TILLERING AND MORTALITY OF THE SALT MARSH CORDGRASS,SPARTINA ALTERNIFLORA

Recruitment and mortality of Spartina alterniflora tillers and culms were estimated from measurements of live standing crops at Wallops Island, Virginia. Stands of tall and medium form grass produced from 452 to 1,227 tillers m⁻² yr⁻¹. Those tillered in spring lengthened into culms their first year, while those tillered in late summer and fall may have overwintered and lengthened into culms their second year. Biennial growth was most prevalent in tall form Spartina. Fifty-six to sixty-one percent of all tillers died before lengthening into culms. Of the remainder, 9 to 56% died as culms before the time of flowering. Sixty-three percent of the surviving culms of tall form Spartina flowered, compared to only 14 to 23% in medium form. Mortality of tillers and culms removed 16 to 35% of the annual production of Spartina biomass from live standing crops before the time of flowering.     

POPULATION,DIET AND THE ANNUAL CYCLE OF THE LAUGHING DOVE AT BAR-BERSPAN,PART 2: DIET

Dean, W. R. J. 1979. Population, diet and the annual cycle of the Laughing Dove at Barberspan, Part 2: Diet. Ostrich 50:215-219.

Laughing Doves Streptopelia senegalensis were collected each month from July 1976 to to June 1977. The diet was found to be mainly seeds of commercial crops (sunflower and make) during most of the year. One species of grass, Eleusine indica, occurred in 100% of stomachs, but often was present only in small amounts. Ovulating females took significantly more animal matter than non-ovulating females.     

Global spread of wheat curl mite by its most polyphagous and pestiferous lineages

The wheat curl mite (WCM), Aceria tosichella, is an important pest of wheat and other cereal crops that transmits wheat streak mosaic virus and several other plant viruses. Wheat curl mite has long been considered a single polyphagous species, but recent studies in Poland revealed a complex of genetically distinct lineages with divergent host‐acceptance traits, ranging from highly polyphagous to host‐specific. This diversity of WCM genotypes and host‐acceptance phenotypes in Europe, the presumed native range of WCM, raises questions about the lineage identities of invasive WCM populations on other continents and their relationships to European lineages. The goals of this study were to examine the global presence of WCM and determine the relatedness of lineages established in different continents, on the basis of phylogenetic analyses of mitochondrial and nuclear DNA sequence data. Host‐range bioassays of a highly polyphagous WCM lineage were performed to supplement existing data on this lineage's ability to colonise graminaceous and non‐graminaceous hosts. Invasive WCM populations in North and South America and Australia assorted with the only three known polyphagous and pestiferous WCM lineages (‘MT‐1’, ‘MT‐7’ and ‘MT‐8’) from a total of eight currently described lineages. These results show that the most polyphagous lineages were more successful colonisers and reflect a need for extensive surveys for WCM on both crops and wild grass species in invaded continents. The most invasive lineage (‘MT‐1’) was shown to successfully colonise all 10 plant species tested in three families and has spread to North and South America and Australia from its presumed origins in Eurasia.     

A host shift from wild blue cohosh to cultivated potato by the phytophagous ladybird beetle, Epilachna yasutomii (Coleoptera, Coccinellidae)

Life history traits of the phytophagous ladybird beetle Epilachna yasutomii were compared between a nonpest population feeding on wild blue cohosh and a pest population feeding on cultivated solanaceous crops, mainly potato. Newly emerged adults of the nonpest population entered diapause early in midsummer when blue cohosh withered, while adults of the pest population were found in tomato and eggplant fields until late autumn. The pest population had larger females, a higher population growth rate, a shorter larval developmental period, and reduced longevity of overwintered females, compared with the nonpest population. ANOVA indicated that all these life history traits were influenced by the food plant, and that the number of eggs laid per female and the longevity of overwintered females were also affected by the population type. These findings suggest that the life history pattern of E. yasutomii changed to high fecundity with a short life span from low fecundity with a long life span as a result of the host shift from wild blue cohosh to cultivated solanaceous crops. Received: May 22, 1998 / Accepted: January 13, 1999     

Responses of holocyclic and anholocyclic Rhopalosiphum padi populations to low‐temperature and short‐photoperiod induction

The different life cycles of aphid species make these organisms good models for studying the short‐term consequences of sex. The bird cherry‐oat aphid Rhopalosiphum padi has a wide geographic distribution and correspondingly different life cycles. In this study, the life cycles of R. padi collected from six different regions in China were characterized experimentally by comparing the responses of holocyclic and anholocyclic populations to low‐temperature and short‐photoperiod induction. Clones collected from Chuzhou, Taian, and Taigu consistently reproduced via obligate parthenogenesis, whereas clones from Hami and Baicheng were holocyclic in their response, and those from Lanzhou were both holocyclic and anholocyclic. Prolonged exposure to low temperature and a short photoperiod (LS) had negative effects on the offspring of anholocyclic aphids with regard to adult lifespan, total longevity, and fecundity compared with aphids maintained at a normal temperature and a long photoperiod (NL). Holocyclic LS R. padi had longer developmental times at all nymph stages, a shorter adult lifespan, shorter total longevity, and a lower fecundity than NL counterparts. The adult prereproduction period of gynoparae was significantly longer than that of virginoparae, and the total longevity of gynoparae was significantly shorter than that of virginoparae. Moreover, the net reproductive and gross reproduction rates, as well as the total fecundity, were roughly fivefold higher in virginoparae than in gynoparae, indicating that there is the short‐term cost of sex. When maintained on their secondary host (Triticum aestivum), gynoparae, males, and oviparae produced by holocyclic populations could survive, and gynoparae produced oviparae. However, under NL conditions, oviparae could not produce overwintering eggs on the secondary host, whereas a few overwintering eggs were generated by oviparae under LS conditions. Taken together, these results illuminate the complexity of insect responses and contribute to a complete understanding of the aphid life cycle and its evolution.     

The distribution and abundance of predatory arthropods overwintering on farmland

Soil samples (0·04 m^-2) were taken from December to January in 1981-82 and 1982-83 from five crop and two non-crop habitats on farmland in Hampshire to quantify the distribution and abundance of overwintering polyphagous predatory arthropods. Total numbers of overwintering polyphagous predators were greater in field boundaries, winter-sown cereals and established grassland than in the other four habitats. Many individual species overwintered in significantly higher densities in field boundaries than in any other habitat; others predominated in woodland, grassland and winter-sown cereals. The carabids Agonum dorsale and Demetrias atricapillus, probably the most important predators of cereal aphids, overwintered almost entirely in field boundaries.     

Plantlet regeneration from somatic hybrids of rice (Oryza sativa L.) and barnyard grass (Echinochloa oryzicola Vasing)

Summary Somatic hybridization of rice (Oryza sativa L.) and barnyard grass (Echinochloa oryzicola), a close relative of barnyard millet, was attempted using electrofusion and a new culture method developed for rice protoplasts (Kyozuka et al. 1987) to incorporate some of the agronomically important characters of the latter species into rice. Selection of hybrids was based on inactivation of rice protoplasts by iodoacetamide and the inability of barnyard grass protoplasts to divide. A total of 166 calli were identified as hybrids by isozyme and chromosome analyses. Hybrid calli were highly morphogenic, and 44 shoots were obtained. Most of them, however, were abnormal, and nine grew to plantlets whose morphology was distinct from that of either parent. Our study clearly demonstrates the totipotency of protoplasts in graminaceous monocots.     

Distribution and abundance of adult Opomyza florum (Diptera: Opomyzidae) in cereal crops and grassland

From 1971 to 1979 counts were made of the numbers of Opomyza florum adults found in vacuum net samples taken from cereal and grass fields on a 62-km² study area in West Sussex. In 2 years, 1974 and 1975, counts were made of the numbers of adults found in similar samples taken from hedgerow grasses. In all years, numbers of adults were highest in winter wheat crops; relatively few were found in other autumn- or spring-sown cereals or in grass fields. Mean numbers (/m²) in winter wheat in June varied from 33.0 in 1974 to only 0.9 in 1977. In most years, the first adults were found in June in winter wheat; numbers reached a peak in July and then declined rapidly. More adults were usually found in winter wheat crops that followed winter wheat in the rotation than in those following grass. Adults were also more abundant in June in winter wheat crops sown relatively early than in those sown late. The current tendencies towards earlier drilling of winter wheat and to the growth of successive wheat crops could increase the pest status of O. florum.     

Specialisation pattern of the aphid Rhopalosiphum maidis is not modified by experience on a novel host

The effect of alternative host condition on the pattern of specialisation of the aphid Rhopalosiphum maidis (Fitch) was studied. R. maidis commonly occurs in Chile on Johnson grass (Sorghum halepense L.) but rarely on contiguous wheat (Triticum durum L.) crops. The performance of 23 clones of R. maidis on S. halepense (established host) and T. durum (novel host) before and after rearing on wheat for 20 asexual generations was evaluated. Prior and after the period of conditioning on wheat no negative correlation of performance parameters between both hosts was found. Only 8 out of 23 clones survived the 20 asexual generations on wheat. Further, after conditioning on wheat, survival of three out of eight clones increased on wheat and three clones improved r _m on Johnson grass but not on wheat. Although some genotypes of R. maidis were able to withstand wheat stressing conditions, the conditioning on this alternative host did not cause a decreased capacity to use Johnson grass as host. The results suggest that R. maidis performance on wheat is not genetically constrained, and also that conditioning on wheat cannot substantially modify this pattern.     

Associations between host plant concentrations of selected biochemical nutrients and Mexican rice borer,Eoreuma loftini,infestation

The Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), is an economic pest of sugarcane (Saccharum spp. hybrids) and other graminaceous host crops, and it attacks grassy weeds. Oviposition preference has been known to be for plants with leaves that form folds. This study is the first to associate the nutritional quality of crop and forage plant hosts with Mexican rice borer injury. Three experiments were conducted to determine the levels of selected biochemical nutrients, including free amino acids and sugars, in four grass weeds [barnyardgrass, Echinochloa crus‐galli (L.); broadleaf signalgrass, Urochloa platyphylla (Munro ex C. Wright); johnsongrass, Sorghum halepense (L.) Pers.; and Vasey's grass, Paspalum urvillei Steud.], a forage grass [sudangrass, Sorghum bicolor (L.) Moensch spp. drummondi (Nees ex Steud.) de Wet & Harlan], three crop species [maize, Zea mays L.; sorghum, Sorghum bicolor (L.) Moench; and sugarcane, Saccharum spp.], and a sorghum‐sudangrass forage hybrid (all Poaceae). Of 16 free amino acids detected among plants in the first two experiments, only high accumulations of free histidine in sudangrass and maize were associated with increased infestations by Mexican rice borer larvae. In all three experiments, high levels of fructose were associated with heightened Mexican rice borer infestation. Ramifications of these findings on the potential dispersal of this invasive pest in the USA and possible applications of fructose in baits are discussed.     

High‐resolution spatial modelling of greenhouse gas emissions from land‐use change to energy crops in the United Kingdom

We implemented a spatial application of a previously evaluated model of soil GHG emissions, ECOSSE, in the United Kingdom to examine the impacts to 2050 of land‐use transitions from existing land use, rotational cropland, permanent grassland or woodland, to six bioenergy crops; three ‘first‐generation’ energy crops: oilseed rape, wheat and sugar beet, and three ‘second‐generation’ energy crops: Miscanthus, short rotation coppice willow (SRC) and short rotation forestry poplar (SRF). Conversion of rotational crops to Miscanthus, SRC and SRF and conversion of permanent grass to SRF show beneficial changes in soil GHG balance over a significant area. Conversion of permanent grass to Miscanthus, permanent grass to SRF and forest to SRF shows detrimental changes in soil GHG balance over a significant area. Conversion of permanent grass to wheat, oilseed rape, sugar beet and SRC and all conversions from forest show large detrimental changes in soil GHG balance over most of the United Kingdom, largely due to moving from uncultivated soil to regular cultivation. Differences in net GHG emissions between climate scenarios to 2050 were not significant. Overall, SRF offers the greatest beneficial impact on soil GHG balance. These results provide one criterion for selection of bioenergy crops and do not consider GHG emission increases/decreases resulting from displaced food production, bio‐physical factors (e.g. the energy density of the crop) and socio‐economic factors (e.g. expenditure on harvesting equipment). Given that the soil GHG balance is dominated by change in soil organic carbon (SOC) with the difference among Miscanthus, SRC and SRF largely determined by yield, a target for management of perennial energy crops is to achieve the best possible yield using the most appropriate energy crop and cultivar for the local situation.     

Factors affecting cold hardiness in the small striped flea beetle, Phyllotreta undulata

The striped flea beetle, Phyllotreta undulata Kutschera (Coleoptera: Chrysomelidae), is a pest of cruciferous crops. It overwinters as an adult. During winter in northern European countries, such as Estonia, it is subject to sometimes severe temperatures that may fluctuate daily, over the season, and between seasons. The objective of this study was to investigate factors that affect its cold hardiness. In a series of five experiments, the effects of food plant, starvation, and acclimatization on the beetles’ ability to supercool and survive exposure to sub‐zero temperatures was investigated. The supercooling points (SCP) of overwintered beetles field‐collected from white mustard and Indian mustard differed from those caught from white cabbage and oilseed rape, but these differences disappeared after a 4‐day period of starvation at room temperature, indicating that gut content probably influences the potential to supercool. The duration and temperature of acclimation affected SCP in overwintered beetles. The decrease in SCP was more rapid at 22 °C than at 0 °C, probably because of faster dehydration and gut evacuation at the higher temperature. Acclimation at 0 °C for a week increased the ability of overwintered beetles to survive sub‐zero temperatures, lowering both SCP and lower lethal temperature (LLT₅₀). Some pre‐freeze mortality occurred; SCP and LLT₅₀ were correlated but the latter was a constant 3 °C higher than the former. The SCP of field‐collected pre‐winter beetles decreased gradually during the autumn. It also decreased when field‐collected pre‐winter beetles were acclimated at 0 °C in the laboratory, attaining its lowest level after 18 days. Phyllotreta undulata is well‐adapted to unstable and sometimes severe winter conditions; its high potential to supercool enhances its cold hardiness and ability to survive short periods at sub‐zero temperatures although it cannot survive freezing of its body fluids.     

CHARACTERIZATION OF TISSUE CULTURE INITIATION AND PLANT REGENERATION IN SPOROBOLUS VIRGINICUS (GRAMINEAE)

Tissue cultures of the halophytic saltmarsh grass Sporobolus virginicus were initiated from unemerged immature inflorescence tissue. Typical graminaceous embryogenic and nonembryogenic callus and cell types were noted. Embryogenic callus was compact golden yellow. Histological evidence indicated that proliferation of the ovary tissue of the immature pistil was the source for embryogenic callus. Plants regenerated after first reducing and then eliminating auxin from the culture medium. Regeneration was observed both through the concerted development of bipolar meristems from somatic embryos and by the formation of multiple shoot meristems that were either connected through callus tissue to root meristems or which later adventitiously rooted. The main mode of regeneration appeared to be somatic embryogenesis with additional multiple shoot formation probably due to precocious germination of somatic embryos. Plants recovered from culture were acclimated to soil, grown up in a greenhouse, and planted in field plots with saline irrigation to ensure stability of salt tolerance.     

Occurrence ofAzospirillum spp. in coconut-based farming systems

Occurrence of Azospirillum was investigated in coconut-based farming systems, such as high-density multispecies cropping (15 crops), multi-storeyed cropping (3 crops), mixed cropping with tea and coffee (2 crops), intercropping with tropical tubers (5 crops), mixed farming with grasses (3 crops) and in 3 crops, arecanut,Mimosa invisa and sugarcane from other plots. A total of 26 plantation crops and intercrops were included in the study. Incidence of Azospirillum was determined by 2,3,5-triphenyl-tetrazolium chloride reduction and by culturing root fragments in N-free semisolid malate medium. Root samples from guava, mango and mimosa did not show any tetrazolium reduction or sub-surface pellicular growth. The extent of occurrence of Azospirillum seemed to depend upon the crop combinations. In a mixed farming system where guinea grass was one of the component crops, more root fragments of coconut and pepper demonstrated tetrazolium reduction activity than when guinea grass was absent.Azospirillum lipoferum andA. brasilense constituted 42% and 45% of the isolates, respectively, in the coconut-based cropping systems. Isolates from guinea grass, sugarcane and jackfruit exhibited higher nitrogenase (C₂H₂ reduction) than those isolated from plantation crops, tuber crops and spices. The large variation in the extent of association and nitrogenase activity of isolates from different crops indicated the need for inoculation with efficient cultures in a number of crops in coconut-based cropping systems.Contribution No. 613 of Central Plantation Crops Research Institute, Kasaragod.     

Influence of host plant odours on invasion of the rice leaf bug Trigonotylus caelestialium into paddy fields

• 1 The host‐odour preferences of the rice leaf bug Trigonotylus caelestialium between the rice plant Oryza sativa L. and four species of graminaceous weeds, Poa annua, Alopecurus aequalis, Digitaria ciliaris and Eleusine indica, were investigated with an olfactometer aiming to clarify the influence of these odours on invasion of the bug to paddy fields at the flowering stage of rice.
• 2 Both female and male adults significantly preferred the graminaceous weed A. aequalis in the flowering stage to rice in the fifth‐leaf stage. The bugs also significantly preferred flowering P. annua and A. aequalis to rice in the panicle‐formation stage. However, the bugs showed no preferences between rice in the flowering and grain‐filling stages and the flowering graminaceous weeds P. annua, D. ciliaris and E. indica.
• 3 The preference of the rice leaf bug for the flowering graminaceous weeds before rice flowering coincides with the fact that these bugs mainly live on these weeds before rice flowering. It is considered that the bug's similar preference for flowering rice panicles as the flowering graminaceous weeds causes the intense invasion of the bug into paddy fields at this rice developmental stage.

     

The Significance of Napier Grass Stunt Phytoplasma and Its Transmission to Cereals and Sugarcane

Production of Napier grass, Pennisetum purpureum, the most important forage crop in Africa, is severely constrained by Napier grass stunt (NGS) disease. Wild grasses have been identified as alternative hosts for NGS, but there is no record regarding the ability of this phytoplasma to infect cultivated food crops. To assess this threat, six crop species were placed around phytoplasma‐infected Napier grass with the vector Maiestas banda (Hemiptera: Cicadellidae) in transmission cages for a period of 30 days. The crop plants were observed for 3 months after removal from the cages for disease development. Polymerase chain reaction based on the conserved 16S gene, primed by P1/P6‐NapF/NapR nested primer sets, was used to diagnose phytoplasma in test plants, before and after transmission experiments. Pennisetum purpureum showed the highest infection level (81.3%), followed by Saccharum officinarum (56.3%), Eleusine coracana (50%), Sorghum bicolor (43.8%), Oryza sativa (31.3%) and Zea mays (18.8%). All the test plants remained symptomless except sugarcane that exhibited mild to moderate symptoms comprising yellow leaves and bright white or yellow midribs. Napier grass, however, was stunted with small yellow leaves. The study showed that food crops could be infected, suggesting their possible role as sources of inoculum for NGS and their contribution in the spread of the disease in regions where these crops are cultivated. This implies an additional disease cycle could occur for NGS disease, highlighting the challenges to development and implementation of management strategies for the disease.     

Fitness comparison between clones differing in their ability to produce sexuals in the aphid Thopalosiphum padi

This study aimed at evaluating the intrinsic rate of increase (r _m ) of clones of the cereal aphid Rhopalosiphum padi (L.) that differ in their ability to produce sexuals. The value of r _m was measured for wingless parthenogenetic females in experiments conducted at two temperatures (15 °C and 20 °C). We studied six holocyclic clones, five androcyclic clones and five anholocyclic clones and showed that life-cycle has no significant effect on the age at first reproduction, on fecundity nor on longevity. As a result there is no influence on (summer) fitness: on the one hand, holocyclic clones do not compensate their lower fitness in mild winters by a higher fitness during the summer. On the other hand, anholocyclic and androcyclic clones do not seem to obtain any advantage from living on a more restricted range of hosts: there appears to be no cost of polyphenism for holocyclic clones.     

Occurrence of Phialophora radicicola var. graminicola and Gaeumannomyces graminis var. tritici on roots of wheat in field crops

Assessments of Phialophora radicicola var. graminicola (PRG) and Gaeumannomyces graminis var. tritici (GGT) were made by culturing and by direct microscopic examination of pieces of seminal roots from 16 winter wheat crops grown in different cropping sequences and with different phosphate manuring. PRG occurred on all wheat crops, but was abundant only on wheat after grass, where it seemed to delay the onset of damaging take-all by 1 yr. Delayed occurrence of take-all by phosphate fertiliser was not related to differences in populations of PRG. Wheat grown in ‘take-all decline’ soils had only small amounts of PRG, indicating that the development and the decline of take-all epidemics may be influenced by different biological control mechanisms; breaking sequences of wheat crops by 1 yr grass leys might harness the advantages of both mechanisms.     

The growth and nitrogen uptake of Rhodes grass grown on soils with varying histories of legume cropping

Summary Rhodes grass (Chloris gayana) was grown under glasshouse conditions on soils that had previously grown from 1 to 6 soybean (Glycine max) or Siratro (Macroptillium atropurpureum) crops. Soil mineral N contents at sowing were higher in Siratro-cropped than soybean-cropped soils and increased with cropping history.Yields of Rhodes grass at the first harvest (14 weeks) were related to soil mineral N levels at sowing by the relationship Y=138.7 (1–0.917e^–0.012x). Grass grown on all soybean soils was responsive to N at both harvests (14 and 28 weeks). Grass grown on soil that had grown three or more Siratro crops was non-responsive to N at the first harvest but responses to N were recorded on all Siratro soils at the second harvest.The amount of N removed by the grass crops was small in relation to the total amount present in the soil system. This resulted in no change in soil total N levels over the two crop periods.These results highlight the need to study N dynamics in crop systems rather than continue to measure N pool sizes when evaluating the contribution of biologically fixed N to the nutrition of subsequent non-legume crops.     

Weed seed banks in arable fields under contrasting pesticide regimes

Viable weed seed banks on a clay soil were estimated for five years from 11 arable fields receiving one of three levels of pesticide application. Low seed densities, averaging 2000 m^-2, were recorded, but dicotyledonous weed seed numbers usually increased following oilseed rape crops, reaching 26 000 m^-2 in one field. Field-to-field variability was large. Samples could be grouped using multivariate techniques into those dominated by grass weed seeds and those containing mainly dicotyledonous weed seeds. Within TWINSPAN classifications, most fields maintained similar seed banks during the study period, indicating relative stability of communities. Differences present at the outset of the experiment, including variation between fields in the same treatment area, were maintained for the five years. Patterns were not associated with straw disposal or spring herbicide application. Oilseed rape crops did not cause qualitative change in seed bank communities, but allowed certain species, notably Stellaria media and Sonchus asper, to increase seed numbers. Smaller and less diverse seed banks in the area receiving most herbicide were not statistically significant. Seed banks gave poor predictions of following weed populations.