Call number: LD2668 .T4 1953 C35 / Master of Science

Call number: LD2668 .T4 1953 K5 / Master of Science

Zoospore production of Pythium dissotocum Drechs. in the nutrient solution of hydroponically-grown lettuce, in the greenhouse, was shown to be cyclic. The number of zoospores detected in the nutrient solution was lowest around noontime, (11:00-14:00 hr) and highest around 20:00 hr. Growth chamber studies were conducted to determine the effect of different light periods on zoospore production. Under continuous light or continuous darkness, the population of zoospores in the nutrient solution decreased. But under 12 hours light, and 12 hours darkness or two periods of light each for 3 hours, zoospore populations decreased during the light period but increased during the dark period.

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Thesis (PhD)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: The population density of the white wax scale, Ceroplastes destructor Newstead, has increased since 1994 in certain areas of Western and parts of Eastern Cape Provinces of South Africa where citrus is grown, particularly on Citrus reticulata (Blanco). A study was conducted to investigate its morphology, biology and ecology as contributions to the development of a sound integrated management programme. Characteristics of the immature stages and adult females were described and illustrated from field-collected and slide-mounted specimens. A key to the different stages and morphometeric characteristics useful for separating them are provided. No significant differences in female fecundity were found between orchards (P > 0.05). However, fecundity varied significantly between female size classes from the same orchard (P < 0.001). Female body-size also differed significantly between orchards (P < 0.05) and was significantly positively correlated with fecundity (P < 0.01). C. destructor has one discrete generation per year in South Africa. Oviposition commenced in November and continued through to the end of December with a few females ovipositing until mid January. Population density of the second instar peaked in February while the third instar extended from March to the end of July, followed by a peak population of adults in August. Seven primary and three secondary parasitoids, as well as four predator species attacking C. destructor were identified. Aprostocetus (= Tetrastichus) ceroplastae (Girault) was the dominant species, accounting for 78.87% of the total primary parasitoids reared. Peak numbers of parasitoids and predators were synchronized with peak emergence of susceptible scale stages, indicating that the host-parasitoid/predator system contained a density-dependent regulatory mechanism. Key mortality factors varied slightly between two of the orchards. Key stage mortality determined from a cohort life table was generally in the third instar (LIlI) and preovipositional female (POF) stage. Significant density-dependent mortality factors were demonstrated for the first instar (LI) and PDF stage. Dispersal of C. destructor is by first instar nymphs and the numbers caught on a series of yellow sticky traps varied significantly between crawler densities at the source, trap distances and trap directions from the source (P < 0.001). The numbers caught were positively correlated to the initial crawler density at the source (P < 0.01), suggesting that dispersal was density dependent. Trap distance and the numbers caught were inversely correlated (P < 0.01). Evaluation of effects of different densities of C. destructor on growth, survivorship and reproduction of scales as well as on leaf bearing ability of trees and area of leaf surface covered with sooty mould fungus was carried out on naturally infested Syzygium (= Eugenia) malaccensis (L.) plants. Scale body size and fecundity were inversely related to scale density (P < 0.01), suggesting density-dependent intraspecific competition. Scale survivorship generally declined with increasing density whereas scale parasitism and predation were positively correlated with density (P < 0.05). At high scale densities production of new leaves was significantly reduced (P < 0.01), reducing the resource base for subsequent generations of scale. Scale density and leaf area covered with sooty mould fungus were significantly positively correlated (P < 0.05). The toxicity of four synthetic insecticides against the three immature stages of C. destructor and of eight insecticides against the parasitoid A. ceroplastae was evaluated. Development of the first and second instars of C. destructor was completely arrested by the chemicals. Female fecundity, fertility and body sizes of survivors of treatments applied at the LIII stage were not significantly affected by any of the chemicals (P > 0.05). All the chemicals exhibited high toxicity to A. ceroplastae and hence are not recommended for integrated management of C. destructor in citrus orchards where A. ceroplastae plays an important role. / AFRIKAANSE OPSOMMING: Die populasiedigtheid van die witwasdopluis, Ceroplastes destructor Newstead, het sedert 1994 toegeneem in sekere gebiede van die Weskaap en Ooskaap provinsies van Suid-Afrika waar sitrus verbou word, veralop Citrus reticulata (Blanco). 'n Studie van hierdie insek se morfologie, biologie en ekologie is onderneem as bydrae tot die ontwikkeling van 'n geïntegreerde bestuursprogram. Die karaktertrekke van die onvolwasse stadia en die volwasse wyfies is beskryf en geïllustreer vanaf eksemplare wat in die veld versamel is en op g1asplaatjies gemonteer is. 'n Sleutel vir die verskillende stadia en morfometriese kenmerke wat nuttig is om hulle te onderskei, word voorsien. Geen beduidende verskille in die vrugbaarheid van wyfies van verskillende boorde is gevind nie (P < 0.05). Vrugbaarheid het egter betekenisvol verskil by die verskillende grootteklasse van wyfies uit dieselfde boord (P < 0.001). Die liggaamsgrootte van wyfies uit verskillende boorde het betekenisvol verskil (P < 0.05) en was betekenisvol positief gekorreleer met vrugbaarheid (P < 0.01). C. destructor het een generasie per jaar in Suid-Afrika. Eierlegging het in November begin en aangehou tot aan die einde van Desember, met enkele wyfies wat nog tot in middel Januarie eiers gelê het. Die populasiedigtheid van die tweede instar het 'n hoogtepunt in Februarie bereik, terwyl die derde instar van Maart tot aan die einde van Julie geduur het, gevolg deur 'n piekbevolking van volwassenes in Augustus. Sewe primêre en drie sekondêre parasitoïde asook vier predator spesies wat C destructor aanval, is geïdentifiseer. Aprostocetus (=Tetrastichus) ceroplastae (Girault) was die dominante spesies wat 78.87% van die totale aantal primêre parasitoïde wat uitgeteel is, uitgemaak het. Die pieke in die getalle van parasitoïde en predatore was gesinchroniseer met pieke in die verskyning van die gevoelige stadia, wat dui op die aanwesigheid van 'n digtheidsafhanklike regulatoriese meganisme. Die sleutel mortaliteitsfaktore het effens gevarieer tussen twee van die boorde. Die sleutelstadium van mortaliteit, soos bepaal m.b.v. 'n kohort lewenstabel, was gewoonlik die derde instar (LIlI) en die preoviposisionele wyfie (POW). Betekenisvolle digtheidsafhanklike mortaliteitsfaktore IS aangetoon vir die eerste instar (LI) en die POW. Die verspreiding van C.destructor vind plaas deur die eerste instar nimfe en die getalle wat op 'n reeks van taai geel valle gevang is, het betekenisvol gewissel volgens kruiperdigthede by die bron, asook die afstand en rigting van die valle vanaf die bron (P < 0.001). Die getalle wat gevang is, was positief gekorreleer met die aanvanklike kruiperdigtheid by die bron (P < 0.01), wat daarop dui dat verspreiding digtheidsafhanklik was. Die afstand van die valle en die aantal wat gevang is, was omgekeerd gekorreleer (P < 0.01). 'n Evaluering van die invloed van verskillende digthede van C. destructor op die groei, oorlewing en reproduksie van dopluise, asook die vermoë van bome om blare te dra en die area van die blaaroppervlak wat met roetskimmel besmet is, is uitgevoer op plante van Syzygium (= Eugenia) malaccensis (L.) met 'n natuurlike besmetting. Die liggaamsgrootte en vrugbaarheid van die dopluise was omgekeerd gekorreleer met hulle digtheid (P < 0.01), wat dui op digtheidsafhanklike intraspesifieke kompetisie. Die oorlewing van die dopluise het oor die algemeen afgeneem met toenemende digtheid, terwyl parasitisme en predasie positief gekorreleer was met digtheid (P < 0.05). By hoë dopluisdigthede het die produksie van nuwe blare betekenisvol afgeneem (P < 0.01), wat die hulpbronbasis vir daaropvolgende generasies van dopluise verswak. Die dopluisdigtheid en blaaroppervlak wat met roetskimmel bedek was, was positief gekorreleer (P < 0.05). Die toksisiteit van vier sintetiese insektemiddels teenoor die drie onvolwasse stadia van C. destructor en van agt insektemiddels teenoor die parasitoïd A. ceroplastae is geëvalueer. Die ontwikkeling van die eerste en tweede instars van C. destructor is heeltemal stopgesit deur die middels. Die fekunditeit, fertiliteit en liggaamsgrootte van wyfies wat toedienings op die LIIl stadium oorleef het, is nie betekenisvol ge-affekteer deur enige van die middels nie (P < 0.05). Al die middels was baie toksies teenoor A. ceroplastae en word dus nie aanbeveel vir die geïntegreerde bestuur van C. destructor waar A. ceroplastae 'n belangrike rol speel nie.

Five field experiments were conducted to evaluate the relative attractiveness of potential beneficial insectary plants to aphidophagous hover flies and parasitic Hymenoptera and the effectiveness of interplanting selected flowering plants in a broccoli field to enhance biocontrol of the cabbage aphid and green peach aphid. In 1996 we established a preliminary screening trial to begin development of our sampling methods and evaluations of the attractiveness of selected flowering plants to hover flies and parasitic Hymenoptera. In 1997, we conducted a field experiment at the Oregon State University Vegetable Research Farm near Corvallis, OR to assess the relative attractiveness of 11 selected flowering plants to hover flies and parasitic Hymenoptera. Six of these plants were also evaluated for attractiveness to aphidophagous hover flies in two on-farm trials. The experimental design was a complete randomized block design, with four replications at the OSU site, and three replications at the two on-farm sites. Attractiveness of flowering plants to hover flies was assessed by conducting weekly timed observations of feeding frequencies. Associations of parasitic Hymenoptera were assessed by weekly timed vacuum sampling from a fixed area in plots of flowering plants. Attractiveness differed by dates and sites. Among early-season flowering species, Coriandrum sativa (cilantro) was highly attractive to aphidophagous hover flies and Brassica juncea (mustard), Fagopyrum esculentum (buckwheat) and Agastache rugosa (Korean licorice mint) were most attractive to parasitic Hymenoptera. Among late-season flowers, Achillea millefolium (yarrow), Foeniculum vulgare (fennel) and Agastache rugosa (Korean licorice mint) were most attractive to hover flies, but attractiveness to parasitic Hymenoptera did not differ. An on-farm trial was conducted in 1997 at Stahlbush Island Farm near Corvallis, OR. The objective of this trial was to test the hypothesis that interplanting either alyssum (Lobularia maritima), or cilantro (Coriandrum sativa), with broccoli (Brassica oleracea) would attract aphidophagous hover fly adults and parasitic Hymenoptera, enhance oviposition in the adjacent crop, and increase larval predation and parasitism in the adjacent crop, resulting in suppressed cabbage aphid (Brevicoryne brassicae) and green peach aphid (Myzus persicae) populations in the broccoli crop. The predominate hover fly species present were Toxomerus occidentalis and T. marginatus. More adult female T. occidentalis were caught in pan traps in plots with alyssum than in cilantro or control plots. More hover fly eggs were found on broccoli leaves and a higher percent of the aphids present were parasitized by Hymenoptera in plots with alyssum than in cilantro or control plots. However, no differences in aphid intensities were found between treatment and control plots. A comparison between the mean number of hover fly eggs found per broccoli leaf and the mean number of aphid counted per broccoli leaf suggests there is an association between the two. There appears to be an aphid density threshold below which few hover fly eggs are laid. Gravid females were present in the crop from the first sampling date on, yet hover fly eggs were not found in the crop until the second to last sampling date. Our results indicate that the presence of alyssum enhanced hover fly activity, but did not result in increased larval predation on aphids in the crop. In 1997 a survey of hover flies was conducted at each of the four experimental sites. Hover flies were captured with sweep nets. Representative specimens were identified to species by Christian Kassebeer, University of Kiel, Germany and subsequent identifications were made from reference specimens and with taxonomic keys. Twenty species were identified, 16 of which are aphidophagous. At the OSU site and the two on-farm sites, where the relative attractiveness of flowering plants was assessed, the six most common aphidophagous species, collected at all three sites, were: Meliscaeva cinctella, Toxomerus marginatus, T. occidentalis, Sphaerophoria sulphuripes, S. pyrrhina, and Scaeva pyrastri. / Graduation date: 1998

4 pp.

2 pp. / Nondormant alfalfa varieties are adapted to mild winter areas in Arizona. An alfalfa variety should be selected based on dormancy class, potential pest problems, university yield trials, and on-farm tests. This publication contains pest resistance ratings and a summary of University of Arizona yield trials for nondormant alfalfa varieties.