T. C. de K. van der Linde

Critical thermal limits and temperature tolerance in the harvester termite Hodotermes mossambicus (Hagen)

Abstract The CTMax of pigmented major and minor workers and unpigmented larvae of Hodotermes mossambicus (Hagen) from laboratory colonies range between 43.5–48.53°C while the CTMin range between 5.3–10.31°C over a range of acclimation temperatures between 15 and 35°C. Major and minor workers have both higher CTMax and lower CTMin than unpigmented large larvae. Temperature tolerance limits are influenced by a number of factors such as the age or maturity of the individual, its size and colour, previous thermal history, the extent of its water reserves as well as environmental conditions.

Genetic (co)variances between wrinkle score and absence of breech strike in mulesed and unmulesed Merino sheep, using a threshold model

Heritability estimates for, and genetic correlations among neck wrinkle score, body wrinkle score, breech wrinkle score and the absence of breech strike were determined on 2918 16-month hoggets from a selection experiment with South African Merinos. Data were obtained from the lamb drops of 9 years between 1998 and 2007, born as the progeny of 247 sires and 1250 dams. All progeny born until 2002 were subjected to the Modified Mules operation as lambs (n = 1799). However, due to international pressure to phase out the Mules operation, animals from 2003 onwards were not mulesed (n = 1119). During both periods, the experimental animals were maintained in single flocks (separated on sex) after weaning. Progeny were subjected to visual appraisal of wrinkle scores (range) on the neck (1–6), body (1–5) and breech (1–6) at an age of ~16 months. The occurrence of blowfly strike in the breech area (breech strike) was recorded in all animals during the wool growth period of a year from shearing as weaners. Two four-trait animal models were fitted to obtain (co)variance components and ratios from the separate datasets with neck wrinkle score; body wrinkle score, breech wrinkle score and the absence of breech strike, all defined as threshold traits. Estimates of heritability for breech wrinkle score amounted to 0.27 and 0.45 for the mulesed and unmulesed groups, respectively. The absence of breech strike on the underlying scale was also heritable for the mulesed (0.33) and unmulesed (0.46) groups; translating to heritability estimates of 0.074 and 0.157, respectively, on the observed scale. The genetic correlations of wrinkle scores with absence of breech strike were generally favourable, but significant only for neck wrinkle score (–0.57) and breech wrinkle score (–0.45) in the unmulesed group. It was concluded that South African Merino sheep would respond to selection against breech strike under conditions of adequate challenge. Based on results from unmulesed sheep, selection for plainer sheep is also expected to result in a favourable correlated response in breech strike.

Influence of divergent selection for reproduction on the occurrence of breech strike in mature Merino ewes

Divergent selection resulted in Merino lines that differ markedly for reproduction. These lines were selected from the same base population from 1986 to 2009. Selection was initially based on maternal ranking values for reproduction in both ewe and ram progeny. The maternal ranking scores were augmented by breeding values from a single-trait repeatability model since 2003. The site and severity of flystrike were recorded for mature breeding ewes in the flock from 2007 to 2009. The following details were captured: animal number, site of the strike (body or breech) and the severity of the strike (1 = mild to 5 = severe). Breech strikes amounted to 92.1% of all strikes and this trend was consistent across years. High-line ewes were less likely (P 20 years were less likely to be affected by breech strike than contemporaries selected for low reproduction.

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.

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.

Physiological changes during colony establishment in the termite Hodotermes mossambicus (Hagen): Energy reserves—neutral lipids and glycogen

Abstract At swarming approximately 60% of the dry mass of the termite Hodotermes mossambicus male and female alates consisted of lipids, the bulk of which was neutral lipids. During the period prior to the appearance of the first larvae (27–32 days after swarming) there was a significant decrease in total lipid content and a dramatic increase in glycogen level. The latter may have been triggered by intake of water. During the period between the appearance of the first larvae and the first workers (42–50 days after swarming) there was a further drop in the lipid content and decrease in the glycogen level. This indicates that lipids are the primary energy source and that they and/or glycogenic amino acids derived from flight muscle degradation could serve as glycogen precursors.

Evaluation of chemical control against the insect complex of groundnuts (Arachis hypogaea L.) in the South African Highveld Region

Chemical compounds representing seed treatments, soil-applied systemic insecticides, contact and systemic corrective sprays were tested on the groundnut pest complex for three seasons in the South African Highveld Region. The efficacy of the compounds was evaluated by determining the effect of the compounds on plant population, leaf feeding injury, incidence of subterranean insects, pod damage, yield, grading quality and shelling percentage. Despite a favourable cost/benefit ratio achieved with all the compounds tested, none of the chemicals gave consistent results for all the parameters evaluated. Groundnut seed treatment with liquid-based compounds cannot be recommended on a commercial scale since these treatments caused testa damage, resulting in split seed.

The response of alate Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae) to volatile substances from four non-host plant extracts under laboratory conditions

Volatile compounds present in essential plant oils are known to influence insect behaviour. They are generally of low molecular weight, highly volatile, have a unique mode of action and are sometimes of low toxicity to non-target organisms. One example of a volatile compound extracted from a plant is methyl salicylate which is being used to reduce aphid infestation on barley in some European countries. The use of such volatile compounds was therefore considered as an alternate control option to be included in the control programme of Diuraphis noxia (Kurd.) after plant resistance-breaking biotypes started to develop in South Africa. The aim of this study was to test the response of alate D. noxia to plant extracts from four plant species in a four-arm olfactometer. Aqueous and light mineral oil extracts of Artemisia afra (Jacq.), Datura stramonium (L.), Tagetes minuta (L.) and Tulbaghia violacea (Harv.), which grow naturally in some dryland wheat production regions of South Africa, were tested. These plants were chosen based on possible insect repelling properties known to occur in other species of the same genera. Aphids were strongly repelled by the oil extract of T. violacea and the aqueous extract of A. afra. The oil extract of A. afra, both T. minuta extracts and T. violacea aqueous extract were less repellent but still elicited an exceptional repelling response. Aphids were not repelled by D. stramonium extracts. The effectiveness of these basic extracts in repelling D. noxia should, however, be tested under field conditions before they can be recommended as a control option.