Abstract

Insecticide resistance in diamondback moth (DBM), Plutella xylostella (L.) is one of the most important problems in the management of DBM. To retard or avoid the development of insecticide resistance in DBM, it is becoming more important to preserve natural enemies. We evaluated the toxicity of several insecticides on cocoons and adults of larval parasitoid, Cotesia plutellae Kurdjumov and effects of insecticides on parasitism. Under the experimental conditions, most insecticides applied at recommended concentrations showed high insecticidal activity against adults, but were less toxic to cocoons. However, parasitism by surviving adults was seriously affected.

Keywords

parasitism, selectivity

Introduction

Diamondback moth (DBM), Plutella xylostella (L.), is one of the most notorious pests in the world because it can easily develop resistance to various types of insecticides (Miyata et al. 1986, Saito et al. 1995). To retard or avoid the development of insecticide resistance in DBM, the selection pressure of insecticides to DBM should be reduced by the rotational use of insecticides which do not show cross resistance. This strategy should be accompanied by monitoring of DBM resistance to insecticides and use of other management techniques such as the introduction of sex pheromone (Miyata 1989, Saito et al. 1996). It is well known that DBM populations can be controlled by natural enemies. Goodwin (1979) stated that there were more than 90 species of DBM parasitoids. In Thailand, Keinmeesuke et al. (1995) reported that the egg parasitoid, Trichogramma confusum Viggiani, the larval parasitoid Cotesia plutellae Kurdjumov and the pupal parasitoid, Diadromus collaris (Gravenhorst) were found in highland areas while the egg parasitoid Trichogrammatoidea bactrae Nagaraja and the larval parasitoid, C. plutellae, were found in lowland areas.

To preserve natural enemies by introducing selective insecticides is one of the most important strategies to retard or avoid the development of resistance of insect pests to insecticides (Saito et al. 1991). Kao and Tzeng (1992) evaluated toxicity of 17 commonly used insecticides to C. plutellae. Among them, seven insecticides were harmful (mortality >99%) to adults of C. plutellae, while the remaining 10 insecticides proved to be harmless (mortality <50%). In this paper, we examine the effects of several insecticides which have been recently introduced to Thailand, against the larval parasitoid, C. plutellae.

Materials and methods

Insecticides

Candidate insecticides used for toxicity tests on C. plutellae were Ascend^® 5% SC (fipronil), Rampage^® 10% SC (chlorfenapyr), Vertimec^® 1.8% EC (abamectin), Polo^® 25% EC (diafenthiuron) and Ripcord^® 15% EC (cypermethrin). Recommended rates of application were 10 ml/L for fipronil, chlorfenapyr and abamectin, 30 ml/L for diafenthiuron and 15 ml/L for cypermethrin, respectively.

Rearing of C. plutellae

Potted common cabbage plants were used for mass rearing DBM larvae. Only the II instar larvae were used as the host larvae for parasitoid rearing. DBM eggs were collected on aluminium foil strips. Ten aluminium foil sheets were prepared at a time. Each sheet was crinkled to form parallel lines. Aluminium foil sheets were dipped into autoclaved cabbage juice (blend 130 g of cabbage leaf material in 1000 ml water and autoclaved at 120ºC, 2-3 atmospheric pressure for 20 minutes) and were allowed to air dry. Each sheet was folded and cut into 30 strips 2.5 cm wide. These strips were stored in the refrigerator at 4ºC until used. Three strips were hung in the oviposition chamber from the lid. About 200 DBM moths were introduced into the chamber and fed with 10% sugar-water solution with yellow food colouring contained in a 50 ml flask. A cotton dental wick was inserted through the flask. Eggs were collected consecutively every 48 hours. The egg sheets were sterilized with 10% formalin for 30 minutes, rinsed under running tap water for 10 minutes and then allowed to air dry.

Egg mass strips were placed onto the cabbage plants for subsequent hatching of larvae. Cabbage plants were placed into a wooden cage covered with fine mesh screen (1 mm²). The potted cabbage plants were covered with aluminium foil sheet just above soil surface level. Plants were watered daily. The II instar larvae of DBM were used for rearing and multiplying C. plutellae.

A potted cabbage plant with about 1000 II instar larvae of DBM was placed into the stock culture cage of C. plutellae. Freshly detached cabbage leaves were placed around the cabbage plant pot on the floor of the cage. This served as food for those larvae of DBM which fell from the plant. The larvae of DBM were withdrawn from the parasitoid cage at 24 hours after introduction to the parasitoid cage. The parasitised larvae of DBM were observed and transferred to parasitoid free cabbage plants for further rearing until pupation, from 8 to 10 days later. The cocoons of C. plutellae were collected using forceps. Meanwhile any surplus cocoons were stored in the refrigerator for delayed emergence without loss of cocoon viability and fecundity of emerging wasps.

Toxicity test of insecticides to adults of C. plutellae

The dry film method was employed as a test method (Kao & Tzeng 1992, Keinmeesuke et al. 1994). Five insecticides were applied at recommended dosage concentrations and a water-treated test tube was provided as a control. Each insecticide solution of 0.05 ml was added to a test tube of 3.5 cm diameter and 15 cm in height. All tests were replicated five times. The insecticide solution was distributed evenly around the inner wall of the test tubes and allowed to air dry. One adult C. plutellae (1-2 day old) was released into each test tube and fed with honey soaked cotton wool. Ten adults of unsexed C. plutellae were used for one treatment. Mortality was determined 24 and 48 hours after treatment. The effect of insecticides on C. plutellae and their selectivity was classified and categorized according to the IOBC/WPRS Working Group method (Hassan et al. 1985). The data obtained were statistically analysed by applying analysis of variance (ANOVA) and Duncan’s multiple range tests (DMRT).

Effects of insecticidal toxicity on cocoons and parasitism by emerged adults of C. plutellae

Five candidate insecticides were diluted with water based on recommended dosage concentrations and 2 ml of each insecticide solution was sprayed onto a cocoon of C. plutellae. Cocoons treated with water were provided as a control. Each treatment was replicated five times. Adult emergence and dead adults at 24 hours after emergence were recorded daily. The mortality data were recorded and calculated by Abbott’s formula. The obtained data were statistically analysed by applying ANOVA and DMRT.

The surviving adults were transferred to a cylindrical cage (30 cm height and 15 cm in diameter) and were fed with 10% honey-solution. One hundred II instar larvae of DBM were provided on the 2^nd, 4^th, 6^th and 8^th day after emergence. The number of DBM larvae parasitised was counted and calculated for each cage. The reduction in parasitism was measured by the following formula:

% reduction in parasitism = total % parasitism in control - total % parasitism in treated parasitoids) * 100

The experiments were conducted at 25˚C and 80% RH with 16L:8D photoperiod.

Results and discussion

Mortality of C. plutellae as a result of exposure to different insecticides is presented in Table 1. Fipronil and chlorfenapyr caused 100% mortality at 24 hours after treatment. Diafenthiuron caused 92% mortality at 24 hours and 100% mortality at 48 hours after treatment. Abamectin caused 68% mortality at 24 hours and 88% mortality at 48 hours. Cypermethrin caused 34% mortality at 24 hours and 58% at 48 hours after treatment.

Table 1. Toxicity of various insecticides to adults of Cotesia plutellae assessed by dry film method

	% Mortality after treatment		Rating
Treatment	24 hours	48 hours
Ascend® 5% SC (fipronil)	100	100	4
Rampage® 10% SC (chlorfenapyr)	100	100	4
Vertimec® 1.8% EC (abamectin)	68	88	3
Polo® 25% EC (diafenthiuron)	92	100	4
Ripcord® 15% EC (cypermethrin)	34	58	2
Control	0	0	-

1 = harmless (< 50%), 2 = slightly harmful (50 – 79%), 3 = moderately harmful (80 – 99%), 4 = harmful (> 99%) (Hassan et al. 1985).

Based on the criteria suggested by Hassan et al. (1985), fipronil, chlorfenapyr and diafenthiuron were rated as harmful, whereas, abamectin and cypermethrin were rated as moderately harmful and slightly harmful to adults of C. plutellae, respectively. These results showed that adults of C. plutellae were extremely affected by fipronil, chlorfenapyr, diafenthiuron and abamectin. Therefore, these four insecticides will not be recommended for use in IPM programs that involve mass release of C. plutellae adults for biological control.

Our toxicity data for abamectin, which showed high mortality to adults of C. plutellae, were different from the report of Keinmeesuke et al. (1994). They demonstrated that at the rate of 2000 dilution times, abamectin (Agrimec^® 1.8% EC) caused 0 and 20% mortality at 24 and 48 hours after treatment. On the other hand, they demonstrated at the rate of 2000 dilution times, cypermethrin (Sherpa^® 25% EC) caused 53.5 and 76.7% mortality at 24 and 48 hours after treatment. The cause in the difference in mortality is not clear, but may be derived from the difference in formulations or differences in the C. plutellae populations used.

Mortality of emerged adults of C. plutellae when cocoons were treated with insecticides was shown in Table 2. Emergence of adults was not affected. However, emerged adults were partially affected within 48 hours after emergence. Based on the criteria suggested by Hassan et al. (1985), five candidate insecticides were considered as harmless to pupae of C. plutellae.

Table 2. Toxicity of various insecticides to cocoons of Cotesia plutellae by the spraying method

Treatment	% Adult emergence	% Mortality of adults after emergencea		Ratingb
		24 hours	48 hours
Ascend®	96	45 d	48b	1
Rampage®	100	44 d	44 b	1
Vertimec®	100	29c	30 ab	1
Polo®	100	14 b	16 a	1
Ripcord®	96	1 a	7 a	1
Control	100	0	0	-

^aMeans followed by a common letter are not significantly different at the 5% level by DMRT.
^b1=harmless (<50%), 2=slightly harmful (50–79%), 3=moderately harmful (80–99%), 4=harmful (>99%) (Hassan et al. 1985).

The effect of insecticides on parasitism by C. plutellae at different intervals was shown in Table 3. Generally, at two days after emergence, C. plutellae showed the highest parasitism activity followed by 4, 6 and 8 days. Fipronil and chlorfenapyr were moderately harmful, abamectin was slightly harmful and diafenthiuron and cypermethrin were harmless.

Table 3. Effects of various insecticides on parasitism of DBM larvae by Cotesia plutellae

Treatment	% Parasitism after emergencea					% Reduction in parasitism
	2 days	4 days	6 days	8 days	Total
Ascend®	5.2 d	1.4 d	0.6 d	0 d	7.2	92
Rampage®	14.2 c	7.2 cd	2.6 d	1.2 d	25.2	88
Vertimec®	18.6 c	14.2 c	9.2 c	5.2 d	47.2	78
Polo®	40.2 b	40.4 b	29.4 b	21.4 c	131.4	39
Ripcord®	47.4 b	47.0 b	33.2 b	28.0 b	155.6	28
Control	64.4 a	59.8 a	50.0 a	41.2 a	215.4	-

^aMeans followed by a common letter are not significantly different at the 5% level by DMRT

Acknowledgements

We would like to express our thanks to staff members of Vegetable and Ornamental Plant Entomology Research Group, Department of Agriculture of Thailand. A part of this work was supported by Grant-in-Aid for Scientific Research by the Ministry of Education, Science, Sports and Culture, Japan.

References