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 (L.), is a pest on crucifer crops worldwide, damaging the leaves, florets and seed pods of many crucifers including cabbage, cauliflower, broccoli and canola. It has been controlled using broad-spectrum insecticides, but this has led to a rapid build-up of insecticide resistance. In the Grahamstown area of the Eastern Cape Province, South Africa, diamondback moth showed resistance to cypermethrin (a pyrethroid) on commercially grown cabbages. Therefore it is imperative that other methods of control be adopted, including both cultural control and biological control using parasitoids, and that these are incorporated into an Integrated Pest Management (IPM) programme. The diamondback moth and its parasitoids were monitored weekly from April 1997 to November 1999 at three sites near Grahamstown. One site was a commercial farm with an active insecticide spraying program; the others were unsprayed. Infestation levels were highest during spring (September to November) and autumn (March to May). Nine species of parasitoids were associated with the diamondback moth, with abundances being highest over spring and early summer (September to December). Cotesia plutellae (Kurdjumov) dominated the sprayed site, while the unsprayed sites yielded a complex of parasitoids, including C. plutellae, Diadegma mollipla (Holmgren), Diadromus collaris Gravenhorst and Oomyzus sokolowsldi (Kurdjumov). Parasitism levels ranged between 10 and 90%. There was a large amount of site-to-site and year-to-year variation. Parasitoids were an effective mortality factor against the diamondback moth. The effects of temperature on development and mortality, and of field size and non-crop plants on the distribution of diamondback moth and its parasitoids, were investigated. The results show that high temperatures can depress pest populations, and that the size and surroundings of fields can be manipulated to improve cultural control of the diamondback moth. Suggestions for effective rPM in the Eastern Cape Province include a reduction in insecticide applications, the use of bioinsecticides, for example Bacillus thuringiensis Berliner (Bt) and the encouragement of indigenous parasitoids by planting suitable nectar sources. Cultural control methods are also important and involve removal of cabbage refuse after harvest, management of wild crucifers around cabbage fields, scouting and monitoring the moth population and determining the optimal field size to assist with control by parasitoids.

Canola, Brassics napus L. is a relatively new crop in South Africa. Insect pests have not yet been a major problem, but the notorious brassica specialist, diamondback moth (DBM), Plutella xylostella (L.) (Plutellidae) is establishing itself as a serious pest of this crop. DBM is the most important insect pest of plants from the family Brassicaceae throughout the world. It has developed resistance to all chemical pesticides used against it in the field and to toxins of the bacterium Bacillus thuringiensis. The pest status of DBM in South Africa is lower than in other countries with similar climates. However, due to indiscriminate use of pesticides, local populations of DBM are showing signs of resistance. An initial survey has indicated that in addition to DBM, canola is also attacked by aphids, Myzus persicae (Sulzer), Brevicoryne brassicae (L.), Lipaphis e1ysimi (Kaltenbach), (Aphiade), thrips, Thrips tabaci (Lindeman) and other pests mostly brassica specialists. The study was initiated to determine the composition of the community of insects found on canola, the seasonal phenology of DBM populations in canola, and the composition, relative abundance and seasonality of its parasitoids. Monitoring of the insects was carried out at weekly intervals for three years at Rietondale and Bapsfontein in Gauteng province of South Africa. Berlese funnels have been found to be useful in extracting insects from plants, and were used to indicate the presence ofDBM larvae and other insects found on canola. Adults of DBM were monitored with synthetic pheromone traps; larval and pupal populations were monitored by scouting canola plants. Samples of larvae, pupae and parasitoid cocoons were brought into the laboratory. Parasitoids that emerged were identified and their incidence recorded. Monolepta cf bifasciata (Chrysomelidae) and Listroderes costrirostris (Schoener) (Curculionidae) were the most abundant of the coleopteran pests. (DBM) and Heliothis armigera (H.) (Noctuidae) were most abundant lepidopteran pests of Canola. There was a high proportion of first and second instar larvae as indicated by the results of the Berlese funnels as compared to visual scouting in Bapsfontein. From May to August the infestation level of DBM was high, reaching the maximum of 0.25 larvae per plant in June 1996, then declined and remained low for the rest of the season in Rietondale. From September to December for all three years of the study, the population levels Of DBM were high, reaching a maximum of 9.6larvae per plant in September 1997, and remained low from January to August in Bapsfontein. The number of adult moths per trap per week ranged from 0 to 91 in Rietondale, peaking in January 1996 and September 1997. There was no correlation between infestation levels and the pheromone trap catches. In contrast to Rietondale, there was a high correlation between pheromone trap catches and subsequent larval infestations at Bapsfontein. Although DBM infestation levels were generally low, parasitism levels often reached 100% caused by a complex of parasitoids. During the period of study, the following hymenopteran parasitoids were recorded: Cotesia plutellae (Kurdjumov) and Apanteles eriophyes (Nixon), Braconidae), both larval parasitoids, Diadegma mollipla (Holmgren) (Ichneumonidae), and Oomyzus sokolowskii (Kurdjumov) (Eulophidae), larval-pupal parasitoids, Diadromus collaris (Gravenhorst) (Ichneumonidae) pupal parasitoid, and the hyperparasitoids Mesochorus sp. (Ichneumonidae) and Pteromalus sp. (Pteromalidae). Cotesia plutellae was the most abundant parasitoid occurring throughout the year..

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)

Thesis (Ph.D.)--University of Adelaide, Dept. of Applied and Molecular Ecology, 2001. / Includes bibliographical references (leaves 204-230).

Includes bibliographical references (leaves 204-230). Investigates the behaviour and ecology of the two major larval parasitoids of the diamondback moth, Plutella xylostella; Cotesia plutellae and Diadegma semiclausum.

Includes bibliographical references (leaves 204-230). / xvi, 230 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Investigates the behaviour and ecology of the two major larval parasitoids of the diamondback moth, Plutella xylostella; Cotesia plutellae and Diadegma semiclausum. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied and Molecular Ecology, 2001

Includes bibliographical references (leaves 204-230). / xvi, 230 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Investigates the behaviour and ecology of the two major larval parasitoids of the diamondback moth, Plutella xylostella; Cotesia plutellae and Diadegma semiclausum. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied and Molecular Ecology, 2001

The susceptibility of a population of diamondback moth, Plutella xvlostella L (Lepidoptera: Plutellidae), collected from Montgomery County, Virginia, and its ability to develop resistance to Bacillus thuringiensis was evaluated. The field population of diamondback moths was found to be susceptible to B. thuringiensis. Selection pressure at 153 I.U. of B. thuringiensis endotoxin per ml for nine generations did not cause any significant difference in mean mortality of third instar diamondback moths although there was a trend towards lower mortality, there was significant negative linear correlation between generation and mean mortality of diamondback moth larvae (P = 0.003, r²= 0.73). LC₅₀ increased from 264 I.U. of B. thuringiensis endotoxin per ml in generation I to 514 LU/ml in generation IX. The interrelationship of B. thuringiensis to diamondback moth and its primary parasitoid, Diadegma insulare Cress (Hymenoptera. Ichneumonidae), was studied by determining: the differential response of third instar diamondback moth, parasitized and unparasitized, to B. thuringiensis, and the ability of D. insulare to discriminate between B. thuringiensis-treated and untreated hosts. There was no significant difference (P > 0.05) between mean mortality of parasitized and unparasitized larvae at each of the three concentrations consisting of 154, 334, and 2237 I.U. of B. thuringiensis endotoxin per ml. The regressions of the response of parasitized and unparasitized larvae, however, were highly significant (P = 0.0001). The LC₅₀s of parasitized versus unparasitized larvae were 373 and 176 I.U./ml B. thuringiensis endotoxin, respectively. Female D. insulare did not discriminate between B. thuringiensis-treated and untreated hosts. The percentage of D. insulare females emerging from B. thuringiensis-treated larvae (41.4%) was not significantly different from that of untreated larvae (32.0 %). Mean mortality of third instar diamondback moth subjected to B. thuringiensis endotoxin at 153, 334, and 2237 I.U./ml were not significantly different at temperatures of 15 and 20 °C, but were significantly lower than that at 30 °C. The effects of B. thuringiensis endotoxin residues on leaves under room conditions [27 ± 1 °C, RH 27 ± 72%, and 8:16 (L:D)] were not significantly different at 2 and 192 hours after treatment. / Master of Science

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)