Running title: Parasitoids associated with the diamondback moth in South Africa

Abstract

Seasonal fluctuations of diamondback moth and its hymenopteran parasitoids were recorded weekly from April 1997 to November 1999 at four cabbage sites in the Grahamstown area of the Eastern Cape, South Africa. Two sites were commercial farms with active spraying programmes; the others were unsprayed. Infestation levels were highest during spring (September to November) and autumn (March to May), where 100% infestation of plants was reached at times. The highest infestation was found during the spring months, where 12 larvae/ plant were found at the unsprayed sites and between 6 and 10 larvae at the sprayed sites. At the unsprayed sites abundance of diamondback moth larvae and parasitoids was high during 1997, but much lower during 1998 and 1999, indicating possible control by the parasitoids.

Nine species of parasitoid were recorded from diamondback moth during this period and four (Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae), Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae), Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae) and Oomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae)) showed potential as biological control agents. The highest rate of parasitism was found from mid-autumn to the beginning of winter (April to June) and from mid-spring to the beginning of summer (October to December). Percent parasitism varied throughout the year, ranging between 10% and 80%. Parasitism of 100% was observed when moth numbers were low. Different species of parasitoids were found to be dominant at different times of the year.

Key Words

Parasitoids, diamondback moth, South Africa

Introduction

Crucifiers, especially cabbage, Brassica oleracea var. capitata, are important crops, forming the staple diet for many South Africans and are grown on both a small and large scale. 80% of small-scale rural farmers grow cabbage as a subsistence crop and commercially, 160,000 tons/ annum are harvested (Charleston 1998). Control of the cosmopolitan cabbage pest, Plutella xylostella (L.) (Lepidoptera: Plutellidae) diamondback moth is difficult as it has become resistant to nearly all insecticides used against it (Talekar and Shelton 1993). For this reason the use of biological control agents, particularly parasitoids has become important.

South Africa is fortunate to have a large number of parasitoids, many of them indigenous, that are associated with the diamondback moth and that can provide suitable control in particular circumstances. Ullyett (1947) recorded 14 species of primary parasitoids in the Pretoria region and more recently Kfir (1998) recorded 22 species. The pest status of diamondback moth is much lower in South Africa than in other parts of the world with similar climates (Kfir 1998), but the pest still causes serious damage.

Materials and Methods

Four study sites were selected. Site 1(33°19'S; 26°36'45" E) and Site 2 (33°19’45”S; 26°38’30”E) were large-scale commercial farms, growing cabbage as a cash crop. At the other two sites, Site 3 (33°29'30"S; 26°09'15"S) and Site 4 (33°18'30"S; 26°33'S) cabbage was grown on a smaller scale and solely for the purpose of the study. All sites were in the vicinity of Grahamstown, Eastern Cape. Site 1 was sampled during 1997 and January 1998, after which cabbages were no longer grown at this site. Sampling continued at the second commercial farm (Site 2) from February 1998 until the end of the study in November 1999. Spraying programs, mainly using synthetic pyrethroids, were in effect at both Site 1 and Site 2. At Site 3, sampling was carried out from April 1997 until November 1999, but rearing of larvae and pupae to determine parasitoid emergence only started in August 1997. Cabbages had not been grown in this area before. Sampling at Site 4 was carried out from August 1997, but rearing of larvae and parasitoids only started in January 1998. Sampling was inconsistent at this site, as there were periods when cabbages were not grown. Site 3 and 4 were not treated with insecticide.

Once a week 30 randomly selected cabbages were sampled at each site. Diamondback moth larvae and pupae and parasitoid cocoons were recorded. Larvae and pupae were collected and reared out in the laboratory, at 24°C and 16:8 hours (L:D), to determine moth emergence and parasitism levels in the field. The date of pupation and emergence of either moth or parasitoid were recorded. Parasitism was related to the sampling date and not the emergence date.

Results

Infestation levels of Plutella xylostella

The percentage of cabbage plants infested by P. xylostella follows a similar pattern at each site with the infestation levels being highest during spring, from September to November. Infestation levels of the plants (Figure 1) at all sites were low until the beginning of August 1997. From then until the end of October 1997 infestation was high, reaching 100% at all sites. These levels decreased over November and December 1997 and remained below 40% at the two unsprayed sites (Site 3 and 4) until the beginning of September 1998. Infestation levels at Site 3 and 4 increased during spring1998, reaching 60% and 97% infestation, respectively. In 1999 infestation levels remained below 40% until August where it increased, reaching 68% and 86%, at Site 3 and 4 respectively. At Site 2, infestation levels remained high during 1998, generally above 50%. In 1999, infestation levels dropped slightly, but by October 1999 infestation reached 100%.

Figure 1. Infestation levels of P. xylostella at four sites in Grahamstown, Eastern Cape, South Africa, over the study period April 1997 to November 1999.

The highest numbers of larvae per plant were recorded during spring (September to November) (Figure 2). At Site 1 and 2, the sprayed sites, 7 larvae/ plant were recorded in 1997 and 10 larvae/ plant in 1998, respectively. At Site 3, the unsprayed site, 12 larvae/ plant were found in 1997. In 1998 and 1999 it dropped to below 2 larvae/ plant. At the second unsprayed site (Site 4) numbers remained below 4 larvae/ plant.

Figure 2. Mean number of larvae/ plant at four sites in Grahamstown, Eastern Cape, South Africa, over the study period April 1997 to November 1999..

Plutella xylostella abundance

The abundance of P. xylostella larvae and pupae varied between sites and between years. Figure 3 shows the abundance of the larvae and pupae at Site 2, a sprayed site and at Site 3, an unsprayed site. Abundance increased during August, reaching a peak over September to November. There were additional peaks in abundance over the autumn months, from March to May at some of the sites. At the sprayed site (Site 2) abundance of larvae and pupae were generally always high. At the unsprayed site (Site 3) abundance of larvae and pupae was high in 1997, but decreased drastically in 1998 and 1999.

Figure 3. Abundance of P. xylostella larvae and pupae at Site 2 and 3 in Grahamstown, Eastern Cape, South Africa, over the study period April 1997 to November 1999.

Parasitoid species present

Nine species of parasitoid wasp were recorded from diamondback moth larvae and pupae (Table 1), including primary and hyperparasitoids. The different species attacked different stages of the P. xylostella life cycle. They differed in abundance throughout the year and between sites. Apanteles eriophyes Nixon, Itoplectis sp. and hyperparasitoids were found in very low numbers.

Table 1. Parasitoids and hyperparasitoids associated with the diamondback moth in the Eastern Cape, South Africa.

Species	Family	Stage of Attack
Cotesia plutellae (Kurdjumov)	Braconidae	Larval
Apanteles eriophyes Nixon	Braconidae	Larval
Diadegma mollipla (Holmgren)	Ichneumonidae	Larval - pupal
Diadromus collaris Gravenhorst	Ichneumonidae	Pupal
Oomyzus sokolowskii (Kurdjumov)	Eulophidae	Larval - pupal
Itoplectis sp.	Ichneumonidae	Larval - pupal
Mesochorus sp.	Ichneumonidae	Hyperparasitoid
Pteromalus sp.	Pteromalidae	Hyperparsitoid
Proconura sp.	Chalcididae	Hyperparasitoid

Abundance of parasitoids

The abundance of the different parasitoid species at two sites, Site 2 and 3, are shown in Figure 4. Cotesia plutellae (Kurdjumov) was most abundant at Site 2, a sprayed site, throughout the study period. At Site 3, an unsprayed site, C. plutellae, Diadegma mollipla (Holmgren) and Diadromus collaris Gravenhorst and Oomyzus sokolowskii (Kurdjumov) were abundant during spring of 1997, but their abundance dropped severely in 1998 and 1999 as a result of the low abundance of P. xylostella larvae and pupae in the field.

Figure 4. Abundance of the four common parasitoids at Site 2 and 3 in Grahamstown, Eastern Cape, South Africa, over the study period April 1997 to November 1999.

Percentage Parasitism

The percentage parasitism by the different parasitoid species varied between the sites. Figure 5 shows the percentage parasitism at Site 2, a sprayed site, and at Site 3, an unsprayed site. The highest rates of parasitism by C. plutellae (88%) were recorded at the sprayed site, Site 2. The rates of parasitism by other species remained below 25%. At the unsprayed site, Site 3, parasitism by C. plutellae was generally below 60%, but parasitism by other species increased. At Site 3 parasitism rates reached 100% at times, probably due to the low infestation levels of diamondback moth larvae and pupae in the field.

Figure 5. Percentage parasitism of the four common parasitoids at Site 2 and 3 in Grahamstown, Eastern Cape, South Africa, over the study period April 1997 to November 1999.

Discussion

The infestation level of diamondback moth at all the sites increased almost simultaneously at the end of the winter period and remained high, in some cases 100% of plant infestation was recorded for the first two months of the spring period in 1997. At Site 2, a commercial farm, infestation remained high over the entire sampling period even with the weekly use of pesticides. Cypermethrin was the common pesticide used and resistance to this pesticide has been found in other parts of South Africa (Sereda et al. 1997). The high infestation levels at Site 2 could indicate pesticide resistance. Ullyett (1947) found that in the Pretoria region of South Africa the numbers of P. xylostella were highest in early summer and reached 11.6 larvae/ plant, but this did not result in serious economic damage. The mean number of larvae per cabbage in the Eastern Cape increased at the end of the winter period and numbers remained at their highest (12 larvae/ plant) until late spring.

Cabbage is grown all year round in the Eastern Cape. Because of this continuous resource and the mild climatic conditions, P. xylostella is present all year, but seasonal variation occurs and low numbers are found in winter months. Both moth and parasitoid abundance is highest in spring, from September to November, with additional peaks in abundance in autumn (March to May). The different parasitoid species show seasonal variation between years and between sites. C. plutellae was the most abundant parasitoid at sprayed sites, Sites 1 and 2, and was found in much lower abundance at unsprayed sites, Site 3 and 4. However, D. mollipla, D. collaris and O. sokolowskii were more abundant at unsprayed sites than at sprayed sites. This suggests that C. plutellae may have developed resistance to pyrethroids used at the sprayed sites.

At Site 3, the unsprayed site, all four major parasitoids were present and showed the highest abundance in the spring of 1997, in 1998 and 1999 the abundance of the moth and its parasitoids decreased drastically. This suggests that the parasitoids were able to establish and reduce the population of diamondback moth to a very low level after the first year. It also suggests that it is important that there is a complex of parasitoids, working together to reduce the diamondback moth population. A similar pattern was found at the second unsprayed site, Site 4, with diamondback moth larvae and pupae decreasing after the first year.

There is a complex of parasitoids in the Eastern Cape region of South Africa that are capable of providing suitable control against P. xylostella if they are allowed to establish in a pesticide-free environment. Hyperparasitoids are present in very low numbers and as a result did not reduce the effectiveness of the primary parasitoids.

Acknowledgements

We would like to thank Dr. Rami Kfir (Plant Protection Research Institute, Agricultural Research Council, Pretoria, South Africa) for identifying the parasitoids and for his continued help and support during this project. Thanks also to Rhodes University and USDA-ARS for financial assistance.

References