H. du Plessis

Effect of exchangeable sodium and phosphogypsum on the hydraulic properties of several South African soils

The susceptibility of soils to the deleterious effects of sodicity was assessed for samples which varied with respect to physical, mineralogical and other properties. For this purpose soil samples were collected from different locations in South Africa. The effect of electrolyte concentration and sodium adsorption ratio (SAR) on hydraulic conductivity (HC) was determined with permeameters, while the combined effects of raindrop impact energy and low electrolyte concentration of rain on infiltration rate (IR) were assessed using a rainfall simulator. Soils proved to be either stable or susceptible to the effects of sodicity as indicated by modulus of rupture, air-water permeability ratio, HC and IR measurements. Sesquioxides and organic matter promoted stability. Final IR was always less than HC due to crust formation even at low exchangeable sodium percentage (ESP) levels as a result of the dispersive effects of both mechanical impact (physical disruption) of rain drops and low electrolyte concentration (...

First report of groundnut leafminer, Aproaerema modicella (Deventer) (Lepidoptera: Gelechiidae) on groundnut, soybean and lucerne in South Africa

The groundnut leafminer, Aproaerema modicella is an important pest of groundnut and soybean in South and South-East Asia. It has also been reported from Uganda, Malawi and Mozambique. During 2000, it was first noticed on groundnut in South Africa. Since then it has spread over the entire groundnut production area of the Free State, Northern Cape, North West and Mpumalanga provinces. Reduction in yield was observed under conditions of severe infestations. During 2001 A. modicella was also observed on soybean in Mpumalanga and lucerne in the Northern Cape province, respectively.The groundnut leafminer, Aproaerema modicella is an important pest of groundnut and soybean in South and South-East Asia. It has also been reported from Uganda, Malawi and Mozambique. During 2000, it was first noticed on groundnut in South Africa. Since then it has spread over the entire groundnut production area of the Free State, Northern Cape, North West and Mpumalanga provinces. Reduction in yield was observed under conditions of severe infestations. During 2001 A. modicella was also observed on soybean in Mpumalanga and lucerne in the Northern Cape province, respectively.

The effect of different host plants on the reproduction and longevity of Nysius natalensis

Nysius natalensis Evans (Hemiptera: Orsillidae) is a pest of sunflower in South Africa. Adults invade sunflower fields from their weedy hosts which occur inside crop fields and on surrounding headlands. The host plant suitability for survival and reproduction as well as the effect of within‐generation host switching was studied on different wild host plants and sunflower. Life history parameters used to assess host plant suitability were F1 adult survival, pre‐oviposition period, fecundity, and longevity. Nymphs and adults were provided with stems and seeds of five host plants, viz., Amaranthus hybridus L. (Amaranthaceae), Portulaca oleracea L. (Portulacaceae), Chenopodium album L. (Chenopodiaceae), Conyza albida Spreng. (Asteraceae), and sunflower, Helianthus annuus L. (Asteraceae). Nymphs were reared on crushed seed of the five plant species. After completion of the nymphal stage, emerging adults of each host plant group were provided with seed of a different host plant species for food. Adults did not survive long on stems only and very few eggs were laid. Seeds of the host plant species were shown to be an essential source of nutrients for N. natalensis reproduction, whereas the vegetative plant parts were unsuitable. Nymphal food and host‐plant switching between the nymphal and adult stages significantly affected the pre‐oviposition period. Nymphal and adult food source also affected female longevity. The number of eggs laid was not influenced by nymphal food, but was influenced by adult food and the switch between nymphal and adult food. The comparative attractiveness of sunflower and wild host plants for oviposition was also investigated and showed that females preferred to lay eggs on wild host plants, compared with sunflower. These results may explain why N. natalensis will lay their eggs on sunflower after weeds in the vicinity are controlled, or senesce toward the end of the growing season.

Distribution and host plant range of Nysius natalensis Evans (Hemiptera: Orsillidae) in the sunflower production area of South Africa

Feeding by Nysius natalensis (Hemiptera: Orsillidae) adults on sunflower heads reduces grain yield, oil content and seed germination. Since the insect is known to feed on several host plants, a study was conducted on its host plant range to assess the role that weeds played in the pest status of this insect throughout the sunflower production area of South Africa. Roadside surveys of hosts, conducted in every quarter-degree grid of the sunflower production area, showed that N. natalensis was ubiquitous. The insect survived, developed and reproduced across a wide range of temperature and rainfall conditions. Feeding of nymphs and adults, as well as the presence of eggs was observed on 26 plant species belonging to eight families. The polyphagous habits of N. natalensis, together with a favourable temperature range for development in its distribution area, are important factors in determining population density of the pest. Late-planted sunflowers were prone to N. natalensis damage because seed-fill coincided with high numbers of the insect that increased during spring and summer. These individuals migrated onto sunflower with senescence of host weeds prior to winter. Recommendations are made on weed management practices to limit damage by N. natalensis.

Using morphological characteristics to distinguish between male and female larvae and pupae of the groundnut leafminer, Aproaerema modicella (Deventer) (Lepidoptera: Gelechiidae)

Abstract The groundnut leafminer, Aproaerema modicella (Deventer) (Lepidoptera: Gelechiidae) is a well known groundnut pest in Asia. It was first reported in east Africa during 1998 after which it rapidly spread throughout East and southern Africa. The adult is a small brownish-grey moth and it difficult to distinguish between the different sexes even under a stereomicroscope. Research on pest behaviour is hampered by the fact that male and female moths cannot be distinguished. The aim of this study was to identify distinguishing morphological characteristics of male and female larvae and pupae that are visible with the naked eye or under a stereomicroscope. The presence of pink coloured gonads which are easily visible through the cuticle was a distinguishing characteristic of male larvae. The position of the male genital aperture is a useful distinguishing characteristic present in male pupae. It is situated further away from the proximal side of the last abdominal segment, compared to the aperture of oviduct and bursa copulatrix of the female.

Calibration models for interpretation of soil salinity measurements using an electromagnetic induction technique

A commercially available salinity sensor (model EM-38 of Geonics Ltd., Canada) has been developed for diagnosing and quantifying soil salinity in the field. There is a need, however, to calibrate the instrument reading against a more commonly used measure of soil salinity. Linear regression equations are reported that allow the interpretation of field instrument readings in terms of electrical conductivity of the saturation extract (ECe) measured in the laboratory. Instrument measurements and soil samples were taken at some 110 sites on various irrigation schemes distributed across South Africa. Samples were analysed in the laboratory and soils categorized according to texture, water status and salinity distribution with depth. Calibration equations were developed firstly by relating field instrument readings, taken either in the vertical or horizontal position, to a depth-integrated ECe value which was weighted for depth according to instrument response. Secondly, the mean of the readings taken in the ve...

On the concept of leaching requirement for salinity control

The concept of irrigating in excess of plant water requirement to restrict soil profile salinity to acceptable limits is well established. The tolerable upper salinity limit is used to calculate the leaching requirement. This upper limit is determined mainly by crop salt sensitivity and acceptable yield decrease. It is proposed that when salinity limits are such that reduced yields result, then plant water use is also reduced. Should the crop be irrigated according to water requirements for non-saline conditions, the leaching fraction will be higher than planned. A degree of self-adjustment in the leaching fraction can thus be expected. Calculations were carried out to confirm and demonstrate the importance and applications of this concept. S. Afr. J. Plant Soil 1986, 3: 181–184

Chemical control of Nysius natalensis Evans (Hemiptera: Orsillidae), a pest of sunflower in South Africa

No insecticide is currently registered for Nysius natalensis Evans (Hemiptera: Orsillidae), control on sunflower [Helianthus annuus L. (Asteraceae)]. Field as well as laboratory trials were conducted to evaluate efficacy of insecticides for N. natalensis control. During budding prior to anthesis, insecticides were applied aerially. These were endosulfan, a mixture of dichlorvos + monocrotophos, mevinphos, a mixture of endosulfan + cypermethrin, methomyl and cypermethrin Compared to the untreated control, all insecticides provided effective control of N. natalensis. However, re-infestation by the pest occurred in all treatments after five days. A laboratory evaluation of the efficacy of the contact action of eleven insecticides in controlling N. natalensis adults on sunflower heads during application or immediately thereafter was conducted. Insecticides were representative of different groups and modes of action. These were carbosulfan, methomyl, endosulfan, demeton-S-methyl, dichlorvos, chlorpyrifos, malathion, profenofos, cypermethrin, fenvalerate and tau-fluvalinate. The insecticides were applied to sunflower heads containing 50 N. natalensis adults in plastic containers. All insecticides controlled N. natalensis effectively. The efficacy of the eleven insecticides against N. natalensis adults that re-infest sprayed fields from three hours post-application onward, were also evaluated. The insecticides were applied as full cover sprays to sunflower plants in the field. Disks were cut from the leaves three hours after treatment and placed in small plastic containers. Adults were transferred to the treated leaves and evaluated against those on leaves treated with water only. Fenvalerate and tau-fluvalinate were less effective in controlling N. natalensis than the other insecticides evaluated. Reduction in efficacy of seven of the insecticides was evaluated over a period of 14 days. Systemic insecticides remained more effective over longer periods than contact insecticides. However, the contact action of endosulfan remained as effective as the systemic insecticide, demeton-S-methyl. Endosulfan has a low toxicity to bees and will therefore control N. natalensis without a detrimental effect on pollination. To limit the number of applications, correct timing of insecticide application is important. The period that sunflower heads take to turn downward after completion of anthesis will determine the period for application of insecticides against N. natalensis during seed fill.

Laboratory assay to determine efficacy of insecticides for control of the spotted maize beetle, Astylus atromaculatus Blanchard (Coleoptera: Melyridae), a pest of sorghum

Spotted maize beetle, (Astylus atromaculatus), African boll- worm, (Helicoverpa armigera) and various aphid species attack sorghum from flowering onwards. The possibility of simultaneous control of these pests with a single application of an insecticide exists. The efficacy of insecticides registered for the control of aphids, A. atromaculatus and H. armigera on grain sorghum was evaluated for control of A. atromaculatus. Significant differences in efficacy were observed but most insecticides were ineffective. Gamma-BHC was only 53 % effective, but methomyl, registered for control of H. armigera was 100 % effective after 72 hours. Since the residual action, based on the withholding period of methomyl is short and the possibility of reinfestation by beetles under field conditions exists, control may not be effective over the long term.Spotted maize beetle, (Astylus atromaculatus), African boll- worm, (Helicoverpa armigera) and various aphid species attack sorghum from flowering onwards. The possibility of simultaneous control of these pests with a single application of an insecticide exists. The efficacy of insecticides registered for the control of aphids, A. atromaculatus and H. armigera on grain sorghum was evaluated for control of A. atromaculatus. Significant differences in efficacy were observed but most insecticides were ineffective. Gamma-BHC was only 53 % effective, but methomyl, registered for control of H. armigera was 100 % effective after 72 hours. Since the residual action, based on the withholding period of methomyl is short and the possibility of reinfestation by beetles under field conditions exists, control may not be effective over the long term.

Resistance Status of Busseola fusca (Lepidoptera: Noctuidae) Populations to Single- and Stacked-Gene Bt Maize in South Africa

Abstract Transgenic Bt maize expressing Cry insecticidal δ-endotoxins of Bacillus thuringiensis has been cultivated in South Africa for the control of Busseola fusca since 98. Busseola fusca is resistant to Cry1Ab Bt maize at many localities throughout the maize production region. Pre-release evaluation (94–96) of the inherent susceptibility and post-release assessments (98–2011) of resistance status of B. fusca focused on a limited number of pest populations. This study reports the current levels of susceptibility of 10 B. fusca populations evaluated between 2013 and 2017 and compared this data with previously reported data on the survival of this pest on Bt maize, including data of pre-release evaluations done during 94 and 95. Larval feeding bioassays in which plant tissue of maize events expressing either Cry1Ab or Cry1A.105+Cry2Ab2 (stacked event) proteins were conducted and survival and different life history parameters recorded. Results show a shift in levels of susceptibility of B. fusca to Bt maize. Pre-release evaluation of the single-gene event showed very low larval survival on Bt maize leaf tissue while studies 10 yr later and the current study reported survival of up to 40% and 100% on Cry1Ab maize, respectively. While no larvae completed their life cycle on the stacked event, higher LT50 values in this study indicate a shift in susceptibility of B. fusca to the stacked-gene event and highlight the importance of baseline information and monitoring of pest populations for their susceptibility to Bt maize.