrogen, and that in its nitrogen nutrition theHippophaë plant shows a close similarity to the nodulatedlegume. It is pointed out that Hippophaë and the otherAngiospermous genera with nitrogen-fixing root nodules are allwoody plants or belong to a family which includes woody types.The possible significance of this is briefly discussed.