Abstract

An insecticidal exclusion method was used to assess the effect of parasitoids on level of infestation by the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), in cabbage. In field trials conducted at Rietondale, Pretoria, Gauteng Province and Brits, North-West Province, South Africa, a selective organophosphate pesticide, with systemic and contact action, dimethoate, was applied twice weekly to three plots in cabbage fields that had been divided into six plots. The three remaining plots served as controls. At weekly intervals, ten plants were randomly selected from each plot and thoroughly scouted for DBM infestation. Infestation levels were recorded and larvae, pupae and parasitoid cocoons were collected and taken to the laboratory. To determine parasitism levels all collected larvae were kept individually in Petri dishes with fresh cabbage leaves, and pupae and parasitoid cocoons were kept individually in glass vials until either parasitoids or moths emerged. Incidences of parasitism were high in the control plots, peaking above 90% on several occasions. The fauna of DBM parasitoids was rich; during the study period 23 species of parasitoids and hyperparasitoids were identified. The most abundant parasitoids were the larval parasitoid Cotesia plutellae (Kurdjumov), the larval-pupal parasitoid Oomyzus sokolowskii (Kurdjumov), the pupal parasitoid Diadromus collaris Gravenhorst, and the hyperparasitoids Mesochorus sp. and Pteromalus sp. Egg parasitoids were not recorded. At both sites, infestation levels in the sprayed plots were significantly higher than those in the control plots. On the other hand, parasitism levels of DBM in the control plots were significantly higher than in the treated plots. It was concluded that the higher infestation level of cabbage by DBM in the sprayed plots was because of partial elimination of parasitoids by the pesticide.

Key words

diamondback moth, Plutella, Cotesia plutellae, parasitoid

Introduction

Diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) is the most damaging insect pest of cole crops in the world (Talekar & Shelton 1993). It has developed resistance to all major classes of chemical pesticides (Talekar et al. 1985) and to the bacterial insecticide Bacillus thuringiensis (Tabashnik et al. 1990). Early in the past century, DBM was studied in South Africa by Gunn (1917) where it was considered to be an important pest of cole crops (Annecke & Moran 1982). However, its pest status in South Africa is much lower than in other countries with similar climates (Kfir 1996). Ullyett (1947) studied the natural enemies of DBM in the Pretoria area in the 1930s and recorded eleven parasitoids, several predators and the fungus, Zoophthora radicans Brefeld (Zygomycetes: Entomophthorales) (recorded as Entomophthora sphaerosperma Fres.). He concluded that DBM is well controlled by its natural enemies in South Africa. Because of the low pest status of DBM in South Africa, almost no research has been conducted on the pest for almost 60 years since Ullyett’s work. A renewed interest started after farmers reported outbreaks of DBM in cabbage fields and difficulties controlling the pest with insecticides. It was shown that because of indiscriminate use of insecticides by farmers in South Africa, local field populations of DBM started to show signs of resistance to synthetic pyrethroids, organophosphates and carbamates (Sereda et al. 1997). In further studies, Kfir (1996, 1997a) recorded 22 species of parasitoids and hyperparasitoids of DBM larvae and pupae in South Africa. Because of the large number of indigenous plants from the Brassicaceae in South Africa, on which DBM can develop, and the large number of parasitoids of DBM in the region, Kfir (1998) speculated that DBM might have originated in southern Africa. This is in contradiction with the widely accepted theory that DBM had originated in the Mediterranean region of Europe, and spread around the world with the cultivated brassicas (Hardi 1938, Harcourt 1954).

The only published study on the effect of eliminating parasitoids on DBM populations is by Lim et al. (1986) in Malaysia. The results showed that Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae) the only important parasitoid at the time of the study, could contribute significantly to the control of DBM. The parasitoid fauna of DBM in South Africa is richer, more diverse and normally a higher proportion of the pest population is parasitised, compared with the situation in Malaysia (Kfir 1997b, Lim 1986). However, no published studies are yet available on the effect of parasitoids on DBM populations in South Africa or in other parts of Africa. High parasitism levels of larvae and pupae of DBM have been recorded in unsprayed cabbage crops in various parts of South Africa (Kfir 1997b, Waladde et al. 2001), but the effectiveness of parasitoids in reducing damage to crops by reducing DBM populations remained unknown.

The aim of this study was to examine the effects of removing parasitoids from a cabbage crop on DBM populations in South Africa. Experimental data on the effect of parasitoids are important for their conservation as resident natural enemies for the control of DBM.

Materials and methods

A plot of 1100 m² was transplanted on 29 July 1998 with cabbage, Brassica oleracea var. capitata L. of the cultivar Green Star at Hartebeespoort Agricultural Research Farm near Brits (25^o38'S; 27^o47'E; elevation 1102 m), North-West Province, South Africa. An identical plot was transplanted on 8 August 1998 at Rietondale Experimental Farm in Pretoria (25°44'S, 28°13'E, elevation 1333 m), in Gauteng Province, South Africa. Previous studies at Hartebeespoort near Brits indicated that the number of diamondback moths caught in pheromone traps and DBM larval infestations in cabbage were low from January to August and much higher during September to December, peaking during the spring months of September-October (Kfir 1997b). The planting dates in this study were chosen to coincide with high populations of DBM in the field to ensure maximum natural infestations. Each plot was divided into six subplots of 160 m² each. The remaining planted area served as buffer between the treated and untreated plots.

To suppress natural enemies, a selective insecticide, dimethoate, an organophosphate compound with both systemic and contact action (emulsifiable concentrate 400 g/ L active ingredient) at a concentration of 4 ml per 10 litres water, was used. Agral^® was added as a wetting agent. In California, dimethoate was used to suppress predators in cotton plots, which in turn caused an increase in abundance of Spodoptera exigua Hübner (Eveleens et al. 1973) and Trichoplusia ni Hübner (Ehler et al. 1973). This was an indication that dimethoate suppresses insect natural enemies, but causes no harm to Lepidoptera. Dimethoate was sprayed twice weekly with a knapsack sprayer on three subplots, whereas the remaining three subplots served as controls. Spraying started two weeks after transplanting the cabbage seedlings in the field and lasted until the trials were terminated.

Ten plants were selected randomly from each subplot and thoroughly scouted for DBM larvae, pupae and parasitoid cocoons. Samples were taken to the laboratory where all live larvae were kept individually in Petri dishes. Pupae and parasitoid cocoons were kept individually in glass vials (2.5 x 10 cm) stoppered with cotton wool. Larvae were provided with fresh cabbage leaves, which were replaced every third day, until larvae pupated or parasitoid cocoons formed. Collected pupae were kept until either parasitoids or moths emerged. Insects were held in the laboratory at 23°C ± 2°C, 60 ± 5% RH. All emergent parasitoids were identified and their incidence calculated. Larvae that escaped or died of unknown causes were disregarded for calculating rates of parasitism. For presentation of results, the data from the three subplots of each treatment were pooled.

For the duration of the trials, three delta-shaped sex-pheromone traps were deployed in each site to monitor the flight pattern of male moths. In the traps, sticky floors coated with a layer of polybutene adhesive were used to trap the moths. The sticky floors were replaced weekly when the traps were examined and moth catches recorded. The pheromone dispensers were placed in the middle of the sticky floor within the metal trap (26 x 9.5 x 13 cm high). The dispensers were replaced once a month.

All voucher specimens have been deposited in the National Collection of Insects, Biosystematics Division, ARC-Plant Protection Research institute, Pretoria.

Statistical analysis

The data were analysed using the statistical program GenStat (GenStat Committee 2000).

A t-test between two independent samples (Snedecor & Cochran 1967) was used to indicate significant differences between DBM population levels (larvae and pupae) on sprayed and control plots at Brits and Rietondale.

The difference between the proportions of infested plants in the sprayed and the unsprayed plots, and the difference between the proportions of parasitised DBM in the sprayed and the unsprayed plots was tested by using the Generalized Linear Model (GLM) (Dobson 1990) with the binomial distribution. The GLM with the binomial distribution was used because the proportions of infested plants and the proportions of parasitised DBM were not normally distributed.

Results

At Brits, the number of male moths caught in the pheromone traps increased gradually from around 4 moths/ trap/ week in July, peaked during the second half of October at 45 moths/ trap/ week and then declined to very low levels during December (Figure 1a). At Rietondale, the number of moths caught was very low during September, increased sharply from the end of October to reach a peak of 60 moths/ trap/ week in the middle of December, and declined sharply to low levels during February (Figure 1b).

Figure 1. Sex pheromone trap catches of diamondback moth, Plutella xylostella, male moths.
Bars represent standard errors (SE) when larger than symbol size
a. At Brits. b. At Rietondale, South Africa.

The peaks of moth flights (Figure 1) coincided with the peaks of larval infestations on the crops (Figure 2). At Brits, populations were relatively low during August and the first half of September, around 1 DBM/ plant, and then increased rapidly from the second half of September, peaking during the second half of October at 40.7 and 12.4 DBM/ plant in the sprayed and control plots, respectively (Figure 2a). At Rietondale, populations were low during September and the first half of October increasing rapidly in the sprayed plots to peak at 27.7 DBM/ plant during the second half of December. In the control plots, populations fluctuated around two DBM/ plant peaking at 4.7 DBM/ plant at the same time as the treated plots (Figure 2b).

Figure 2. Abundance of diamondback moth, Plutella xylostella, larvae and pupae on sprayed (triangles) and control (circles) cabbage. Bars represent standard errors (SE) when larger than symbol size
a. At Brits. b. At Rietondale, South Africa.

At the two study sites, population levels of DBM on the sprayed plants were significantly higher than on the control plants (Figure 2). At Brits, a total of 8205 DBM larvae and pupae were collected from the sprayed plants and 1607 from the control plants (t = -16.59, 4 d.f., P = < 0.001). At Rietondale, 3648 DBM were collected from the sprayed plants compared with 734 DBM from the control plants (t = -16.28, 4 d.f., P = < 0.001).

The proportion of infested plants in the sprayed plots at the two sites was higher than in the unsprayed plots (Figure 3). At Brits, 100% of infested plants was recorded in the sprayed and the control plots. However, in the sprayed plots (seasonal mean of 81.8%) this was reached by the second half of September and lasted for the remaining of the season, a period of 12 weeks. In the control plots (seasonal mean of 56.7%), however, 100% infestation was also recorded by the second half of September, but it lasted only to the second half of October, a period of five weeks, and then declined rapidly (Figure 3a). At Rietondale, proportion of infested plants in the sprayed plots (seasonal mean of 73.5%) peaked at 100% from the middle of November to early in January whereas in the control plots (seasonal mean of 52.4%) proportion of infested plants peaked at about 93% in the middle of December (Figure 3b).

Figure 3. Proportion of sprayed (triangles) and control (circles) cabbage plants infested by diamondback moth, Plutella xylostella, larvae and pupae
a. At Brits. b. At Rietondale, South Africa.

The GLM analysis indicated that at Brits, the sprayed cabbage plants were about three and a half times (3.426) more likely to be infested with DBM than the unsprayed plants, whereas at Rietondale the sprayed plants were two and a half times (2.515) more likely to be infested than the unsprayed plants. At both sites the sprayed and control plots were highly significantly different (P<0.001) (Table 1).

Table 1. Mean proportions and Standard errors of the mean (SEM) of cabbage plants infested by diamondback moth, Plutella xylostella, in control and sprayed plots at Brits and Rietondale, South Africa

	Brits		Rietondale
	Mean proportion	SEM	Mean Proportion	SEM
Control plots	0.5667	0.0208	0.5246	0.0209
Sprayed plots	0.8175	0.0162	0.7351	0.0185

Percent parasitism of DBM at both sites throughout the season was higher on the unsprayed plots (Figure 4). At Brits, in the sprayed plots, percent parasitism fluctuated around 5% throughout the season (seasonal mean of 4.9%). In the control plots, however, parasitism levels increased rapidly to above 90% towards the end of the season (seasonal mean of 65.9%) (Figure 4a). At Rietondale, parasitism in the sprayed plots fluctuated around 10% with a peak of 17.9% in middle of December (seasonal mean of 12.8%) (Figure 4b) coinciding with peak of larval infestation (Figure 1b). In the control plots, parasitism rose quickly and remained high (70-95%) from the middle of November to the middle of January (seasonal mean of 64.9) (Figure 4b).

Figure 4. Percentage parasitism of diamondback moth, Plutella xylostella, larvae and pupae on sprayed (triangles) and control (circles) cabbage. Numbers represent sample size.
a. At Brits. b. At Rietondale, South Africa.

At Brits, the GLM analysis indicated that DBM infesting the sprayed plants were about 59 times (1/0.01704) less likely to be parasitised than DBM infesting control plants. At Rietondale, insects on sprayed plants were 20 times (1/0.0497) less likely to be parasitised than on control plants. At both sites the proportions of parasitised DBM in the sprayed and unsprayed plots were significantly different (P < 0.001) (Table 2).

Table 2. Mean proportions and Standard errors of the mean (SEM) of parasitised diamondback moth, Plutella xylostella, infesting cabbage plants in control and sprayed plots at Brits and Rietondale, South Africa

	Brits		Delmas
	Mean Proportion	SEM	Mean Proportion	SEM
Control plots	0.7096	0.0113	0.7725	0.0155
Sprayed plots	0.0400	0.0022	0.1444	0.0058

No egg parasitoids were recorded in the current study. Two egg-larval parasitoids were recorded; Chelonus curvimaculatus Cameron and Chelonus sp. (Hymenoptera: Braconidae). The most abundant larval parasitoid was the solitary endoparasitoid, Cotesia plutellae. Other larval parasitoids were Apanteles halfordi Ullyett (Hymenoptera: Braconidae), Cotesia sp., Habrobracon brevicornis (Wesmael) (Hymenoptera: Braconidae) and Peribaea sp. (Diptera: Tachinidae). Recent taxonomic studies suggest that A. halfordi is a senior synonym of Apanteles eriophyes Nixon, a matter that will be dealt with in the taxonomic literature (G.L. Prinsloo, personal communication). This species is specific to P. xylostella and is known only from South Africa (Walker & Fitton 1992). Three larval-pupal parasitoids were recorded. The most abundant was Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae), which is the only gregarious primary parasitoid of P. xylostella. Oomyzus sokolowskii occasionally acted also as a hyperparasite and emerged from cocoons of C. plutellae. The other larval-pupal parasitoids were Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae) and Itoplectis sp. (Hymenoptera: Ichneumonidae). Recently, Azidah et al. (2000) taxonomically revised the species of Diadegma attacking P. xylostella and reported that D. mollipla is an Afrotropical species occurring also on some Indian and South Atlantic islands. It is also a parasitoid of the potato tuber moth, Phthorimaea operculella (Zeller). Broodryk (1971) studied the biology of D. mollipla in South Africa. The most abundant pupal parasitoid was Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae). Other pupal parasitoids were Brachymeria sp. and Hockeria sp. (Hymenoptera: Chalcididae), Tetrastichus howardi (Olliff) (Hymenoptera: Eulophidae) and an unidentified ichneumonid. Tetrastichus howardi is an introduced species in South Africa (Kfir et al. 1993). Hyperparasitoids were active when P. xylostella populations were high and primary parasitoids abundant. The most abundant hyperparasitoids were Mesochorus sp. (Hymenoptera: Ichneumonidae) and Pteromalus sp. (Hymenoptera: Pteromalidae). Both emerged from cocoons of their primary parasitoid hosts. Other hyperparasitoids were Aphanogmus fijiensis (Ferrière) (Hymenoptera: Ceraphronidae), Eurytoma sp. (Hymenoptera: Eurytomidae), Tetrastichus sp. (Hymenoptera: Eulophidae), Hockeria sp., Brachymeria sp. and Proconura sp. (Hymenoptera: Chalcididae). All these hyperparasitoids are solitary except A. fijiensis. Between four to seven A. fijiensis emerged from each cocoon of C. plutellae or A. halfordi.

Discussion

Ullyett (1947) and Kfir (1997a, 1997b) have studied the parasitoids of DBM in South Africa. Le Pelley (1959), Kibata (1997) and Oduor et al. (1997) have studied them in Kenya. A very rich fauna of indigenous parasitoids was recorded from DBM larvae and pupae in South Africa (Kfir 1997a). High parasitism levels, often above 90%, were recorded on unsprayed cabbage crops in North-West Province (Kfir 1997a) and in the Eastern Cape Province (Waladde et al. 2001) of South Africa.

During this study it was revealed that although dimethoate had substantial adverse effects on the parasitoids’ populations it could not completely eliminate them. Similar observations were reported from the eastern Cape Province of South Africa where parasitism by C. plutellae was observed in cabbage plots treated regularly with chemical insecticides such as methamidophos, mercaptothion, cypermethrin and others (Waladde et al. 2001). This might indicate some level of tolerance or resistance by the local populations of C. plutellae in South Africa to chemical pesticides. In Malaysia, Lim et al. (1986) recorded similar results when Sevithion (carbaryl + malathion) was sprayed on a cabbage field and did not completely eliminate the C. plutellae population.

Methods for evaluation of natural enemies have been developed, i.e. introduction and augmentation, cages or other barriers, removal of natural enemies, prey enrichment, direct observations and evidence of feeding (Luck et al. 1988). However, these methods are unique to interactions of particular parasitoid-host or predator-prey species groups (Luck et al. 1988). These methods are more suitable for the assessment of the effects of natural enemies of insects that form large colonies such as scale insects, aphids, whiteflies and mealybugs. Evaluation of parasitoids of Lepidoptera on the other hand is more complicated because of the relative low densities and the mobility of the pests and their parasitoids. Removal of natural enemies with insecticides, first described as the insecticidal check method by DeBach (1946), is a good experimental technique for evaluating the efficacy of natural enemies (Luck et al. 1999). It can be used to determine the level of control provided by parasitoids (Jones 1982, Kenmore et al. 1984, DeBach & Rosen 1991). The technique was used successfully in West Africa to evaluate the efficacy of the introduced parasitoid, Apoanagyrus lopezi (De Santis), in controlling the cassava mealybug, Phenacoccus manihoti Matile-Ferero without affecting the mealybug (Neuenschwander et al. 1986).

Insecticidal applications can stimulate Lepidoptera populations by altering the plant physiology. This could result from affecting the photosynthetic rate of plants (Jones et al. 1983) making them more attractive oviposition sites for Lepidoptera such as Helicoverpa zea Boddie and Heliothis virescens Fabricius. Although this aspect cannot be ruled out completely, there are no indications that the sprayed plots in this study attracted more oviposition by DBM than the control plots.

The five-fold increase in the DBM population levels in the treated plots after the partial removal of the parasitoids indicates that parasitoids play an important role in curtailing DBM populations in South Africa and without their activities, annual yield losses would be much higher.

Conclusion

The findings from the current study clearly demonstrate that the parasitoid complex in South Africa contributes significantly to the control of DBM. The higher infestation level of cabbage by DBM in the insecticide-treated plots was caused by partial removal of parasitoids.

Acknowledgements

I wish to thank Edith van den Berg (ARC- Biometry Unit) for the statistical analysis.

References