J. B.J. van Rensburg

First report of field resistance by the stem borer, Busseola fusca (Fuller) to Bt-transgenic maize

Reports of severe damage caused by the African stem borer, Busseola fusca (Fuller) to Mon810-transgenic maize (Bt) prompted a study in which the survival of progenies of diapause larvae collected from both a Bt and non-Bt planting were compared when feeding on various Bt and non-Bt hybrids. Field and greenhouse grown plants were artificially infested with neonate larvae. Larval mass was recorded at two-day intervals for three weeks. Data were subjected to simple regression analyses followed by pair wise comparison of the slopes. The two borer populations showed similar larval mass gains on non-Bt hybrids but differed in the response to Bt-hybrids. Appreciable numbers of larvae from the non-Bt derived population survived only to the eighth day. In contrast, substantial numbers of larvae of the Bt-derived population survived over the entire trial period. The mean larval mass of the Bt-derived population at the conclusion of the experiment was less on Bt-hybrids than on their susceptible counterparts. This i...Reports of severe damage caused by the African stem borer, Busseola fusca (Fuller) to Mon810-transgenic maize (Bt) prompted a study in which the survival of progenies of diapause larvae collected from both a Bt and non-Bt planting were compared when feeding on various Bt and non-Bt hybrids. Field and greenhouse grown plants were artificially infested with neonate larvae. Larval mass was recorded at two-day intervals for three weeks. Data were subjected to simple regression analyses followed by pair wise comparison of the slopes. The two borer populations showed similar larval mass gains on non-Bt hybrids but differed in the response to Bt-hybrids. Appreciable numbers of larvae from the non-Bt derived population survived only to the eighth day. In contrast, substantial numbers of larvae of the Bt-derived population survived over the entire trial period. The mean larval mass of the Bt-derived population at the conclusion of the experiment was less on Bt-hybrids than on their susceptible counterparts. This indicates that the Bt-derived population has attained a level of resistance where some larvae are able to survive in the presence of the Bt-toxin but not without some detrimental effect on larval growth rate. Since producers are inclined to provide refugia under rain fed conditions in the immediate vicinity of irrigated plantings rather than as part of irrigated fields, the known preference of moths for high humidity could have contributed to increased selection pressure towards the development of resistance to the Bt-toxin.

Transgenic Bt maize: farmers’ perceptions, refuge compliance and reports of stem borer resistance in South Africa

Based on surface area, South Africa is currently ranked the eighth in planting genetically modified (GM) crops in the world. Bt maize (MON810) has been grown to control lepidopterous stem borers in South Africa since its first release during 1998. The first report of resistance of the African maize stem borer, Busseola fusca (Lepidoptera: Noctuidae) and poor compliance to refuge requirements that could have contributed to resistance development prompted this study which was done in the main maize production area of South Africa. Objectives were to evaluate farmer’s perceptions of the regulatory aspects guiding the planting of Bt maize and refugia and how it was applied between 1998 and 2010. This study involved 105 commercial farmers covering 87 778 ha of maize throughout the main maize production region on the Highveld of South Africa. A questionnaire survey was conducted and addressed signing of technical agreements upon purchasing GM seed, refuge compliance, pest management practices, perceived benefits and risks relating to Bt maize. Compliance with refugia requirements was low especially during the initial 5–7 years after release. An alarmingly high number of farmers applied insecticides as preventative sprays on Bt maize and refugia irrespective of stem borer infestation levels. A large proportion of farmers reported significant borer infestation levels on Bt maize and between 5% and 93% farmers in all districts applied insecticides to Bt maize to limit borer damage, indicating that the occurrence of resistance is more wide‐spread in the country than previously thought. This study shows irresponsible management of GM crop technology by farmers, chemical and seed companies. Concerns and perceptions of farmers in South Africa seem to differ from those in Europe. South African farmers perceived little, if any, negative impact on non‐target organisms and remain positive about the technology in spite of resistance development. Other studies indicated that European farmers were concerned about these aspects. A shared concern, however, was consumer acceptance of GM maize and marketability on the export market.

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.

Evaluation of Bt.-Transgenic maize for resistance to the stem borers Busseola fusca (Fuller) and Chilo partellus (Swinhoe) in South Africa

The gene Cry1Ab derived from the entomopathogenic bacterium Bacillus thuringiensis is currently deployed commercially for control of Pyralid stem borers of maize in the USA. In this study various events of the Bt-gene were evaluated in maize inbred lines and hybrids for leaf feeding resistance to the local stem borers Busseola fusca and Chilo partellus, using artificially infested plants under greenhouse and field conditions. The event MON810 proved superior to all other events tested. C. partellus was more susceptible than B. fusca to the same events. Significant maternal effects were observed. Efficacy was not negatively affected by variations in the time of infestation. Hybrids did not respond equally to transformation with the same event, which was attributed to differences in adaptation to adverse environmental conditions.

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.

Larval mortality and injury patterns of the African stalk borer, Busseola fusca (Fuller) on various plant parts of Bt-transgenic maize

During the 1998/99 season considerable stem damage was caused by B. fusca in commercial plantings of Bt-maize, without leaf feeding damage during the vegetative stages of plant development. This indicated that larvae may survive on some less toxic plant part subsequent to the vegetative stages. In this study a laboratory bioassay was conducted, using forced feeding of larvae on various parts of Bt-maize. In a field trial larval survival and plant damage were recorded following artificial infestation of Bt- plants one week before, and one and three weeks after tasseling. It appears that there is a window of approximately 14 days immediately preceding tasseling, in which relatively late infestations may result in increased larval survival, due to the availability of silks as a primary food source to neonate larvae. The Bt-protein concentration in silks appears to be low enough to allow survival of some larvae until completion of the first two instars, after which the ear tips and husk leaves serve as important feeding sites. The upper stem appears to be less toxic than the lower stem, providing a site for early penetration of stems by-young larvae, culminating in the eventual successful penetration of the lower stem at later stages of plant development. Increased larval survival resulted in a significant increase in the incidence of ear damage, which appears to be most pronounced within the first 14 days after infestation. Observed plant damage did not result in significant yield losses, but does not exclude the possibility that currently used Bt-hybrids may suffer economically important yield losses at the high levels of natural infestation often experienced with late planting.During the 1998/99 season considerable stem damage was caused by B. fusca in commercial plantings of Bt-maize, without leaf feeding damage during the vegetative stages of plant development. This indicated that larvae may survive on some less toxic plant part subsequent to the vegetative stages. In this study a laboratory bioassay was conducted, using forced feeding of larvae on various parts of Bt-maize. In a field trial larval survival and plant damage were recorded following artificial infestation of Bt- plants one week before, and one and three weeks after tasseling. It appears that there is a window of approximately 14 days immediately preceding tasseling, in which relatively late infestations may result in increased larval survival, due to the availability of silks as a primary food source to neonate larvae. The Bt-protein concentration in silks appears to be low enough to allow survival of some larvae until completion of the first two instars, after which the ear tips and husk leaves serve as import...

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

Effect of Busseola fusca on the incidence of maize ear rot caused by Fusarium moniliforme and Stenocarpella maydis

The influence of Busseola fusca incidence and damage on Fusarium moniliforme and Stenocarpella maydis ear rots was studied in the field using artificial infestation with B. fusca, mechanical damage and pathogen inoculation. B. fusca infestation increased the incidence of F. moniliforme infected maize ears irrespective of artificial or natural F. moniliforme infection. Mechanical damage plus F. moniliforme inoculum also resulted in a high incidence of F. moniliforme-infected ears suggesting that physical damage to the ear will increase F. moniliforme-infected ears. Fungus-infected rotten kernel percentages of B. fusca-damaged treatments did not differ significantly from the control. The insignificant correlation (r = −0.206; P = 0.695) between F. moniliforme- infected ears and per cent rotten kernels indicates that in the case of the former, when used as a disease criterion, damage was localized and did not result in significant grain quality reductions. Neither B. fusca nor physical damage significantly a...

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.

Reproductive biology of Bt-resistant and susceptible field-collected larvae of the maize stem borer, Busseola fusca (Lepidoptera: Noctuidae)

The maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae), is of economic importance throughout sub-Saharan Africa. The seasonal activity pattern of this pest is characterized by two to three distinct generations during spring and summer followed by a diapause period of approximately six months during autumn and winter. Genetically modified Bt maize (MON810) that expresses an insecticidal Cry1Ab protein has been deployed in South Africa since 1998 to manage stem borers. The first report of field resistance of B. fusca to Bt maize was made during the 2006 cropping season. Resistant strains of this pest are expected have reduced fitness compared to susceptible individuals. Information regarding fitness of resistant individuals that survive on Bt maize could contribute to the understanding of resistance evolution as well as to development of improved resistance management strategies. Life history parameters of different B. fusca populations were compared in a laboratory study using diapause (spring) as well as second-generation (summer) populations. Sex ratio, pupal mass, fecundity and longevity of moths of field-collected Bt-resistant and susceptible B. fusca populations were compared. Slight adverse effects of Bt maize on fitness of the resistant summer-population were observed. The sex ratio was biased towards males in some resistant populations and towards females in susceptible populations. The resistant population had a lower mean pupal mass, shorter longevity of moths and reduced fecundity.