M. C. Van Der Westhuizen

Development rate, fecundity and lifespan of apterae of the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae), under controlled conditions

The rate of development, fecundity and lifespan of apterae of Diuraphis noxia (Mordvilko) were determined at five sets of temperature and daylength conditions. The time required to complete pre-imaginal growth at mean daily temperatures of 10, 13, 14, 17·25 and 20°C was 19·70, 12·09, 11·25, 9·88 and 8·17 days, respectively. The threshold for development was estimated to be 0·54°C and the number of day-degrees C required to complete development was 158·73.

Chilo partellus (Lepidoptera: Pyralidae) moth and larval response to levels of antixenosis and antibiosis in sorghum inbred lines under laboratory conditions

Two experiments were conducted to determine Chilo partellus (Swinhoe) moth and larval response on sorghum lines differing in levels of antibiosis and antixenosis. The inbred lines were E 302, IS 2205, IS 2122 and SA 2681, all reportedly varying in levels of antibiosis and ovipositional and larval antixenosis to C. partellus . Ovipositional antixenosis was evaluated in choice tests under cage conditions and significant differences were observed in the average number of egg batches per line. E 302 received the greatest number of egg batches. Antibiosis and larval antixenosis was evaluated with artificial infestation in a greenhouse. Significant differences occurred in larval numbers and mean larval mass on different lines, with E 302 possessing the greatest level of antibiosis resistance. This study indicates that resistant genotypes such as E 302 may exhibit high levels of antibiosis or larval antixenosis, but may also be preferred for oviposition. The size of egg batches was not influenced by ovipositional antixenosis. The potential effect of ovipositional antixenosis mechanisms on pest populations under field conditions is discussed.

A six week screening method for mechanisms of host plant resistance to Diuraphis noxia in wheat accessions

The Russian wheat aphid Diuraphis noxia (Kurdjumov) is a serious pest of wheat in South Africa. The use of D. noxia-resistant cultivars may reduce the impact of this pest on wheat production and at the same time reduce environmental risks and control costs. The mechanisms of resistance in two new sources of resistance were compared by using various methods, in order to establish a rapid and relatively accurate screening protocol. The resistant lines OSU ID 2808 and Aus 22498 were compared to the susceptible cultivars Betta and Tugela. The predominant mechanism of resistance in OSU ID 2808 was antibiosis, although a low level of antixenosis may also be present. The line Aus 22498 was primarily a tolerant type, with a moderate level of antibiosis and a low level of antixenosis. A six week screening procedure is described using the colony count technique for antibiosis, a completely random free choice experiment for antixenosis and a three week test measuring initial and final plant height, initial and final D. noxia infestation, damage rating, leaf area and dry plant mass for tolerance.

Physiological changes during colony establishment in the termite Hodotermes mossambicus (Hagen): Water balance and energy content

Abstract The changes in dry mass, wet mass and energy content were determined during the establishment of the incipient colony. The winged reproductives of H. mossambicus are weak fliers. It is postulated that the alates compensate for the large quantity of reserve material by reducing their body water. The wet mass of both males and females increased significantly (twice original mass) from emergence until the first larvae appeared which tends support to this view. Furthermore, although the termites maintained in groups imbibed water, it is also clear that the reproductives, as long as they remain social, maintain the same low body-water content. This desiccated physiological condition appears to be and adaptation which assists flight and dispersal. In contrast with sterile eggs, fertile eggs absorbed water during development. The paired females (normal and homosexual) maintained the same energy/unit wet mass from egg production until the first workers appeared. Thus the possibility exists that the hydrophobic reserves (e.g. triglycerides) were converted to the hydrophilic reserves (e.g. carbohydrates). The same applied to the eggs and offspring. The changes in dry mass, wet mass, body water and energy content of females per unit of eggs (specific mass) produced were 0.005, 8.696, 21,276 and 12.820 respectively. The small specific dry mass value can probably be ascribed to the use of small amounts of glycerides and the increase in dry mass by the conversion from relatively light and “compact” glycerides to relatively heavy and “bulky” glycogen. Females utilised far more dry mass and energy in comparison to males to survive until the first workers appeared. Wet mass, body water and energy used in rearing of the offspring shows clearly that males made a larger contribution to the water and energy requirements of the offspring.

Aspects of foraging in the harvester termite, Trinervitermes trinervoides (Sjöstedt) (Termitidae: Nasutitermitinae)

Foraging behaviour and the diurnal and seasonal foraging periodicity of the southern African snouted harvester termite, Trinervitermes trinervoides (Sjöstedt), are described. The termite usually foraged during summer nights when ambient temperatures were 10–14 °C. Foraging occurred at temperatures between 13–25 °C under laboratory conditions. A foraging party emerging from a single hole harvested over an area of approximately 0.78 m2. A colony exploited a total area of approximately 214 m2 and foraging areas of adjacent colonies overlapped. Colonies occupy single mounds.

Mechanisms of resistance of lentil Lens culinaris Medikus, genotypes to the pea aphid Acyrthosiphon pisum Harris (Hemiptera: Aphididae)

Glasshouse studies were conducted to determine mechanisms of resistance of five genotypes of lentil Lens culinaris (Medikus) previously identified as resistant and moderately resistant to the pea aphid Acyrthosiphon pisum (Harris). A susceptible standard, ILL 8127, was also included. French Indigo had high levels of antibiosis, demonstrated by reduced reproductive period, fecundity and longevity, while Spanish Brown had high levels of antixenotic and tolerance components of resistance. Based on overall resistance, French Indigo was the most resistant genotype followed by Spanish Brown. Lhese genotypes could be a source for resistance to A. pisum. All five genotypes had varying levels of antibiosis, antixenosis and tolerance components of resistance compared with ILL 8127.RésuméDes expérimentations en serre ont été conduites afin de déterminer les mécanismes de résistance de cinq génotypes de lentille Lens culinaris (Medikus) précédemment identifiées comme résistants et modérément résistants au puceron du pois Acyrthosiphon pisum (Harris). Un génotype standard sensible, ILL 8127, a également été testé. Le génotype French Indigo présente un niveau élevé d’antibiose en relation avec une réduction de la période reproductive, de la fécondité et de la longévité, alors que Spanish Brown présente des niveaux élevés d’antixénose et de tolérance. Si l’on considère la résistance totale, French Indigo est le génotype le plus résistant suivi par Spanish Brown. Ces génotypes pourraient constituer des sources de résistance contre A. pisum. Les cinq génotypes présentent des niveaux variables de résistance antibiotiques, antixénotiques et de tolérance comparé à ILL8127.

A simple key for the diagnosis of the instars of the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae).

The morphology of the antennae, caudae and wingbuds of Diuraphis noxia (Mordvilko) was used in conjunction with ratios between antennal segment lengths to draw up a key for diagnosis of the instars.

Tolerance and recovery resistance of grain sorghum genotypes artificially infested with Busseola fusca (Fuller) (Lepidoptera: Noctuidae)

Sixty artificially infested grain sorghum genotypes were evaluated for resistance against Busseola fusca. Evaluation of leaf feeding damage indicated levels of resistance ranging from intermediate to susceptible. Genotypes were categorized according to the presence or absence of both antibiotic and tolerant characteristics. Genotypes (17%) exhibited both antibiosis against leaf feeding and tolerance for borer damage. Borer damage resulted in a 4% increase in grain yield of one genotype and a 50% increase in the number of heads of a second genotype. Tolerance and recovery resistance were the most important factors limiting yield loss in grain sorghum genotypes. In this study, the ability of sorghum for tillering (recovery resistance) was separated from the ability to produce heads on infested stems (tolerance). The importance of distinguishing between tolerance and recovery resistance in the evaluation of sorghum germplasm for borer resistance is emphasized.RésuméSoixtante genotypes de graine do sorgho, infestés articiellement ont éte évalués pour leur résistance aux Busseola fusca. L’evaluation de dégâts dans l’alimentation des feuilles a révelé des niveaux de résistance allant d’intermédiaire à susceptible. Des génotypes ont été classés d’aprés la présence ou l’absence de traits caractéristiques d’antibiotiques et de tolérance. Dix-sept pour cent des génotypes ont montré de l’antibiose centre l’alimentation des feuilles ainsi que la tolérance aux dégâts causées par des perçeurs. Ces dégâts s’élèvent à une augmentation de 4% dans la production de graines d’un genotype, et de 50% dans la guantité d’epis d’un deuxième genotype.La tolérance et la résistance de rétablissement ont été les facteurs les plus importants qui limitentent la perte de production de génotypes de graine de sorgho. Dans cette recherche, la capacité de sorgho pour le tillage (résistance de rétablissement) a éte séparée de la capacité de production d’epis sur des tiges infestées (toléerance). L’importance de distinguer entre la tolérance et la résistance de rétablissement dans le cas de l’évaluation de plasme germinal de sorgho pour la résistance aux perceurs a été accentuée.

Measurement of exchangeable aluminium in soils and clay minerals by isotopic exchange

Isotopically exchangeable Al was determined in soils and clay minerals by equilibration with 28Al. Best results were obtained with a weak extractant and an equilibration time of eight min. The calculated amount of isotopically exchangeable Al was independent of the amount of carrier-Al added with the 28Al. In some soils isotopically exchangeable Al did not appear to be related to the amount of Al which could be extracted by various electrolyte solutions. This technique provides an improved means of studying the exchange reactions of Al in acid soils and clay minerals. S. Afr. J. Plant Soil 1984, 1: 57–60

Utilisation of prey by antlion larvae (Neuroptera: Myrmeleontidae) in terms of energy and nutrients

Abstract Prey utilisation at low prey densities was determined for third instar Cueta sp., Furgella intermedia (Markl) and Palpares annulatus (Stitz) larvae in terms of wet weight, dry weight, energy and nutrients. Prey utilisation was similar to other insects on a wet weight (42–47%), dry weight (46–49%), energetic (40–58%) and nutritive basis (62–79%). Lipids (33–36%) provided energetically the highest contribution of the nutrients ingested. The quantities of water, proteins, nucleic acids, lipids and carbohydrates extracted by the antlion larvae were in proportion to their availability in their prey, the Hodotermes mossambicus larvae. The quantities of nutrients extracted by the antlion larvae at low prey densities were not significantly influenced by the differences in mandible size, antlion body weight or the trapping method (building a pit or not) of the antlion species. It is proposed that a low metabolic rate and the accumulation of fat reserves, and not the extent of prey utilisation, enable P. annulatus larvae to tolerate a 123-d starvation period in which 22.3% of their body weight is lost.