The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is a major pest of Brassica crops in many parts of the world. Because of its ability to develop resistance to virtually all major groups of insecticides, including Bacillus thuringiensis Berliner (Bt), much attention has therefore been given to biological control using parasitoids. South Africa has an abundance of parasitoids attacking this pest. Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae) is the most abundant larval parasitoid of P. xylostella in South Africa. In East Africa, its role in biological control of P. xylostella is insignificant, and the most abundant parasitoid there is Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae), a larvalpupal parasitoid. In South Africa, however, D. mollipla is out-competed by C. plutellae. Total parasitism of P. xylostella in East Africa rarely exceeds 15%, therefore there is a need to introduce more effective and heat-tolerant parasitoids of P. xylostella to that region. This study was therefore initiated to examine the potential of C. plutellae and D. mollipla as biological control agents by studying certain aspects of their biology in the laboratory, as well as the suitability of C. plutellae for introduction into East Africa. Biological aspects studied were: (i) host instar preference, fecundity, and searching efficiency of C. plutellae and D. mollipla at different host and parasitoid densities; (ii) effects of temperature on parasitism of P. xylostella by C. plutellae and D. mollipla, and on their rates of development and emergence. In addition, the role of parasitoids in controlling P. xylostella on unsprayed cabbage plots was investigated. Both C. plutellae and D. mollipla preferred to attack second and third instar hosts than fourth instars in choice and no-choice tests. However, D. mollipla attacked more fourth instar hosts than C. plutellae. Cotesia plutellae laid mainly female eggs in second and third instar hosts than in fourth instars, whereas D. mollipla laid more female eggs in fourth instar hosts than in second and third instar hosts. Diadegma mollipla was more fecund [82.57 ± 32.87, (mean ± s.d.) than C. plutellae (42.13 ± 12.2), and was long lived (7.13 ± 3.69 days) compared to the latter (5.23 ± 2.7 days). An increase in host density resulted in the reduction in the area of discovery (a) and the killing power (K) for both parasitoids. No significant differences were detected between the searching efficiency (t = -1.42NS, d.f. = 48, P < 0.001) of the two parasitoids. An increase in parasitoid density also resulted in a decline in searching efficiency, but not the killing power, of both parasitoids. Cotesia plutellae completed development at all temperatures tested (21-33°C), whereas D. mollipla completed development at temperatures from 18-30°C, and C. plutellae had a lower threshold for development (8.14°C) compared to D. mollipla (10.23°C). At all tested temperatures, the generation time for C. plutellae was shorter compared to D. mollipla. The possible reasons for the dominance of C. plutellae over D. mollipla in the field are: shorter generation time, high production of female progeny in younger hosts, low interference among searching females, and relatively wide thermal tolerance. The role of parasitoids in regulating diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), populations was studied for two years (February 2000-January 2002) on unsprayed cabbage fields at Brits, North West Province, South Africa. Cabbage seedlings were transplanted in three consecutive times each year. Cabbage infestations by P. xylostella larvae and pupae, and their parasitoids, were monitored at weekly intervals. The flight activity of P. xylostella male moths was monitored using sex-pheromone traps. Trap catches indicated that the moths were active throughout the year. The flight activity of the moths corresponded with infestations on the crop. Trap catches and infestation levels were generally low from December to August and high from September to November. Eight hymenopteran parasitoids were reared: the larval parasitoids Cotesia plutellae (Kurdjumov) (Braconidae) and Apanteles halfordi (Ullyett) (Braconidae); the larval-pupal parasitoids Oomyzus sokolowskii (Kurdjumov) (Eulophidae) and Diadegma mollipla (Holmgren)(Ichneumonidae); the pupal parasitoid Diadromus collaris (Gravenhorst) (Ichneumonidae); and the hyperparasitoids Eurytoma sp. (Eurytomidae), Mesochorus sp. (Ichneumonidae), and Pteromalus sp. (Pteromalidae). Cotesia plutellae was the most abundant parasitoid of P. xylostella followed by O. sokolowskii, D. collaris, A. halfordi and D. mollipla. Parasitism of P. xylostella larvae was high reaching 100% on several occasions during late spring to end of autumn (November-May) each year. Parasitism was lower (<50%) in winter and early spring (June-September). Apanteles halfordi was absent in winter but re-appeared in spring. Parasitism of P. xylostella pupae by D. collaris was high only during spring (September-October). Hyperparasitism was generally low except when primary parasitoids were abundant in spring (September-November) and summer (December-February) when up to 25% of P. xylostella larvae and C. plutellae cocoons yielded hyperparasitoids. The role of other biotic and abiotic mortality factors on the population dynamics of P. xylostella is discussed.

The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a cosmopolitan insect pest of Brassica crops. In South Africa, there are no action thresholds for its chemical control which makes it difficult for growers to make informed decisions on when to apply insecticides and how frequently to apply them in order to achieve optimal crop yield. To contribute towards optimum application of insecticides against P. xylostella, this study compared the impact of weekly and bi-weekly applications of a selective insecticide Dipel® (Bacillus thuringiensis Berliner var. kurstaki) applied at 250 g/ha, and a broad-spectrum insecticide Dichlorvos (an organophosphate) applied at 1 ml/L against biological control (Control) on the pest population density on cabbage during October– December 2011 and March–May 2012. The use of both selective and broad-spectrum insecticides for experiments enables us to understand if efforts to optimise cabbage yield depend mainly on effective suppression of P. xylostella densities. Furthermore, investigations were carried out to determine the impact of these chemicals on parasitism rates of P. xylostella and species richness of its primary parasitoids. During the October–December 2011 growing season, the lowest infestation of P. xylostella occurred on cabbage plots that received weekly application of Dipel and the highest on untreated control plots. Cabbage weights were negatively related to infestation levels, implying that weekly application of Dipel yielded bigger cabbage heads. During March– May 2012, P. xylostella infestations were again higher on the control followed by weekly and bi-weekly treatments of Dichlorvos, then weekly and bi-weekly applications of Dipel. Despite the significant differences observed, infestation levels were much lower (< 1 P. xylostella per plant on average) in all treatments during this season. Consequently no significant differences in cabbage weights were observed among the treatments. The lower infestation levels were attributed to higher parasitism levels (≥50 %), especially during the early stages of crop development. A total of four parasitic Hymenoptera species were recorded from P. xylostella larvae and pupae during October–December 2011, while three species were recorded during March– May 2012. However, Cotesia vestalis (Haliday) (Braconidae) accounted for >80 % of total parasitism levels in all treatments. Parasitism levels were not significantly different among the treatments in both seasons. Parasitoid species richness was highest on the control. Although two parasitoid species were recorded in all Dipel and Dichlorvos treatments during October–December 2011, only one parasitoid species was recorded in the Dipel treatments during March–May 2012 compared to two species in Dichlorvos treatments. Although weekly applications of Dipel ensured good yield and crop quality during October–December, weekly applications of the chemical did not lead to better quality crop during March–May crop growing season. Thus, it is not necessary to apply insecticides during periods in which natural mortality of P. xylostella is high due to parasitoids. Since P. xylostella abundance was a determining factor of crop quality, these results imply that insect pest management should focus mainly on suppressing its numbers. Furthermore, there was no evidence that application of either insecticide type had a negative impact on parasitism rates of P. xylostella. The lower parasitoid species richness on Dipel treated plots was the consequence of its higher efficiency in suppressing the pest population which substantially reduced availability of potential hosts for parasitoids, hence only the efficient C. vestalis was recorded at low host densities / Agriculture and  Animal Health / M.Sc. (Agriculture)

The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a cosmopolitan insect pest of Brassica crops. In South Africa, there are no action thresholds for its chemical control which makes it difficult for growers to make informed decisions on when to apply insecticides and how frequently to apply them in order to achieve optimal crop yield. To contribute towards optimum application of insecticides against P. xylostella, this study compared the impact of weekly and bi-weekly applications of a selective insecticide Dipel® (Bacillus thuringiensis Berliner var. kurstaki) applied at 250 g/ha, and a broad-spectrum insecticide Dichlorvos (an organophosphate) applied at 1 ml/L against biological control (Control) on the pest population density on cabbage during October– December 2011 and March–May 2012. The use of both selective and broad-spectrum insecticides for experiments enables us to understand if efforts to optimise cabbage yield depend mainly on effective suppression of P. xylostella densities. Furthermore, investigations were carried out to determine the impact of these chemicals on parasitism rates of P. xylostella and species richness of its primary parasitoids. During the October–December 2011 growing season, the lowest infestation of P. xylostella occurred on cabbage plots that received weekly application of Dipel and the highest on untreated control plots. Cabbage weights were negatively related to infestation levels, implying that weekly application of Dipel yielded bigger cabbage heads. During March– May 2012, P. xylostella infestations were again higher on the control followed by weekly and bi-weekly treatments of Dichlorvos, then weekly and bi-weekly applications of Dipel. Despite the significant differences observed, infestation levels were much lower (< 1 P. xylostella per plant on average) in all treatments during this season. Consequently no significant differences in cabbage weights were observed among the treatments. The lower infestation levels were attributed to higher parasitism levels (≥50 %), especially during the early stages of crop development. A total of four parasitic Hymenoptera species were recorded from P. xylostella larvae and pupae during October–December 2011, while three species were recorded during March– May 2012. However, Cotesia vestalis (Haliday) (Braconidae) accounted for >80 % of total parasitism levels in all treatments. Parasitism levels were not significantly different among the treatments in both seasons. Parasitoid species richness was highest on the control. Although two parasitoid species were recorded in all Dipel and Dichlorvos treatments during October–December 2011, only one parasitoid species was recorded in the Dipel treatments during March–May 2012 compared to two species in Dichlorvos treatments. Although weekly applications of Dipel ensured good yield and crop quality during October–December, weekly applications of the chemical did not lead to better quality crop during March–May crop growing season. Thus, it is not necessary to apply insecticides during periods in which natural mortality of P. xylostella is high due to parasitoids. Since P. xylostella abundance was a determining factor of crop quality, these results imply that insect pest management should focus mainly on suppressing its numbers. Furthermore, there was no evidence that application of either insecticide type had a negative impact on parasitism rates of P. xylostella. The lower parasitoid species richness on Dipel treated plots was the consequence of its higher efficiency in suppressing the pest population which substantially reduced availability of potential hosts for parasitoids, hence only the efficient C. vestalis was recorded at low host densities / Agriculture and  Animal Health / M.Sc. (Agriculture)

The diamondback moth, Plutella xylostella (Linnaeus 1758.) (Lepidoptera: Plutellidae), is a major insect pest of Brassica crops in many parts of the world leading to economic losses amounting to an estimated US$ 4-5 billion. Although parasitoids (biological control agents) play a major role in suppressing the pest populations during November – May in South Africa, the pest reaches outbreak status during September and October due to low impact of parasitoids, which has necessitated regular application of insecticides. However, insecticide applications have often resulted in the pest developing resistance, and when coupled with the negative effects of several insecticides on parasitoids, integration of the two pest control strategies for effective management of P. xylostella population density has been difficult to achieve. One approach that has received little attention is integration of host plant resistance (bottom-up effect) and biological control (top-down effect) for effective management of P. xylostella. However, the interaction between host plants, the insect pest, and parasitoids is not simple and straight forward, as strong negative impact of host plants on fitness of the insect pest can be cascaded up the food chain and have a negative impact on a given parasitoid, which in turn may reduce the desired complementary effect between the two pest control strategies. To identify optimal interactions between cabbage (Brassica oleracea L. var. capitata, Brassicaceae), P. xylostella and its larval parasitoid Cotesia vestalis (Haliday 1834) (Hymenoptera: Braconidae), this study investigated (i) the effects of seven cabbage cultivars (Empowa, Hollywood F1, Megaton, Leano, Menzania, Beverley Hills and Karabo) on fitness parameters (survival, developmental time, pupal weights, longevity without food and oviposition rates) of P. xylostella; (ii) the influence of the same host plant cultivars on fitness parameters (developmental time, pupal weights, longevity xi without food, fecundity, emergence rate and sex ratio) of C. vestalis. Furthermore, net reproductive rates and the intrinsic rates of natural increase were calculated for C. vestalis that emerged from hosts fed on each of the cultivars. All experiments were conducted in climate-controlled laboratory rooms maintained at 22 ± 1 ºC (mean ± S.D.), 60 ± 5 % RH and 16L: 8D photoperiod. Under the no choice test, overall survival of P. xylostella immature stages was highest on Karabo (67.26%) and lowest on Megaton (44.92%). The larval and pupal developmental period, and generation time was prolonged on Empowa (18.48 days), Karabo (14.64 days) and Beverly Hills (17.48 days), while developmental period on Hollywood F1 (13.79 days) was shortest. Male and female P. xylostella pupal weights were lighter from larvae that fed on Megaton (4.13 and 4.65 mg), Menzania (4.53 and 4.91 mg), and Hollywood F1 (4.11 and 5.08 mg), whereas pupal weights from Karabo (6.0 and 6.82 mg) were the heaviest. Unfed female moths originally reared on Beverley Hills had the highest longevity (5.05 days), whereas those reared on Leano (3.54 days) and Megaton (3.89 days) had the shortest life span. Under the choice-test, P. xylostella moth laid significantly more eggs on Empowa (48.8%) and Hollywood F1 (45.6%) and least on Menzania (11.8%) and Leano (10.6%). Megaton was more resistant to P. xylostella due to lower survival rates of immature stages, lower pupal weights and moth longevity. The generation time of C. vestalis was shortest on Karabo (10.10 days) and Leano (10.38 days), and longest on Megaton (12.57 days) and Empowa (12.80 days). The highest pupal weight of C. vestalis was obtained from parasitoids reared from P. xylostella fed Menzania (5.4 mg), Megaton (5.25 mg) and Beverly Hills (4.85 mg) and the lightest on Karabo (3.8 mg). Parasitoids reared on larvae that fed on Hollywood F1 lived the longest (2.28 days) followed by Menzania (1.94 days) and Beverly Hills (1.8 days), whereas those whose hosts fed on Leano had shortest life span (0.83 days). Despite the parasitoids from Megaton hosts being heavier, their fecundity and number of female progeny per female (16.87 and 3.60, respectively) were lowest. Cotesia vestalis fecundity and daughters produced per female were highest on hosts fed on Menzania (38.00 and 9.13, respectively) and Beverly Hills (32.87 and 9.07, respectively). As a consequence, the net reproductive rate (R0) and intrinsic rate of increase (r) were higher on Menzania (7.87 and 0.58, respectively) and Beverly Hills (8.29 and 0.62, respectively). As survival and overall fitness of P. xylostella was lower on Megaton, this cultivar can play a major role in restricting population growth of this pest and thus generational number of eggs deposited on it during September and October. However, this strong bottom-up effect of Megaton on P. xylostella was cascaded up the food chain, as overall fitness of C. vestalis was lower on hosts developing on it. In contrast, the overall fitness of C. vestalis was higher on hosts that developed on Menzania and Beverly Hills. As these cultivars showed potential to sustain population density of C. vestalis at higher levels, it is also assumed that the period required for the parasitoid to reach the critical density to suppress the host population at a lower average density will be reached quicker than on other cultivars. Thus, their cultivation may improve biological control of P. xylostella during November–May in South Africa / Agriculture, Animal Health and Human Ecology / M. Sc. (Agriculture)

The diamondback moth, Plutella xylostella (Linnaeus 1758.) (Lepidoptera: Plutellidae), is a major insect pest of Brassica crops in many parts of the world leading to economic losses amounting to an estimated US$ 4-5 billion. Although parasitoids (biological control agents) play a major role in suppressing the pest populations during November – May in South Africa, the pest reaches outbreak status during September and October due to low impact of parasitoids, which has necessitated regular application of insecticides. However, insecticide applications have often resulted in the pest developing resistance, and when coupled with the negative effects of several insecticides on parasitoids, integration of the two pest control strategies for effective management of P. xylostella population density has been difficult to achieve. One approach that has received little attention is integration of host plant resistance (bottom-up effect) and biological control (top-down effect) for effective management of P. xylostella. However, the interaction between host plants, the insect pest, and parasitoids is not simple and straight forward, as strong negative impact of host plants on fitness of the insect pest can be cascaded up the food chain and have a negative impact on a given parasitoid, which in turn may reduce the desired complementary effect between the two pest control strategies. To identify optimal interactions between cabbage (Brassica oleracea L. var. capitata, Brassicaceae), P. xylostella and its larval parasitoid Cotesia vestalis (Haliday 1834) (Hymenoptera: Braconidae), this study investigated (i) the effects of seven cabbage cultivars (Empowa, Hollywood F1, Megaton, Leano, Menzania, Beverley Hills and Karabo) on fitness parameters (survival, developmental time, pupal weights, longevity without food and oviposition rates) of P. xylostella; (ii) the influence of the same host plant cultivars on fitness parameters (developmental time, pupal weights, longevity xi without food, fecundity, emergence rate and sex ratio) of C. vestalis. Furthermore, net reproductive rates and the intrinsic rates of natural increase were calculated for C. vestalis that emerged from hosts fed on each of the cultivars. All experiments were conducted in climate-controlled laboratory rooms maintained at 22 ± 1 ºC (mean ± S.D.), 60 ± 5 % RH and 16L: 8D photoperiod. Under the no choice test, overall survival of P. xylostella immature stages was highest on Karabo (67.26%) and lowest on Megaton (44.92%). The larval and pupal developmental period, and generation time was prolonged on Empowa (18.48 days), Karabo (14.64 days) and Beverly Hills (17.48 days), while developmental period on Hollywood F1 (13.79 days) was shortest. Male and female P. xylostella pupal weights were lighter from larvae that fed on Megaton (4.13 and 4.65 mg), Menzania (4.53 and 4.91 mg), and Hollywood F1 (4.11 and 5.08 mg), whereas pupal weights from Karabo (6.0 and 6.82 mg) were the heaviest. Unfed female moths originally reared on Beverley Hills had the highest longevity (5.05 days), whereas those reared on Leano (3.54 days) and Megaton (3.89 days) had the shortest life span. Under the choice-test, P. xylostella moth laid significantly more eggs on Empowa (48.8%) and Hollywood F1 (45.6%) and least on Menzania (11.8%) and Leano (10.6%). Megaton was more resistant to P. xylostella due to lower survival rates of immature stages, lower pupal weights and moth longevity. The generation time of C. vestalis was shortest on Karabo (10.10 days) and Leano (10.38 days), and longest on Megaton (12.57 days) and Empowa (12.80 days). The highest pupal weight of C. vestalis was obtained from parasitoids reared from P. xylostella fed Menzania (5.4 mg), Megaton (5.25 mg) and Beverly Hills (4.85 mg) and the lightest on Karabo (3.8 mg). Parasitoids reared on larvae that fed on Hollywood F1 lived the longest (2.28 days) followed by Menzania (1.94 days) and Beverly Hills (1.8 days), whereas those whose hosts fed on Leano had shortest life span (0.83 days). Despite the parasitoids from Megaton hosts being heavier, their fecundity and number of female progeny per female (16.87 and 3.60, respectively) were lowest. Cotesia vestalis fecundity and daughters produced per female were highest on hosts fed on Menzania (38.00 and 9.13, respectively) and Beverly Hills (32.87 and 9.07, respectively). As a consequence, the net reproductive rate (R0) and intrinsic rate of increase (r) were higher on Menzania (7.87 and 0.58, respectively) and Beverly Hills (8.29 and 0.62, respectively). As survival and overall fitness of P. xylostella was lower on Megaton, this cultivar can play a major role in restricting population growth of this pest and thus generational number of eggs deposited on it during September and October. However, this strong bottom-up effect of Megaton on P. xylostella was cascaded up the food chain, as overall fitness of C. vestalis was lower on hosts developing on it. In contrast, the overall fitness of C. vestalis was higher on hosts that developed on Menzania and Beverly Hills. As these cultivars showed potential to sustain population density of C. vestalis at higher levels, it is also assumed that the period required for the parasitoid to reach the critical density to suppress the host population at a lower average density will be reached quicker than on other cultivars. Thus, their cultivation may improve biological control of P. xylostella during November–May in South Africa / Agriculture, Animal Health and Human Ecology / M. Sc. (Agriculture)

The seasonal abundance of the imported cabbageworm, <i>Artogeia rapae</i> (L.)), cabbage looper <i>Trichoplusia ni</i> (Hubner)) and diamondback moth (<i>Plutella xylostella</i> (L.)) was determined in 1982 to 1984 on cabbage in southwest Virginia. The imported cabbageworm was the most important pest. It had one and a partial second generation in 1982 and 1983, and two and a partial third generation in 1984. Market Prize and Green Winter varieties had lower <i>A. rapae</i> infestations than Abbott & Cobb #5 or Rio Verde. Two generations of the diamondback moth, which preferentially fed on cabbage leaves versus heads, were seen in 1983 and 1984. <i>Diadegma insularis</i> (Meus.) (Hymenoptera: Ichneumonidae) parasitized 46% and 69% of the diamondback moth in 1983 and 1984, respectively. Higher parasitization of <i>P. xylostella</i> was found in pupae collected from Abbott & Cobb #5. Two generations of the cabbage looper were seen in 1983 and a single generation in 1984. <i>Pteromalus puparum</i> (L.) (Hymenoptera: Pteromalidae) parasitized 64.1% and 32.5% of imported cabbageworm pupae in 1983 and 1984; parasitism was most pronounced in the latter part of the season. High percentages of parasites successfully emerged as adults, indicating efficient host utilization. Most parasite mortality within the host occurred in the larval stage; mean adult parasite emergence per host for 1983 and 1984 was 52.3 with a 1:1 sex ratio. <i>P. puparum</i> parasitized either host sex, producing equal proportions of male and female parasites. Adult female <i>P. puparum</i> were not adversely affected by residues of methomyl (Lannate®), permethrin (Pounce®) and fenvalarate (Pydrin®), but males showed significantly higher mortality than controls following exposure to methomyl after 12 h, and methomyl and permethrin after six days. <P. puparum</i> adults were successfully refrigerated for up to 15 days at 3°C in individual gelatin capsules. Survival beyond 15 days was better at l5°C and 23°C. <i>A. rapae</i> larvae could be reared at densities of 30 to 60 larvae per 70±10 g on a high wheat germ artificial diet. Pupae from the lowest density were larger in length and weight than those reared at the two higher densities, and all laboratory-reared pupae were smaller than either normal or parasitized field pupae. / Ph. D.