INTEGRATED CONTROL OF Plutella xylostella (L) (LEPIDOPTERA: YPONOMEUTIDAE) IN MAURITIUS

C Dunhawoor and D Abeeluck

Agricultural Research and Extension Unit

ABSTRACT

The diamondback moth (DBM), Plutella xylostella, is one of the most important pests of cruciferous crops in Mauritius. It is particularly damaging to cabbage and cauliflower. Development of resistance to insecticides was suspected in the early 1980’s when growers failed to obtain satisfactory control of the pest. Great efforts were made on screening of insecticides so as to provide growers with effective products. Recently, studies have been geared towards the development of an integrated pest management (IPM) programme. Natural enemies were introduced and parasitism up to 96% has been recorded in the field. Action-thresholds have been worked out to treat on a need basis. Observations on the seasonal occurrence in insecticide-free cabbage plots have shown that larval populations are relatively low in certain areas and periods of the year. Bacillus thuringiensis (Turex, Xentari and Dipel) has proved to be effective and eventually F1 sterility will be included in the IPM programme on DBM.

INTRODUCTION

Crucifers, particularly cabbage (Brassica olearacea var. capitata L.) and cauliflower (Brassica olearacea var. botrytis L.) are important vegetables grown in Mauritius; they are cultivated on about 400 hectares of land which produce about 9000 tons annually (Anon 1984). The cropping system varies from backyard gardens to large-scale farms. Among the pest complex of crucifers, Plutella xylostella, commonly known as the diamondback moth (DBM) is the most destructive. DBM was first recorded in Mauritius in 1945. Until 1980, this pest was controlled successfully by synthetic insecticides (Seewooruthun et al. 1984). In 1985, most insecticides were ineffective against DBM (Table 1) and the crucifer growers incurred heavy crop losses, especially in Moka and Plaines Wilhelms districts where cabbage and cauliflower are mostly grown.This led the growers to increase dosages of pesticides, frequencies of applications and even to use mixtures of insecticides to achieve synergistic effects. The DBM project was thus initiated in 1994 with the support of International Atomic Energy Agency (IAEA) to develop environmentally safe control measures that could be integrated and recommended as an integrated pest management (IPM)programme package to growers to reduce the pesticide load on these crops.

BIONOMICS OF DBM

DBM is a greyish microlepidopteran, 10 to 15 mm long, with pale triangular markings on the inner edge of each forewing, forming a diamond pattern when wings are folded at the back. The life history of DBM has been studied under laboratory conditions ( 19C to 25C ) and relative humidity 90 to 95 % (Seewooruthun et al. 1984). Under these conditions, females lay about 125 eggs in their lifetime. The eggs are yellow and laid on the main vein, on the upper and lower surfaces of leaves. The incubation period is 3 to 5 days. The four larval instars last for 1, 3-4, 2-3, and 1-3 days respectively and feed on leaves. The third and fourth instars are voracious feeders. The pupal period is 4 to 6 days. Adults fed on honey, live up to 31 days.

DISTRIBUTION AND SEASONAL ABUNDANCE OF DBM

Mauritius has a variable climate with four distinct climatic zones,and two seasons, summer (October–March) and winter (April–September). Crucifer production is principally concentrated in two regions within the super humid zones, namely Vacoas and Nouvelle Decouverte at altitudes of 428 and 520 meters respectively. But DBM occurs in all localities where crucifers are grown. Dry weather (summer) favours DBM build-up whereas during winter the population is reduced. Rainfall causes high mortality in all stages of DBM and thus decreases the pest population during the year. The young leaves of the host plants also influence the DBM population on the crop. DBM is most abundant forty five days after transplanting when many young leaves are present. The DBM population was evaluated on untreated plots in 2 localities, Plaisance and Curepipe during 1995 and 1996. Ten plants were chosen at random from a plot of 50 cabbage plants from each locality. The number of larvae per plant was recorded every 7 days. Monthly average results are summarised in Figure 1.

ACTION THRESHOLD

As a first step to reduce the frequency of insecticide applications, an action-threshold was worked out. Two types of thresholds were defined: presence of 0.5 larva per plant and 30 per cent of plants infested. Results were compared with weekly and fortnightly treatments and untreated control. The trials were arranged in a randomised block design with 40 plants per plot. Larval counts were recorded weekly on 20 plants per plot. When thresholds were reached, Abamectrin (Vertimec 1.8 EC) was applied at the rate of 4.5 g active ingredient per hectare. Assessment of the damage at harvest, modified from Beck and Cameron (1992) was as follows: wrapper leaves score 1-3, head score 1-6. A perfect undamaged cabbage with no feeding holes had a score of 2. Head scores of 2-3 gave reasonably good cabbages, scores of 4-5 were acceptable and scores of 6 were unmarketable. Heads were also weighed.

Two trials were performed at Plaisance and Reduit in October 1995. Results show that the number of sprays could be reduced. No significant difference was observed in the total damage index between weekly spraying and spraying at 30 % infestation (Table 2) and between weekly spraying and spraying at 0.5 larva per plant in one case. For scouting purposes, the percentage infestation method would be more practical for growers.

INTEGRATED PEST MANAGEMENT PROGRAMME OF DBM

As a first step towards a biological programme, the search for endemic natural enemies of DBM was started in 1985. One species of parasitoid, Diadegma sp (Hymenoptera: Ichneumonidae) was found. However, the incidence of this species on the pest in cabbage and cauliflower was very low (Anon 1985). Successful cases of biological control of DBM have been reported in other countries (Imam et al.1986; Sastrosiswojo and Sastrodidhardjo 1986). Two exotic parasitoid species, Cotesia plutellae, Kurdjumov (Hymenoptera: Braconidae) and Diadegma semiclausum Hellen ((Hymenoptera: Ichneumonidae) were introduced from the Asian Vegetable Research and Development Centre, Taiwan in 1991. These parasitoids were reared in the laboratory and released in selected crucifer growing areas. From 1992, releases of parasitoids were extended to all crucifer-growing areas. The levels of parasitism by C. plutellae were highest during April and during September (Figure 2).

EVALUATION OF BIOPESTICIDES

To maximise the use of biological control agents, it was necessary to replace synthetic chemicals with biological insecticides. Bacillus thuringiensis (Bt) is a bacterial pesticide toxic to a limited number of pests, non-toxic to most beneficial insects and completely safe for man. When used properly, Bt is reported to be effective against DBM. Marketed strain of Bacillus thuringiensis (Turex) was tested in the laboratory.

Laboratory screening of Turex

Bacillus thuringiensis (Turex) was tested against DBM in the laboratory using the FAO method (FAO Plant protection method No. 35). A single leaf was dipped in diluted Turex (2 and 3 g l-1 of water) and allowed to dry. Ten larvae of 3rd and 4th instar were then exposed to each leaf (3 replicates per each concentration). Mortality was recorded every 24 hours. Feeding was noticed during the period from 24 to 48 hours. After 48 hours, larval mortality was above 86% in both cases (Figure 3).

On-farm trial of biopesticides and insecticides

Three strains of Bacillus thuringiensis (Turex, Xentari and Dipel) and one synthetic insecticide (Mospilan 20 W/W ) were tested against DBM. A field was selected at Carreau Laliane and divided randomly into 4 replicates, each containing 50 plants. Ten plants were randomly selected from each plot and examined. There were about 5 larvae per plant on the 3rd week after transplanting. Treatments were applied on the 3rd and 5th weeks after transplanting as follows: Turex 2 g l-1, Xentari 2 g l-1, Dipel 2 g l-1, Mospilan 1 g l-1 of water, and a control (untreated). Larval counts showed that Xentari and Mospilan were most effective (Figure 4).

DISCUSSION

The exotic parasitoid, C. plutellae, is well established in Mauritius and the level of parasitism has reached up to 96 % in 1994. But the farming community still relies on synthetic chemicals that have deleterious effects on natural enemies. The concept of treating only when the threshold level is exceeded is fundamental to IPM. Growers’ education in the implementation of a biopesticides-IPM system would no doubt help in maximising the use of biological control agents. Research on new technologies such as the use of F1 sterility technique in conjunction with a biological control programme and a trap crop is being carried out.

ACKNOWLEDGEMENTS

The authors are most grateful to Professor E L. Roberts. His criticism on the manuscript is greatly appreciated. They also extend their gratitude to Mr R. Ramnauth who willingly provided statistical advice. They also acknowledge the assistance of Mr R. Padaruth and Mr Y. Mungroo in the preparation of the manuscript.

REFERENCES

ANON. 1984. Digest of Agricultural Statistics. Statistical Office, Ministry of Agriculture and Natural Resources, Mauritius, p 55.

ANON. 1995. Plutella xylostella. Annual Report Ministry of Agriculture and Natural Resources, Mauritius. (internal / unpublished)

BECK NG and CAMERON PJ. 1992. Developing a reduced spray program for Brassicas in New Zealand. p. 341 - 350. In: TALEKAR NS ed. 1992. Diamondback moth and other crucifer pests. Proceedings of the second international workshop, Tainan, Taiwan, 10 - 14 December 1990. AVRDC. AVRDC Publication no. 92-368.

IMAN M, SOEKARMA D, SITUMORANG J, ADIPUTRA IMG, and MANTI I. 1986. Effects of insecticides on various field strains of Diamondback moth and its parasitoids in Indonesia. p 315-323. In: AVRDC. Diamondback Moth Management- Proc First International. Workshop, Taiwan, Asian Vegetable Research and Development Center.

SASTROSISWOJO N and SASTRODIDHARDJO S. 1986. Status of biological control of diamondback moth by introduction of parasitoid, Diadegma eurocephaga in Indonesia. p 185-194. In : AVRDC. Diamondback Moth Management- Proc First International Workshop, Taiwan, Asian Vegetable Research and Development Center.

SEEWOORUTHUN SI, DUNHAWOOR C and LI TIN WAI M. 1984. Plutella xylostella. Annual Report of the Ministry of Agriculture and Natural Resources, Mauritius. (internal/ unpublished ).

COMMENTS

Q. Is there anything that the farmer need to do to increase the level of parasitism?
A. At the Ministry, we are raising parasites at our own costs for release. We intend to ask farmers not to use insecticides but use B. thuringiensis instead.

Q. In your studies have you seen any preference shown by Cotesia plutellae for 3rd and 4th larval stages?
A. 3rd and 4th larval stages appear to be preferred as these have enough food material to support the development of the parasite.