Abstract

This study reports a field trial to measure the impact of beneficial parasitoid wasps on diamondback moth, Plutella xylostella, in Perth, Western Australia. Cabbage seedlings each inoculated with a set number of P. xylostella eggs were planted amongst a commercial cabbage crop in April 2001. Inoculated plants were enclosed using one of four different cages designed to give naturally-occurring wasps varying levels of access to the test plants, ranging from restricted to full access. Test plants were protected from insecticidal sprays applied by the farmer. Three weeks later when most of the P. xylostella had pupated, numbers on test plants were counted and collected. These remaining P. xylostella were reared in the laboratory on canola to determine levels of parasitism within the different cages. Diadegma rapi and Diadegma semiclausum made up 47% and 39% respectively of the parasitoids found in addition to low numbers of Apanteles ippeus and Diadromus collaris. Plutella xylostella survival was highest (42%) in cages where wasps had no access to test plants and lowest (17%) where wasps had the greatest access to plants. In the treatment where wasps had the greatest access to test plants, 100% of remaining P. xylostella were parasitised after three weeks. High rates of parasitism by wasps, particularly by D. semiclausum and D. rapi, were encouraging despite regular applications of insecticide sprays to plants surrounding the test plants.

Keywords

parasitoids, Diadegma, Apanteles

Introduction

The caterpillar of the diamondback moth, P. xylostella, is the most destructive pest of crucifer vegetables in Australia and worldwide (Takelar and Shelton1993). To improve control of this pest, growers are utilising Integrated Pest Management (IPM) techniques, which includes the tactic of encouraging natural enemies.

The natural enemies of P. xylostella include a range of parasitoid wasp species (Hassell and Waage, 1984). Six species of parasitoid wasp, of which Diadegma semiclausum and Apanteles ippeus were the most common, were recorded in an earlier survey of farms surrounding the Perth Metropolitan area. Although these beneficial insects are often mentioned in the context of IPM, few studies have quantified their impact on pest populations. This report outlines a preliminary experiment conducted on a farm near the Perth Metropolitan Area which aimed to measure the impact of naturally occurring parasitoid wasps on P. xylostella populations.

Materials and Methods

The trial site was situated in a commercial cabbage crop located in Mandogalup, 50 km south of the Perth CBD, Western Australia.

Plants and insects

We used the same cabbage seedling variety (Green Coronet) as that planted in the test crop. Four-week old seedlings were exposed to egg lay by a laboratory colony of P. xylostella adults for a 48 h period. Each plant was then carefully examined and surplus eggs were removed to leave 20 eggs per seedling. Seedlings were then transplanted into 12.5 cm pots and planted amongst cabbage rows at the farmer's property (5 April 2001), spaced 1.5 m apart. Each inoculated plant was caged using exclusion cages designed to allow one of four different levels of natural enemy access to the P. xylostella on the test plants.

All cages consisted of a central frame (45 cm high x 45 cm diameter) made from trellis. Modifications to the fine nylon netting sleeve covering the central frame allowed construction of cages with the following treatments:

a) Full cage covered. The central trellis frame covered the plant and was totally covered with a nylon mesh sleeve. This treatment totally excluded natural enemies.

b) Frame only. The cage consisted of only the central trellis frame and gave total natural enemy access

c) Partial cage + sticky barrier. Nylon netting sleeve was used to partially cover the central trellis frame, and the rim of the pot was treated with a sticky barrier (Tac-gel^®). This limited natural enemy access to flying insects.

d) Partial cage only. Same as c) above, but pot rims without sticky barrier.

The treatments were arranged in a randomised block design, replicated 10 times. Treatment (c), where the central trellis frame was partially covered by the nylon netting, was designed to allow natural enemy access to the cage while creating ambient environmental conditions similar to that within the completely sealed cages. The sticky barrier prevented access to the plants by ground-dwelling natural enemies.

Counts of field insect populations were taken every seven days. The surrounding crop was treated for pests at the farmer’s discretion. Sprays were applied weekly during the trial, starting on 30 March 2001 and were (in order of application) Bacillus thuringiensis (Delfin^®, Novartis) + alpha-cypermethrin (Dominex^®, Cropcare), emamectin benzoate (Proclaim^®, Novartis), fipronil (Regent^®, Rhône-Poulenc) + methomyl (Electra^®, Farmoz) and Delfin^® + Dominex^®.

All cages were covered with plastic bags before spray application to protect test plants and insects from insecticide sprays. We terminated the experiment after three weeks (26 April, 2001) when most of the insects had pupated. At this time the appearance of each test plant was ranked as follows: 1 = no damage; then depending on per cent of leaf area skeletonised, 2 = 1 - 10%; 3 = 11 - 30%; 4 = 31 - 60%; 5 = 61 - 90%; and 6 = 91 - 100%.

All P. xylostella larvae collected at the end of the experiment were reared on cabbage leaves in the laboratory at 21°C. Any 3^rd instar or smaller larvae originating from wild P. xylostella eggs that were found on plants on 26 April 2001 were also recorded, but not retained. Mature wasps and P. xylostella moths that emerged were preserved and identified. Differences in P. xylostella survival and plant damage ratings amongst treatments were analysed using analysis of variance (ANOVA) tests.

Results

More P. xylostella survived in the treatment where natural enemies were excluded (a) (p = 0.002) and, of these, only 1% were parasitised (Table 1). Ground-dwelling natural enemies observed included carabid beetles and predatory mites (not identified to species level) which presumably also had some effect on the P. xylostella.

Table 1. Numbers of P. xylostella recovered 3 weeks after the trial commenced, % survival of original infestation and P. xylostella damage to leaves from different exclusion treatments using field cages.

The plants in the cages where the parasitic wasps could freely access the P. xylostella (treatment b) suffered significantly less damage (p = 0.005) than plants in the cages where the parasitic wasps had been excluded (treatment a) (Table 1).

Fewer P. xylostella survived in cages where both ground-dwelling and aerial natural enemies had access to test plants: frame only cage (b) or partial cage only (d) (17-19% survival). The sticky barrier of treatment c prevented the ground-dwelling natural enemies from accessing the eggs or larvae and P. xylostella survival was higher at 31% after three weeks. Of these, 40.9% and 13.1% respectively were parasitised by Diadegma rapi and D. semiclausum (Table 2). Diadegma semiclausum and D. rapi were the dominant species found across the trial and occurred in similar proportions.

Table 2. Species of parasitoid wasps recovered and % parasitism of surviving P. xylostella from different exclusion treatments using field cages.

*parasitism of >100% could be due to more than one wasp egg being laid developing in a single P. xylostella.

Discussion

Although the barrier cage (full cage treatment “a”) protected P. xylostella from parasitoids, 58% of the P. xylostella were not recovered and were assumed to have been killed by unknown factors. Of the survivors, only 1.2% were parasitised. Fully enclosing plants resulted in low levels of parasitism as wasps either had no access or very limited access (on the occasions when cages were briefly opened to examined test plants) to P. xylostella. In contrast, P. xylostella survivorship (17%) was lowest with the highest levels of parasitism (100%) in the frame treatment (b) because of constant exposure to natural enemies. Increased exposure to wind and rain may also have contributed to the lower survivorship of the DBM in the frame treatment (b).

It was encouraging that all the P. xylostella in one treatment were parasitised, demonstrating the effectiveness of the parasitoids. In addition, the two dominant wasp species, D. rapi and D. semiclausum, which attack all larval stages, particularly 2^nd instar larvae, can reduce the amount of damage the larvae can cause. In contrast, Diadromus collaris which attacks prepupae and pupae, gives the pest more time to chew and damage the plant leaves. Diadegma rapi was more common in this study than in 1998-1999, reflecting seasonal population variation.

Plants with the greatest exposure to parasitic wasps (treatment b) were least damaged by P. xylostella compared with the three other treatments. However, this level of damage was greater than that currently accepted by fresh produce markets. As smaller wild P. xylostella larvae were only found in low numbers (0 –3/ plant), we assumed they had a negligible effect on plant damage rankings.

Further work is required to determine how to best utilise natural enemies of P. xylostella in commercial crops sprayed with insecticides. High rates of parasitism by wasps, particularly by D. semiclausum and D. rapi were recorded, in spite of regular applications of insecticide sprays to the surrounding cabbage crop. Perhaps the species of wasps found may have developed some tolerance to insecticides used, or the wasps moved into the trial from unsprayed areas. Neither of these possibilities was tested. Nonetheless, beneficial insects such as parasitic wasps should be encouraged by using less harmful insecticides. Parasitic wasps should be used in conjunction with ”soft” insecticides to develop an IPM program for P. xylostella.

Further work is also needed to measure the effects of seasonal differences on parasitoid wasp activity and their impact on P. xylostella in other Brassica vegetables. This study complements other studies being conducted in Queensland and Tasmania.

Acknowledgments

We thank White and Sons for use of their property in conducting this trial. Funding was provided by HAL as part of the National project VG00055 and by the Department of Agriculture, Western Australia, Horticultural program project HBG. Mike Furlong kindly provided technical advice and Andras Szito helped to identify the parasitoid wasp species.

References