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luminescens (Enterobacteriaceae) against pupae of the Diamondback Moth,. Plutella xylostella (L.) By A. S. Abdel-Razek. Abstract. The relationship between ...
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A. S. Abdel-Razek: Effects of some bacteria on Plutella xylostella (L.)

Anz. Scha¨dlingskunde ⁄ J. Pest Science 76, 108–111  2003, Blackwell Verlag, Berlin ISSN 1436-5693 National Research Centre, Dept. of Pests and Plant Protection, Dokki, Cairo, Egypt

Pathogenic effects of Xenorhabdus nematophilus and Photorhabdus luminescens (Enterobacteriaceae) against pupae of the Diamondback Moth, Plutella xylostella (L.) By A. S. Abdel-Razek Abstract The relationship between Xenorhabdus nematophilus and Photorhabdus luminescens, the bacterial symbionts of the entomopathogenic nematodes Steinernema carpocapsae and Heterorhabditis bacteriophora, against the diamondback moth, P. xylostella (L.), pupae showed the pathogenic capability of P. luminescens to be over that of X. nematophilus. They gave 60 and 40 % mortality with LC50 values of 5 · 104 and 5.5 · 105 cells/ml, respectively. The number of bacterial cells influences the rate of killing the pupae of P. xylostella and a direct correlation between mortality of the pupae, percentage of deformed adults and the spraying dose with significant differences were observed. An inverse correlation found between the no. of eggs laid/ female, percentage of hatching, adult survival and the spraying dose. These bacterial effects have been attributed to different toxic bacterial enzymes which damage the haemocytes and inhibit activation of the humoral immune system of the insect.

1 Introduction One of the many important lepidopterous insect pests of cultivated plants is the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera, Yponomeutidae). This insect is widely distributed and is a serious pest of cruciferous crops in many parts of the world (Salinas, 1977). The adaptability of the insect to different climatic conditions and its recognized status as a major pest in temperate and tropical regions make the study of DBM important from the economic as well as the biological point of view. The primary control strategy of P. xylostella has been the application of insecticides. Because of the development of insect resistance to insecticides and concern about environmental damage resulting from chemical pesticides, there is an increased interest in biological control (Verkerk and Wright, 1996). The entomopathogenic nematodes of the genera Steinernema and Heterorhabditis possess most of the characteristics of an ideal biological agent for insect control in the soil environment (Klein, 1990). Their use in the foliar environment has been limited, and most attempts met with very little success (Begley, 1990). This is due to abiotic factors: UV radiation, temperature and desiccation. The pathogenicity of entomopathogenic nematodes to P. xylostella has been demonstrated (Mason and Wright, 1997). They reported that nematode infectivity differs considerably depending on nematode species and developmental stage of the insects. Differences in U.S. Copyright Clearance Center Code Statement:

infectivity of a nematode bacterial complex to Heliothis zea pupae due to pupal age were observed by (Kaya and Hara, 1981). Also, Kaya and Hara (1980) reported that prepupae of G. mellonella, S. exigua, and Mythimna unipuncta were found to be highly susceptible to S. carpocapsae and its associated bacterium X. nematophilus. On the other hand, pupae of these 3 species differed significantly in their susceptibility (100 %, 75 %, and 54 % M, respectively). Neither the age of the pupae nor the dosage of nematode affected significantly the mortality of S. exgua or M. unipuncta. Raulston et al. (1992) reported that a Steinernema sp. nematode was the major natural mortality factor of prepupae and pupae of corn earworm and fall army worm, S. frugiperda. Cabanilas and Raulston (1995) demonstrated that S. riobravis applied to soil is capable of affecting corn earworm population before adult emergence under field conditions. Entomopathogenic nematodes, after entering the haemocoel of a host insect, continue their development and release symbiotic bacteria into the haemolymph. These symbiotic bacteria cause septicemia, insect disease and death of the insect host. So, if there is a pathogenic relationship, it is important to know how many cells/ larva or pupae are necessary to kill at least 50 % or more of the target stage of the insect host. The purpose of this study was to determine the pathogenicity of the bacterial symbionts from two entomopathogenic nematode species against P. xylostella pupae under laboratory conditions and to estimate the lethal concentrations. 2 Materials and methods The experimental individuals (pupae of the DBM, Plutella xylostella) were taken from a standard laboratory colony maintained at 23–25C on the Chinese cabbage plant, Brassica pekinensis, which was grown in individual pots in multipurpose compost in a greenhouse maintained at 15–20C, with a light regime of 16:8 h (light:dark). Plants were 4 to 6 weeks old at time of use. Susceptibility of mature pupae of P. xylostella (more than 3days old) was evaluated using the two bacterial symbionts, Xenorhabdus nematophilus and Photorhabdus luminescens. These bacterial symbionts were isolated indirectly from dead infected larvae of G. mellonella with the entomopathogenic nematodes, Steinernema carpocapsae and Heterorhabditis bacteriophora, respectively (Woodring and Kaya, 1988). The bacterial symbionts were grown and produced in a previous work.

1436–5693/2003/7604–108 $15.00/0

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A. S. Abdel-Razek: Effects of some bacteria on Plutella xylostella (L.)

Table 1. Effect of the bacterial symbiont, Xenorhabdus nematophilus on the mortality and development of the mature pupae of Plutella xylostella (L.). Bacterial Dose (Cells/ml) 1 · 105 5 · 104 1 · 104 5 · 103 1 · 103 Control

% Mortality

40 28 24 16 12 0

% of deformed adults 48 16 28 28 12 0

Fecundity* No. of Eggs/Female 57.5 ± 11.8a 66 ± 11.8a 81.2 ± 17.5b 84.6 ± 13.9bc 94.4 ± 25.1bc 105.2 ± 31.4c

% of Hatching

Longevity* M.

73 80 85 89 92 100

4.1 4.3 4.9 5.6 6.2 9.2

± ± ± ± ± ±

F.

(days)

0.7a 0.5a 0.9a 1.1a 0.9ab 1.5c

3.9 4.1 4.1 4.1 5.7 6.3

± ± ± ± ± ±

0.2a 0.7a 0.6a 1.0a 0.8a 0.9a

*Means followed by the same letter within columns are not significantly different (P < 0.05) Duncans multiple range test.

Mature pupae were placed separately in plastic pots (D · H ¼ 9 · 3 cm) over a Chinese cabbage plant leaf and sprayed with 1ml suspension of the different concentrations of the bacterial symbionts (0, 1 · 105, 5 · 104, 1 · 104, 5 · 103, 1 · 103 cells/ml of Phosphate Buffer Saline, PBS) and kept in an incubator maintained at 22–23C. Replication was 5 fold (10 pupae per treatment) in a completely randomized design. Pupae were observed daily for visual changes after spraying. Emerging adults were also observed for physical abnormalities. The normal moths which emerged were coupled in glass jars, a piece of green paper toweling provided oviposition sites. Observations were taken for pupal mortality, moth abnormalities, fecundity of females, and percentage of hatching among deposited eggs. The data presented for the biological effects were analyzed for significance of the main effects by analysis of variance (Anova), Duncans multiple range test was used for multiple comparison (P ¼