CONTROL OF COTTON APHID AND GREENHOUSE WHITEFLY WITH A FUNGAL PATHOGEN Jeong Jun Kim, Min Ho Lee, Cheol-Sil Yoon1, Hong-sun Kim, Jai-Ki Yoo, and Kyu-Chin Kim2 Division of Entomology, National Institute of Agricultural Science and Technology, (NIAST), RDA, Suwon, 441-707 Korea 1Research Institute of Engineering and Technology, Korea University, Seoul 136-701, Korea 2Department of Agrobiology, Chonnam National University, Kwangju, 500-757, Korea ABSTRACT This Bulletin reports on the control of aphid and whitefly using a domestic strain of the fungal pathogen V. lecanii in Korea. Of the six isolates collected in Korea, V. lecanii CS-625 caused the highest mortality of cotton aphid (Aphis gossypii). Strain CS-626 was selected as a biological control agent for whitefly, based on bioassay and field tests. The pesticides dimethomorph and procymidone did not affect spore germination and mycelial growth of V. lecanii CS-625, provided they were used at the recommended concentration. INTRODUCTION by rapid sporulation and germination, high virulence, and active discharge of conidia, all attractive traits Aphid and whitefly are serious insect pests in in a fungal pathogen. However, mass production greenhouses all over the world. They damage a and storage of these fungi are difficult. range of crops, especially cucumber, pepper, and In contrast, hypho-mycete fungi such as tomato. They rapidly increase in numbers, and they Verticillium lecanii and Beauveria bassiana require transmit plant viruses. To control them, farmers several days of high humidity for sporulation and apply high doses of pesticides. infection. They also depend on the movement of The overuse of pesticides has resulted in insect aphids to come into contact with their conidia, and resistance to insecticides. Environmental residues a much greater number of conidia are needed to are also a concern, because consumers demand transmit infection. Nevertheless, hyphomycete fungi pesticide-free food. Thus, many countries are trying are cheap to mass produce, easy to store, and effective to reduce their use of pesticides. Biological control over a wide range of temperatures and humidity is an alternative control method. This includes levels. They also provide a rapid kill at economical entomopathogenic fungi (i.e. fungal pathogens doses. which affect insects) (Latge and Papierok 1988, Two isolates of V. lecanii were introduced by Hajek and St. Leger 1994). They are found in the Koppert into the Netherlands in 1981 and 1982. field under natural conditions. There is also interest One controls aphids on chrysanthemum, while in the commercial development of selected strains another controls whitefly on cucumber and tomato of such fungi (Milner 1997). Fungi which cause (Milner 1997). In Korea, eight species of fungi disease in insect pests include Erynia neoaphidis which infest insects, including Pandora neoaphid and Zoophthora radicans. They are characterized and Zoophthora radicans, were found on several Keywords: biological control, cotton aphid, fungal pathogen, Korea, V. lecanii, whitefly, 7 species of aphid (Yoon et al. 2000). This Bulletin which was found in both the conidia and the reports on laboratory virulence tests against cotton blastospores. The time taken for V. lecanii CS-625 aphid, using indigenous strains of hyphomycete conidia and blastospores to inflict LT50 of A. gossypii fungi, and field tests of selected isolates of V. lecanii was shorter than for the other isolates, at 2.74 and CS-626 to control whitefly. 3.31 days, respectively. These findings suggest that V. lecanii CS-625 is ideal for further development CONTROL EFFICACY OF THE FUNGAL as a microbial pesticide to control aphids. PATHOGEN AGAINST COTTON APHID Effect of temperature on virulence Testing for virulence Tests to study the effect of temperature on Cotton aphid (Aphis gossypii) is one of the virulence were carried out. A suspension of conidia most important insect pests of vegetable crops in was sprayed onto cucumber plants, which were then Korea. We tested the potential for biological control kept at different temperatures (20, 25, 30 and 35°C). of several species of hyphomycete fungi native to The mortality rate among the cotton aphids was Korea. These included two isolates of Beauveria recorded each day for five days. bassiana, one isolate of Verticillium lecanii and The mortality rates varied greatly according three isolates of Paecilomyces spp.. to the temperature. They were lower at 20°C than at The time intervals required for a mortality 25 and 30°C (Fig. 1). However, because V. lecanii CS-625 showed the highest germination rate at rate of 50% (LT50) among the A. gossypii varied with different fungal strains (Table 2). V. lecanii temperatures of 20 - 25°C (Yoon 2000), temperatures CS-625 had the highest virulence against A. gossypii, of around 20°C are considered optimal for the use of Table 1.Original hosts and geographical location of isolates examined Table 2.Corrected mortality and LT of A. gossypii 5 days following treatment, using native strains of entomogenous50 fungi (mean ± SD) 8 this biocontrol agent. concentration should therefore be used. The type of formulation must be studied in future to improve Effect of concentration of the inoculum efficacy in the field. Tests were made to identify the most effective Influence of fungicides on V. lecanii concentration of conidia in the inoculum. CS-625 Application of inoculum at a concentration of 104 - 107 conidia/mL gave relatively low mortality. A Each crop suffers from many pests and higher concentration of 108 conidia/mL gave nearly diseases, which are controlled by chemical as well 100% mortality after five days (Fig. 2). This higher as biological methods. For this reason, it is necessary Fig. 1. Effect of temperature on virulence of conidia of Verticillium lecanii CS-625 on nymphs of Aphis gossypii Fig. 2. Virulence of Verticillium lecanii CS-625 on Aphis gossypii on cucumber at different conidial concentrations 9 to assess the effect of pesticides on biological agents. CONTROL EFFICACY OF THE FUNGAL The fungicides tested were wettable powders. To PATHOGEN AGAINST WHITEFLY study the influence of pesticides on mycelial growth, a 6 mm plug of V. lecanii was transferred onto a Two species which infest field crops, Bemisia plate containing a pesticide, and incubated at 25±1°C tabaci (sweetpotato or silverleaf whitefly) and for seven days. Colony diameters were measured Trialeurodes vaporariorum (greenhouse whitefly) after 7 days. may cause severe damage in any country with a Significant differences were found in the effect suitable climate. The two species are both present of various fungicides on the spore germination and in South Korea, but T. vaporariorum is the more mycelial growth of V. lecanii CS-625 (Table 3). important pest of vegetable crops. In recent years, Two fungicides, Fenbuconazole + thira and propineb, many of the conventional insecticides used to control strongly inhibited both spore germination and whitefly have come under scrutiny, because of mycelial growth. Two others, azoxystrobin and resistance problems and/or undesirable effects on chlorothalonil, strongly inhibited spore germination natural enemies and the environment. New but did not seriously impair mycelial growth. Two approaches are being tried, including the use of fungicides, dimethomorph and procymidone, did entomopathogenic fungi such as Verticillium lecanii. not affect either spore germination or mycelial For this purpose, five strains of V. lecanii growth. Tests carried out by Hall (1981) included a from Korea and other countries were tested and different range of fungicides. He found that benomyl compared. The strain CS-626 was selected as being inhibited mycelial growth, but only slightly reduced highly virulent against greenhouse whitefly at spore germination. Wilding (1972) also found that various temperatures (Table 4). benomyl inhibited the mycelial growth of V. lecanii Mortality of the CS-626 strain was nearly on agar. As in our study, Hall (ibid) found that 100% at 25°C, and 80% at 15 and 35°C on the 7th clorothalonil, dimethomorph, and thiram were toxic day. This result indicates that the CS-626 strain to V. lecanii. Hall found that carbendazim + should be effective in Korean greenhouses, in which kasugamycin and fenarimol strongly affected the temperature during spring and fall normally mycelial growth but did not impair spore ranges from 20°C to 35°C (Yoon et al. 1999). germination. Since the virulence of the strain changes Table 3.Inhibition by fungicide of spore germination and mycelial growth of Verticillium lecanii 10 according to both the concentration of conidia and Experiments in whitefly control environmental conditions, we need to determine the optimal concentration (Fargues 1972, Hall and Pot and greenhouse tests were carried out to Papierok 1982). Table 5 shows the effect of different test the effectiveness of V. lecanii strain CS-626 in concentrations of the CS-626 conidial suspension controlling whitefly. Tomato plants infested with on the mortality of adult whitefly. The LC50 was more than 75 second instar larvae of whitefly (T. estimated at 2.3 x 106 conidia/mL. Low vaporariorum) per leaf were sprayed with inoculum concentrations of 1x103 - 107 conidia/mL showed containing various concentrations of conidia (106 - relatively low mortality. Higher concentrations of 109 conidia/mL). All the conidia were the CS-626 1x109 and 108 conidia/mL gave almost 100% strain. The same strain was used on tomato plants in mortality after 7 and 9 days, respectively. Since the two greenhouse tests. difference at LT50 between the two concentrations The number of adult whitefly decreased by was only one day (Table 5), spraying a conidial 60.9% in plants grown on higher shelves and 72% in suspension at a concentration of 108 conidia/mL plants grown on lower shelves, compared to the seems to be an economical and effective way of control (Fig. 3). controlling whitefly. However, these data may not Fig. 4 and Fig. 5 show the control efficacy of be directly applicable to the prediction of mortality the same CS-626 strain against whitefly in a in greenhouses, which needs further research.