Host Plant Conditions Research Articles

Distribution of Glassy-Winged Sharpshooter and Threecornered Alfalfa Hopper on Plant Hosts in the San Joaquin Valley, California

Homalodisca vitripennis (Germar) and Spissistilus festinus (Say) populations were surveyed bimonthly for 14 mo in Kern County, CA, at five agricultural sites made up of a variety of potential host plants. In addition, S. festinus populations were surveyed in four alfalfa, Medicago sativa L., fields in Kern and Tulare counties. Insects were collected by beats-sweeps and sticky traps. Data on host plant condition and phenology, and ground cover presence and composition were collected at the five agricultural sites, whereas data on mowing and insecticide use were collected at the four alfalfa sites. Populations of both insects persisted at the five agricultural locations despite insecticide applications applied as part of a H. vitripennis areawide management program and standard commercial operations. Plants colonized by H. vitripennis included eucalyptus (Eucalyptus L'Hér.), jojoba [Simmondsia chinensis (Link) C. K. Schneid.], and citrus (Citrus spp.). Populations of S. festinus were much greater in collections from alfalfa fields than from the five agricultural sites. Insects collected from the five mixed agricultural sites were negative for presence of X. fastidiosa. In laboratory tests, S. festinus did not acquire or transmit X. fastidiosa in tests with infected grape (Vitis spp.) as an acquisition source and grape, almond [Prunus dulcis (Mill.) D.A.Webb], and alfalfa as inoculation hosts. Recommendations for vector control, vegetation management, and targeted monitoring to reduce insect populations and inoculum potential are discussed.

Integration among the Different Aspects of the Phenotypic Plasticity in Aphis fabae, Black Bean Aphid

Recent studies showed the crucial role of different aspects of phenotypic plasticity when organisms exposed to various environments. Presented study tested integrations among the phenotypic plasticities in fitness, morphological features and host choice behavior of the Aphis fabae genotypes exposed to two different host plants. Results indicated that different aspects of phenotypic plasticities in black bean aphid were not systemically affected by host plant conditions as these features have independent regulation. Host plant conditions affected types of phenotypic plasticities in a different way. However, particularly plasticity in ultimate rostral segment positively integrated with plasticity in fitness on novel host plant.

Soilborne microorganisms of Euphorbia are potential biological control agents of the invasive weed leafy spurge

Leafy spurge ( Euphorbia esula- virgata), a native of Eurasia, is a serious invasive weed of grasslands of the northern Great Plains of the U.S. and prairie provinces of Canada. Leafy spurge is very difficult to control with herbicides, insect biological control agents, and other cultural practices. Previous field investigations revealed pathogen–insect interactions on the roots of leafy spurge leading to mortality. In order to exploit this synergistic relationship as an effective biological control strategy, we undertook an exploration of Europe for soilborne fungi and rhizosphere bacteria on Euphorbia spp. growing in a wide variety of soils in different landscapes. All microbial cultures were screened for growth suppressive or disease potential on leafy spurge plants or callus tissue. Study objectives were to determine relationships of some edaphic factors and host plant conditions with biological control activity, and to screen rhizobacteria isolated from Euphorbia spp. for traits that might contribute to suppression of leafy spurge growth. The most virulent soilborne fungal strains of Fusarium and Rhizoctonia species, based on greenhouse pathogenicity tests, were isolated from roots of Euphorbia spp. with insect feeding damage. High proportions (>50%) of rhizobacteria were classified as deleterious rhizobacteria (DRB) using a callus tissue bioassay. Euphorbia spp. at sites with high DRB numbers displayed severe fungal disease symptoms and supported insect infestations. Selected soil properties were not correlated with potential biocontrol activity of microbes on leafy spurge; however, insect presence and disease ratings were associated with incidence of growth-suppressive microbes. Certain physiological traits (i.e., exopolysaccharides and hydrogen cyanide production) were good indicators of deleterious activity of rhizobacteria. Our study illustrates that the most effective condition for inducing disease and subsequent mortality of leafy spurge includes a synergism between plant-associated microorganisms and root-damaging insects. Furthermore, the results are valuable for identifying sites for collecting soilborne microorganisms on weeds in their native range for evaluation as biocontrol agents in their invasive range.

Preliminary studies on mycorrhizal symbiosis in plant conservation, forestry and farming systems in Kenya

The paper presents studies on mycorrhizal symbiosis in plant conservation, agriculture and forestry in Kenya. Mycorrhizal studies in conservation of rare, threatened and endemic plant species of Kenya were undertaken with aim of establishing fungal species associated with the plant species. Mycorrhizal studies in farming system evaluated the impact of different farming practices on mycorrhizal symbiosis, while in forestry the mycorrhizal studies evaluated fungal species in exotic and indigenous forests. The four studies show affinities of plant species to specific mycorrhizal types. Association with plant species seems to be a heritable trait. Mycorrhizal species composition was affected by the habitat, moisture content, soil nutrients and root morphology. In forestry, indigenous plant communities maintained high species diversity than the exotic species and the species were specific to forest type. Farming systems affected the abundance of spores of Mycorrhizae with spore abundance higher in improved tree fallow than natural fallow and continuous cropping systems. Studies on orchids showed each orchid species to associate with specific Rhizoctonia isolate and taxonomic trends were observed in the association. The four studies show changes in host plant and soil conditions to greatly determine mycorrhizal symbiosis, species composition and abundance. Journal of Tropical Microbiology Vol.3 2004: 48-62

Some ecological factors influencing the breeding success of the Brenton Blue butterfly, <i>Orachrysops niobe</i> (Trimen) (Lepidoptera: Lycaenidae)

The Brenton Blue butterfly, Orachrysops niobe (Trimen, 1862) (Lepidoptera:Lycaenidae), is endemic to the southern Cape and is currently listed as Endangered. This study looks at some of the key ecological factors influencing the breeding success of the species—host plant abundance and condition, nectar sources, climate/ microclimate, and vegetation management techniques. The adult butterfly population was monitored over an entire breeding season; host plants were identified and individually monitored; and egg counts were done. This enabled the effects of a number of different management techniques to be evaluated (burning, cutting, physical removal of invasive elements, and combinations thereof). A fivefold increase in the population of O. niobe was observed over the breeding season. This increase was positively correlated to a similar increase in host plant abundance in the areas where cutting and physical removal of invasive elements was practiced. Burning, by contrast, appeared to have a negative impact on host plant and butterfly abundance over the same period. Impacts of other factors such as climate, nectar sources and the natural strength of the second brood are discussed. A hypothesis, of megaherbivore activity as the principal historical disturbance mechanism promoting locally favourable conditions for O. niobe to establish and maintain colonies, is proposed. Recommendations for reserve management and future research are made.

Association of declining red pine stands with reduced populations of bark beetle predators, seasonal increases in root colonizing insects, and incidence of root pathogens

Declining red pine, Pinus resinosa (Aitman), stands in the Great Lakes region appear to arise from a complex of biotic and abiotic factors. We monitored stem and root colonizing beetles in declining and healthy plantations in Wisconsin from 1997 to 1999. We also conducted systematic field excavations and laboratory isolations of various root pathogens. Multiple funnel traps were baited with the synthetic aggregation pheromones of the two most common bark beetle (Coleoptera: Scolytidae) species in the region, Ips pini (Say) and Ips grandicollis (Eichhoff). Lower-stem flight traps were baited with 1:1 (+)-α-pinene:75% ethanol, and pitfall traps were baited with 1:1 (−)-α-pinene:75% ethanol. Healthy stands had higher populations of predators, particularly Thanasimus dubius (F.) (Coleoptera: Cleridae), Platysoma cylindrica (Paykull) (Coleoptera: Histeridae), and Platysoma parallelum Say than declining stands. Seasonal abundance patterns of predators and Ips also varied between declining and healthy stands. Declining stands had higher numbers of Ips and lower numbers of predators early in the season, whereas healthy stands had higher predator populations in the early season. Declining stands had more lower stem infesting bark beetles Dendroctonus valens LeConte (Scolytidae) and weevils (Curculionidae) Pissodes spp. Surprisingly, healthy stands had more root weevils, Hylobius pales (Herbst) and Hylobius radicis Buchanan, and root bark beetle Hylastes porculus Erichson, over the entire season. However, spatial by temporal patterns again were important. Populations of root colonizing insects were higher in healthy stands early in the season, but higher in declining stands throughout much of the ovipositional period. These results suggest dispersal patterns and overwintering behaviors are important in the interactions among herbivores, predators, and host plant condition in Red Pine Decline. The principal fungi isolated from roots were Leptographium procerum (Kendr.) Wingfield and Leptographium terebrantis Barras & Perry. These were more prevalent in declining than healthy stands. Staining fungi were more frequently isolated from roots of trees at the pocket margin than from trees in the asymptomatic portion of declining stands. Heterobasidion annosum (Fr.) Bref. was not isolated from root samples or stem disks, despite 40% recovery in positive controls using the latter method. Likewise, Armillaria spp. were not associated with declining plantations. Our results further support the view that forest declines are due to complex interactions among multiple biotic and abiotic stresses, and exhibit particular spatial and temporal patterns.

Tending Ants (Hymenoptera: Formicidae) RegulateDalbulus quinquenotatus(Homoptera: Cicadellidae) Population Dynamics

Population dynamics of colonies of the mymecophilous leafhopper Dalbulus quin- quenotatus DeLong & Nault were recorded in the laboratory in the presence and absence of the tending ant, Formica fusca (L.). These experiments used colonies of leafhoppers started with five male and five female adults on a single Tripsacum dactyloides L. plant. The first experiment consisted of eight leafhopper colonies, four with tending ants and four without. The second experiment used 16 leafhopper colonies, eight with tending ants and eight without, with four of each of these ant treatments also with Nabis americoferus Carayon predators. The numbers of young nymphs, older nymphs, and adult leafhoppers, along with host plant condition were recorded every 2—3 d for 62 d for both experiments. Nontended leafhopper colonies exploded and overwhelmed their host plants, killing them in an average of 39.0 d, resulting in the extirpation of those leafhopper colonies. Ants maintained leafhopper colonies by regulating population dynamics, with colonies of leafhoppers tended by ants being significantly smaller (P , 0.02) and developing from egg to adult significantly slower (P , 0.001) than colonies of nontended leafhoppers. Ant-tended leafhopper colonies did not overwhelm their host plants during either experiment, as the ants removed excess honeydew and leafhopper exuvia, thus maintaining a constant supply of honeydew. Ants were observed protecting leafhoppers from nabid predators, although this protection did not significantly influence leafhopper population size or length of egg to adult development.

INSECTS AND TEMPERATURE—DIFFERENTIAL EFFECTS OF EXPERIMENTAL CONDITIONS ON GROWTH AND DEVELOPMENT

AbstractCohorts of Pieris rapae (L.) (Lepidoptera: Pieridae) were reared at different temperatures on three substrates: plants in growth chambers, plants in field cages, and artificial medium. Pupal weight and larval development times were measured. The data were analyzed to determine the effect of temporal changes in substrate on pupal weight. When cohorts were reared on whole plants, average pupal weight declined with increasing time to pupation. For cohorts on potted plants in growth chambers, the rate of decline decreased as temperature increased from 15 to 27 °C. But, for cohorts on plants in field cages, the rate of decline increased as temperature increased from 16 to 25 °C. We speculate that the former trend resulted from plant deterioration, especially at low temperatures, and the latter from larval competition at high temperatures. When larvae were reared on potted plants to the late II or early III instar and then transferred to artificial medium, deterioration of the medium at temperatures below 20 and above 27 °C caused pupal weight to decline significantly with increasing time to pupation, whereas between 20 and 27 °C, pupal weight was constant or increased with increasing time to pupation. Development times of larvae from cohorts reared on plants in growth chambers showed no consistent differences from those of larvae from cohorts reared on artificial medium, at any temperature (overall averages: 187 and 184 day-degrees above 10 °C, on plants versus medium). Pupal weights are determined by the ratio of growth rate to development rate (Appendix). Therefore, the observed differences in pupal weight reflect differences in growth rates rather than differences in development rates. The results show that, depending on temperature, substrate deterioration may seriously affect laboratory studies of herbivore growth and that there are important interactions between temperature and host-plant growth conditions that may make extrapolation of laboratory estimates of herbivore growth to field conditions tenuous.

Reproductive schedule and factors affecting soldier production in the eusocial bamboo aphid Pseudoregma bambucicola (Homoptera, Aphididae)

The reproductive characteristics of the soldier-producing aphid Pseudoregma bambucicola were studied in Kagoshima, Southern Japan, to know the factors affecting soldier production of eusocial aphids. The soldier proportion in aphid colonies was highest from October to November. In some large colonies, soldiers were observed in all seasons except in July when colony size was relatively small. Multiple regression analysis showed that the colony size was a principal factor affecting soldier proportion throughout a year. Other social or environmental factors such as aphid composition, host plant conditions and predator abundance were not always significant. Rearing experiments revealed that large colonies (≥1,000 individuals) produced soldiers in almost all seasons while small colonies ( 10 and >20, respectively. Because they have a small number of ovarioles (<15 on average), females should alter caste production within the same ovarioles according to changes in environmental conditions. Artificial removal or introduction of predators and reduction of colony size did not affect soldier production over two successive generations, revealing maternal effects on soldier production. Females cannot shift caste production quickly in response to changes in predator abundance and colony size. This is probably due to early developmental determination of castes within the mother's body.

Rainfall, nitrogen and host plant condition: consequences for the processionary caterpillar, Ochrogaster lunifer

1. At the end of November in subtropical areas of Australia, second‐instar larvae of the processionary caterpillar Ochrogaster lunifer (Lepidoptera: Thaumetopoeidae) initiate feeding on the peripheral shoots of acacias (first‐instar larvae do not feed). Field surveys at ten localities in south‐east Queensland showed that larval survival was highly variable both among and within localities. Within‐locality variation in larval growth was low compared with variation among localities. Larval growth and survival rates were higher at coastal and island localities, where November rainfall was high, than at drier inland localities.2. Potted Acacia concurrens were grown in the greenhouse under high and low watering regimes, with and without nitrogen‐rich fertilizer. Plant vigour (height, foliar water content and quantity of flush growth) was significantly greater in high‐water treatments than in low‐water treatments. Watering also affected foliar nitrogen, with plants in the high‐water/no fertilizer treatment having similar nitrogen levels to those in fertilized treatments. Fertilizer increased foliar nitrogen levels of plants in low‐water treatments and increased the number of shoots in high‐water treatments. Different treatments had no effect on leaf toughness. After the first 3 weeks of feeding, size and survival of larvae were significantly reduced on the small, less vigorous plants in low water treatments. These results do not support the plant stress hypothesis.3. Early‐instar larvae (instar II–IV) developed more quickly and grew larger when reared on flush leaves than when reared on senescent leaves of A. concurrens. As water uptake affects the quantity of flush growth available to early stage larvae as well as foliar quality, rainfall and water availability may have important consequences for the distribution and population dynamics of the moth at local and regional scales.

Microcosm experiments on the development of different plant parasitic nematode fauna in two soils from the Soudanese-Sahelian zone of West Africa

To test the hypothesis that the structure of plant parasitic nematode communities is affected by soil characteristics, experiments were conducted in a greenhouse with two soils with different physical and chemical characteristics and land management histories (fallow and a cultivated field) from adjacent plots. The cultivated soil was more sandy and had lower organic matter and nutrient contents than the fallow soil. Four nematode assemblages of Scutellonema cavenessi, Helicotylenchus dihystera and Tylenchorhynchus gladiolatus were inoculated in the soils. The pot experiment was conducted on millet during 2 months. Multiplication rates of H. dihystera were not significantly different in the two soils. T. gladiolatus had a lower multiplication rate in the fine-textured soil. S. cavenessi seemed to reproduce better in the coarse-textured soil when inoculated in low density with H. dihystera. The presence of plant parasitic nematodes in the cultivated soil caused a significant decrease of millet biomass, whereas plants in the fallow soil were less sensitive to nematode damage and were only affected when the soil was inoculated with T. gladiolatus alone. This experiment did not explain the distribution of plant parasitic species observed in the field. However, parameters other than the presence of a favourable host plant and micro-climatic conditions were found to induce differences in the reproductive rates of several species of plant parasitic nematodes.

Response of Bemisia argentifolii (Homoptera: Aleyrodidae) to Water and Nutrient Stressed Cotton

Water stress usually increases plant susceptibility to the whitefly Bemisia argentifolii Bellows & Perring, whereas nutrient stress generally reduces the level of whitefly infestation. This study examined the effect of water and phosphorus stress on plant physiological traits to help explain the response of B. argentifolii to host plant conditions. Host preference was determined by examining oviposition rates on water- and phosphorus-stressed cotton, Gossypium hirsutum L., in the greenhouse, and by studying the effect of irrigation frequency on oviposition and B. argentifolii developmental rate in the field. In the greenhouse, oviposition was reduced 9% ( P = 0.09) by water stress but was not affected by phosphorus deficiency. In the field, oviposition was 6 times greater in the biweekly compared with weekly irrigated plots. In no choice experiments, however, oviposition was unaffected by irrigation frequency, with the exception of lower leaves in the canopy which had been previously damaged by B. argentifolii , where oviposition was reduced 63% in the biweekly plots where damage was the greatest. Irrigation frequency had no effect on developmental rate from egg to adult. In the greenhouse, oviposition was affected most by leaf position on the plant, and was >2 times as great on tile youngest compared with the oldest leaf on a 4-leaf seedling. Multiple regression analysis suggested that oviposition was not independently affected by transpiration rate or leaf temperature. Leaf water potential, sucrose concentration, and amino acid concentration made the greatest contribution to B. argentifolii host selection, with oviposition being greatest on leaves with low water potential and low bulk leaf sucrose and amino acid concentrations.

Natural Control of Helicoverpa armigera in Cotton: Assessment of the Role of Predation

Irreplaceable mortality of Helicoverpa armigera due to natural enemies was studied in cotton in western Kenya. Field populations of H. armigera were followed in three types of subplots: where crawling predators were suppressed, where both crawling and flying predators were suppressed and where predators were unaffected. Ants were the predominant crawling predators, whereas anthocorids were the predominant flying predators. H. armigera mortality from egg to late larval stage was very high (96.4-99.7%) and was greater in the second than in the first generation. Suppression of the different groups of predators did not increase the density of the pest. It is argued that the high level of background mortality obscured the role of predators. The possible role of host-plant condition on background mortality is discussed.

Exclusion cage studies on the impact of predation onHelicoverpa armigerain cotton

The impact of predation on Helicoverpa armigera was studied in four field cage exclusion trials on cotton in Kenya. H. armigera egg cohorts were introduced inside predator‐free and open control cages, and the impact of local predator populations on the cohort was examined. Fourteen days after inoculation, exclusion cages had four times more larvae than controls, indicating a strong impact of predation. Ants and Anthocoridae were the predominant predator groups. Exclusion cages had more damaged fruiting plant parts (squares, flowers and bolls) than the control. In the absence of predators, natural mortality of H. armigera was greater as cotton matured, and is likely to be linked to the host plant condition.

Comparative Studies on Field Performance of Heliothis Larval Parasitoids Microplitis croceipes and Cardiochiles nigriceps at Varying Densities and under Selected Host Plant Conditions

Field performance of the Heliothis (Lepidoptera: Noctuidae) larval parasitoids Microplitis croceipes (Cresson) and Cardiochiles nigriceps (Viereck) (Hymenoptera: Braconidae) was assessed at various parasitoid densities and on different plants. Laboratory-reared M. croceipes were released in field plots of soybeans and peanuts at levels of ca. 400 and 1,350/ha. Observed and calculated data indicate that over a 4-day period parasitism levels of 45 and 95%, respectively, would be obtained with M. croceipes at these levels on soybeans. Performance of M. croceipes on peanuts was less consistent, but in one study, 71% parasitism was obtained during a 2-day interval after releases of 1,344 females/ha. The searching behavior and efficiency of C. nigriceps females were studied on the hyacinth bean (Dolichos lablab L.) and Florida beggarweed (Desmodium purpureum (Mill.)). Observed and calculated data indicate that populations of 2,622 and 5,371 females/ha on the hyacinth bean and beggarweed, respectively, would yield 80% parasitism, as compared to earlier studies showing that 988-1,142 females/ha yield this level of parasitism on cotton. These findings show that the efficiency of the parasitoids varies considerably on different host plants. The distribution of searching parasitoids within the experimental plots and the distribution of their eggs among the host larvae did not differ significantly from a random (Poisson) pattern.

Influence of temperature and host plant condition on preimaginal development and survival in the sorghum shootfly Atherigona soccata

Temperature-time and temperature-velocity equations are given for egg, larval and pupal stages of Atherigona soccata Rond., based on stduies in Kanya. Preimaginal development (from egg deposition to adult emergence) on sorghum CSH-1 seedlings lasted 19.5, 26.3, 33.8 and 76.8 days at 30, 25, 20 and 15°C, respectively. A. soccata showed optimal development and survival at 30°C. The lower threshold temperatures were 13.8, 10.2 and 11.8°C for the egg, larva and pupa, respectively. The condition of the food-plant affected larval development and survival; pupal weight was higher and larval mortality lower in insects reared on fertilised and normally watered seedlings than on unfertilised and water-stressed seedlings

POPULATION DYNAMICS OF THE WHEAT STEM SAWFLY, CEPHUS CINCTUS (HYMENOPTERA: CEPHIDAE), IN WHEAT

AbstractThe effects of the major factors on populations of the wheat stem sawfly, Cephus cinctus Norton, were studied in the field at Lethbridge, Alberta between 1970 and 1977. Fecundity of the females, which contain an average of 32.7 eggs, is influenced by the condition and variety of host plants in which they develop as larvae. Populations of the adults that emerge from infested crops cut 7–9% by the larvae in one year may infest 70–80% of the stems of adjacent crops in the following year. Increased adult densities, however, fail to increase markedly the percentage of stems infested because the adults concentrate at field margins when they reinfest the most preferred stems. Because only one larva can mature in each stem, competition within stems that receive more than one egg causes a major drop in the population but not in the percentage of infested stems cut. Another major drop is caused by host plant resistance, which is influenced in both susceptible and resistant wheats by the weather during the growing season. On an average, all of the larvae die in 28% of the infested stems of two susceptible wheats and in 67% of those of a resistant wheat. The mortality in resistant wheats, however, is generally not sufficient to prevent economic infestations in subsequent years. Parasitism, which was a major factor in only 7 of 36 years in southern Alberta, is increased by weather conditions that cause delayed ripening of the wheat crops. The mortality of mature larvae and pupae averaged 21.3%. It is affected by extremes of moisture and temperature and can be increased slightly by shallow tillage of infested stubble.

On the life-history and fecundity of the cochineal insect, Dactylopius austrinus De Lotto (Homoptera: Dactylopiidae), a biological control agent for the cactus Opuntia awantiaca

AbstractThe life-history of Dactylopius austrinus De Lotto on the weed Opuntia aurantiaca, is described. The male moults four times and the female twice. The instars are illustrated. Fecundity is shown to be a function of female density on the plant and of host-plant condition. The sexes were produced in a ratio of about two males to one female.

Erwinia chrysanthemi . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Erwinia chrysanthemi . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: On a number of widely different plants, including Aechmea fasciata, Aglaonema pictum, Ananas comosus, Begonia bertini, Chrysanthemum spp., Colocasia esculenta, Dahlia spp., Dianthus caryophyllus, Dieffenbachia spp., Euphorbia pulcherrima, Musa sp., Parthenium argentatum, Philodendron spp., Saccharum officinarum, Saintpaulia ionantha, Syngonium podophyllum and Zea mays . DISEASE: Symptoms are stunting, yellowing, wilting, necrosis of parenchyma or soft rotting according to host plant, pathotype and environmental conditions. Stunting and wilting usually appear on Begonia, Chrysanthemum , carnation, Dahlia and poinsettia. Rots appear on leaves of Aechmea, Philodendron and Saintpaulia , while various other parts show rotting with other hosts. On sugarcane a leaf mottle is produced. GEOGRAPHICAL DISTRIBUTION: The species is probably world wide, but the different distributions of the strains or pathotypes (see below) must be distinguished as follows: Dianthus : Europe (CMI Map 425, ed. 1, 1967) to which should be added Italy (46, 3461), USA (47, 2498) and New Zealand (52, 3341); Zea : Egypt, Sudan, Rhodesia, S. Africa, India, USA, France, Italy, possibly Greece; Chrysanthemum : UK, Germany, Italy, Canada, USA, Japan; Dahlia : Netherlands, Romania; Dieffenbachia : Germany, Italy, USA; Saintpaulia : France, Germany, USA; Begonia : Netherlands; Aechmea : Italy; Ananas : Malaysia; Colocasia : Solomon Islands; Musa : Central America and probably most other banana-growing areas; Philodendron, Parthenium, Euphorbia and Aglaonema : all confined to USA; Saccharum : Australia (Qd., NSW). TRANSMISSION: Usually in or on vegetative propagating material. Cuttings and setts from infected plants are frequently infected. Cutting knives may carry inoculum to healthy material. Transmission in soil and planting medium is also possible. Serious outbreaks have occurred after holding a large number of cuttings together in water prior to planting. The sugarcane pathogen is rapidly spread by flood water (48, 2533). Maize stalk rot is carried by water splash. It is often associated with overhead irrigation (48, 1675; 55, 204).

Effects of groundnut crop density on the population dynamics of Aphis craccivora Koch (Hemiptera, Aphididae) in Malawi

AbstractThree plant spacings, 30 cm, 7·5 cm and 2·5 cm within rows 90 cm apart, equivalent to about 36, 143 and 430 thousand plants/ha respectively, were used in three field trials on the population dynamics of Aphis craccivora Koch on groundnuts at Chitedze, Malawi in 1964–1967. Aphid density per infested plant was consistently higher, while numbers of aphid-infested plants per unit area were consistently lower, in 30 cm than in 2·5 cm spacing. Aphid numbers per unit area increased to similar levels at all spacings in crops under 40–50 days old, but subsequently aphid density was consistently lower in 2·5 cm than in 30 cm spacing. This effect could not be related to the intensity of fungus disease infection, parasitism, predation, the availability of feeding sites or the rate of emigration. Aphid groups were, however, consistently smaller in 2·5 cm than in 30 cm spacing in one trial. The potential rate of increase was studied in two trials using the methods of Hughes (1962), and was found to be consistently lower in 2·5 cm than in 30 cm spacing. The rate of plant growth was relatively depressed in 2·5 cm spacing, particularly after 40 days. The effect of spacing on aphid numbers was attributed to the effect of host-plant condition on the potential rate of increase.