J. H. Giliomee

Life history and life tables of western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae), on English cucumbers

The life history of Frankliniella occidentalis (Pergande) was studied at six different temperatures on English cucumbers, cv. Pepinex. Life tables constructed indicate that the highest population growth rate (r m ) and net reproduction rate (R 0 ) occur at 30°C and the lowest at 15°C, where the population remains nearly stable. Generation time (T) was the shortest at 30°C and increased as temperature decreased. At 30°C oviposition increased, females matured and reproduced earlier and the time specific fecundity (m x ) was higher than at lower temperatures. Adult longevity was shortest at 30°C and longest at 15°C. The threshold value for development was determined as 9.4°C. Average duration of each developmental stage and the sex ratio at six different temperatures are given.

Ecology of the maize stalk borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae).

New information on the intraseasonal progression of larval infestations of Busseola fusca (Fuller) in South Africa was obtained through repeated sampling in maize plantings of different planting dates over various seasons. Due to the occurrence of distinct periods of moth flight, variation in planting date had a marked influence on levels of larval infestation. Also, plants were found to be most attractive as sites for oviposition between three and five weeks after emergence, resulting in a definite pattern in the time distribution of different larval instars in different plant parts. In the pre-tassel stage of plant development, most larvae occurred in localized groups within the whorls, reaching maximum numbers eight weeks after plant emergence. The emergence of the tassel forced some larvae to migrate to adjacent plants, resulting in an increase of internally damaged plants without an increase in larval numbers. It is shown that scouting for eggs over a limited period of plant development can lead to better timing of spray applications and thus to more cost-effective control measures.

Natural enemies of three mealybug species (Hemiptera: Pseudococcidae) found on citrus and effects of some insecticides on the mealybug parasitoid Coccidoxenoides peregrinus (Hymenoptera: Encyrtidae) in South Africa.

The population density of mealybug species in some South African citrus orchards has increased to pest status in recent years. The characterization of the natural enemy complex and quantification of their contribution to the control of Planococcus citri (Risso), Pseudococcus longispinus (Targioni-Tozzetti) and Pseudococcus calceolariae (Maskell) on Citrus limon (L.) and Citrus reticulata (Blanco) was investigated through intensive sampling. Eight primary and four secondary parasitoids, and two predator species were identified from P. citri and P. calceolariae. Anagyrus pseudococci (Girault) and Coccidoxenoides peregrinus (Timberlake) were the most common species, accounting for 44% and 21% of the total. Of the five primary parasitoids reared from P. longispinus, A. pseudococci and Anagyrus sp. were predominant, comprising 41% and 30%. Nymphal and adult parasitism (range = 0-26% vs. 0-66%) and predation (range = 0-5.6% vs. 0-4.1%) varied significantly between host trees and mealybug species (P < 0.001). The numbers of nymphal instars and adult stages of P. calceolariae and P. longispinus and the nymphal stage of P. citri that were parasitized and killed by predators correlated significantly with the total number of hosts on which they acted (P < 0.01), suggesting a density-dependent association. Laboratory bioassay of nine contact insecticides (methidathion, methomyl, methyl-parathion, parathion, profenofos and prothiofos) against C. peregrinus indicated that all were highly toxic, causing 98-100% mortality in < 6 h of treatment. The IGRs fenoxycarb and triflumuron did not cause significant parasitoid mortality (P > 0.05). However, a mixture of pyriproxyfen and mineral oil caused a marginally significant mortality (P < 0.05).

Resistance of some rose cultivars to the western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae)

Some rose cultivars such as City of Belfast are more resistant to the western flower thrips, Frankliniella occidentalis (Pergande) than others. Data from life tables and behavioural studies indicate that the reason for resistance may be physiological-chemical as well as the physical structure and colour of the flower. Some rose odours repelled F. occidentalis .

Aspects of the injuriousness of the maize stalk borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae)

Through repeated sampling in maize planted on different dates over different seasons in South Africa, it was shown that damage by Busseola fusca (Fuller) to plant parts other than the whorl had an important influence on yield. Damage to plants in the period after tasselling was shown to be important. The number of larvae in the plant was a weak estimator of expected yield losses.

Geographical variation in the seasonal moth flight activity of the maize stalk borer, Busseola fusca (Fuller), in South Africa

The seasonal abundance of Busseola fusca moths at five localities in the maize production area of South Africa was monitored by means of Robinson light traps. Geographical variation in the flight pattern was shown to exist between localities from east to west. Both the time and magnitude of the three seasonal moth flights seem to be governed by climatic factors. It is pointed out that the severity and the time of occurrence of larval infestations in different localities are largely predetermined by the time of planting. S. Afr. J. Plant Soil 1985, 2: 123–126

Plant population and cultivar effects on yield losses caused by the maize stalk borer, Busseola fusca (Lepidoptera: Noctuidae)

Maize plants were subjected to controlled infestations of Busseola fusca (Fuller). Variations in inter- and intra- row spacing had a pronounced influence on the damage caused by the pest. Both the rates of dispersal and the survival of the larvae were enhanced by increased plant population, with a concomitant increase in the number of damaged plants. High yielding, longer growing season maize cultivars suffered more severe yield losses under identical circumstances of infestation than shorter season cultivars of moderate yield potential.

The influence of rainfall on the seasonal abundance and flight activity of the maize stalk borer, Busseola fusca in South Africa

This study attempts to demonstrate the extent to which seasonal fluctuations in the number of Busseola fusca (Fuller) moths can be explained by precipitation data. Daily flight activity is shown to be enhanced by cool, humid conditions and restricted by rain. Humidity was shown experimentally to be of importance in the survival of moths under laboratory conditions. It is suspected that the observed relationship between seasonal moth numbers and rainfall is an indirect one and that survival of moths is rather determined by the direct influence of humidity. More serious infestations can therefore be expected during years with favourable rains.

Resistance of South African maize hybrids to maize streak virus

Data from maize cultivar trials at Vaalharts over a five-year period were used to evaluate the level of resistance to maize streak virus (MSV) in commercial maize hybrids, and to determine the potential value of such resistance. Maize hybrids presently grown in South Africa showed varying degrees of resistance to MSV. In general, yellow maize hybrids displayed a wide range of genetic variability, from resistant to highly susceptible, whereas white maize hybrids were mostly highly susceptible. Calculated resistance indices on a scale from 1 (resistant) to 5 (highly susceptible) proved very useful in quantifying genetically-based plant reaction to MSV infection.

Population dynamics of the white wax scale, Ceroplastes destructor (Hemiptera: Coccidae), on citrus in South Africa, with implications for biological control.

The population dynamics of the white wax scale, Ceroplastes destructor Newstead, was studied intensively in four easy-peel citrus orchards in the Western Cape Province of South Africa over three consecutive years (1997-1999). Key factor analysis was used to determine and quantify the contribution of individual mortality factors to the total generation mortality. Key stage mortality, determined from a cohort life table, was in the third instar and pre-ovipositing female stages. Mortality of C. destructor was caused primarily by parasitoids, predators and miscellaneous factors. Parasitoids and miscellaneous factors acted as density-dependent regulatory agents during the pre-ovipositional and first instar stages respectively. This has implications for biological control of C. destructor. Some of the mortality factors acted either randomly with no reference to the population densities or in an inverse density-dependent manner during the egg-crawler, second or third instar stages.