Pathogenicity of Indigenous Strains of Three Entomopathogenic Fungi to the Sisal Weevil, Scyphophorus Acupunctatus (Gyllenhal) (Coleoptera: Curculionidae)
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Hellenic Plant Protection Journal 8: 46-54, 2015 DOI 10.1515/hppj-2015-0007 Pathogenicity of indigenous strains of three entomopathogenic fungi to the sisal weevil, Scyphophorus acupunctatus (Gyllenhal) (Coleoptera: Curculionidae) V.T. Gkounti, D. Markoyiannaki and D.Ch. Kontodimas* Summary The pathogenicity of indigenous isolates of Beauveria bassiana, Metarhizium anisopliae and Isaria fumosorosea was evaluated in the laboratory against larvae and adults of the sisal weevil, Scyphophorus acupunctatus. Inoculation was achieved via immersion of individuals into conidia sus- pensions of diff erent concentrations. All three fungal species proved high pathogenicity against lar- vae of the weevil, causing 100% mortality in most of the treatments. Beauveria bassiana caused the highest mortality of the adults (86.67±12%), followed by M. anisopliae (46.67±17.8%) and I. fumosoro- sea (40±17.5%). Mean survival time also diff ered signifi cantly among treatments and life stages of the weevil. In total, larvae survived signifi cantly fewer days than adults post infection. Results of the pres- ent study indicate the potential of indigenous strains of entomopathogenic fungi as biological control agents against the invasive weevil. Additional keywords: agave, Beauveria bassiana, Curculionidae, Isaria fumosorosea, Metarhizium anisopliae Introduction by plant’s fi bers to complete their devel- opment (Lock, 1965). In addition to the de- The sisal weevil, Scyphophorus acupuncta- structive activities of the larvae, the adults tus (Gyllenhal), is amongst the most severe are often natural vectors of plant pathogen- pests of both cultivated and ornamental ic bacteria, such as Erwinia carotovora (Dye) agave plants (Pott, 1975; Valenzuela-Zapata, (Aquino Bolanos et al., 2011). In Central and 1994; Camino Lavin et al., 2002). In its place South American agave cultivations chem- of origin, the Nearctic region, it has been ical insecticides are commonly applied to found to cause severe economic losses by control the sisal weevil, however there are strongly damaging Agave tequilana (We- many problems and controversies connect- ber) the tequila producing agave (Solis et ed to their use (Solis et al., 2001; Terán-Var- al., 2001). The sisal weevil was introduced gas et al, 2012). Firstly, as the weevil com- in Europe around 1980, when it was fi rst re- pletes its development within a protected corded on imported Yucca sp. in the Neth- habitat, inside the agave leaves, insecticides erlands (van Rossem et al., 1981). Since then have a low eff ectiveness (Figueroa-Castro et it has been found in many European coun- al., 2013). Secondly, their extended applica- tries (i.e. Italy, France, Greece) on ornamen- tion harbors the risk of resistance develop- tal plants (Colombo, 2000; EPPO 2008; Kon- ment in the pest species (Terán -Vargas et todimas and Kallinikou, 2010). al., 2012). Finally, there are certain restric- The females use the basal parts of the tions in the use of synthetic insecticides in agave leaves for feeding and as oviposition urban landscape areas, where the weevil is sites. Furthermore, once the neonate lar- mainly found, due to their eff ects on the en- vae hatch they start to bore galleries with- vironment and health risks. Therefore, the in the plant’s head and form a cocoon made development of biological and biotechnical control methods is of great environmental and economic importance. Department of Entomology and Agricultural Zoolo- Although a few arthropod natural ene- gy, Benaki Phytopathological Institute, 8 St. Delta Str., GR-145 61 Kifi ssia, Attica, Greece mies of the sisal weevil could potentially be * Corresponding author: [email protected] regarded as biological control agents in its © Benaki Phytopathological Institute Pathogenicity of entomopathogenic fungi on sisal weevil 47 site of origin (Velazquez et al., 2008), these digenous isolates of fungal entomopatho- arthropods are not found in the invaded re- gens is advantageous, as they are already gions. Entomopathogenic fungi and nema- adapted in specifi c environmental condi- todes have been reported as potential bio- tions and could be part of a conservation bi- logical control agents of the weevil (Hueso ological control strategy. et al., 2005; Velazquez et al., 2008). Both the fungi and the nematodes are of great im- portance for the biological control of S. acu- Material and Methods punctatus, as they are able to overcome the barrier of the concealed environment in Entomopathogenic fungi which the weevil develops. While the nem- All entomopathogenic fungi strains used atodes are able to move within the agave in the laboratory assays were obtained from leaves and are easily able to reach the con- the entomopathogenic fungi collection of cealed pest (Dembilio et al., 2010a), the en- the Benaki Phytopathological Institute. The tomopathogenic fungi infect their hosts strain of B. bassiana IBB 010 was initially iso- by direct contact and subsequent penetra- lated from a Rhynchophorus ferrugineus (Ol- tion of their cuticle (Dembilio et al., 2010b). ivier) cadaver, found in Ellinikon region This makes insects susceptible both to di- (Athens, Greece). Both strains IBB020 of I. fu- rect contact with the pathogen, but also to mosorosea and TMB04 of M. anisopliae were passive, horizontal transmission, between isolated from soil samples with the Galleria healthy and infected individuals vector- bait method (Zimmerman, 1986) from the ing spores (Klein and Lacey, 1999; Quesada- Agios Stefanos and Marathon regions (At- Moraga et al., 2004). tica, Greece), respectively. The molecular It has been widely accepted that ento- identifi cation of the strains of I. fumosorosea mopathogenic fungi show high intra-spe- and M. anisopliae was previously conduct- cifi c diversity, which results in diff erent lev- ed by Beris et al. (2013). The fungal isolations els of pathogenicity among diff erent strains were cultured on Sabouraud dextrose agar of the same species (Ferron, 1978). Further- (SDA) at 25°C, under dark regulated condi- more, the virulence of a specifi c strain to tions. Spore (conidia) suspensions were pre- specifi c insect genera has been proved to pared by scraping the surface of 2 week old depend upon the host insect identity from cultures into an aqueous solution of 0.2% which it was isolated (Fargues and Robert, Tween 80, after which the suspensions were 1983; Maurer et al., 1997). This is attributed vortexed for 60 s and fi ltered twice using a to the strong selective pressure of the host sterile nylon membrane to remove myceli- insect environment on the pathogen (Roy um. Subsequently, the concentration of the et al., 2006) through which the origin of the spore suspensions was determined by the entomopathogens could aff ect their patho- use of a standard haemocytometer. The genicity against pests (Cherry et al., 2005). concentrations of spore suspensions used Despite the importance of alternative in bioassays were 2.12 x 107 and 2.12 x 106 methods for the control of the sisal weevil, spores/ml for B. bassiana, 1.27 x 107 and 1.27 research on the evaluation of potential bio- x 106 spores/ml for M. anisopliae and 7.45 x logical control agents of the pest is very lim- 106 spores/ml for I. fumosorosea. Spore con- ited (Molina-Ochoa et al., 2004; Valdes Estra- centrations are not equal due to the diff er- da et al., 2014). In the present study we aim ent developmental rate of each fungus (Kon- to evaluate the virulence of locally isolat- todimas and Gkotsi, 2009). Spore viability ed strains of Beauveria bassiana (Balsamo), measured over 96% for all selected strains. Metarhizium anisopliae (Metchnikoff ) So- rokin and Isaria fumosorosea (Wize) to adults Insects and larvae of the introduced weevil S. acu- Both adult and larvae of the sisal wee- punctatus. It is expected that the use of in- vil were collected from ornamental aga- © Benaki Phytopathological Institute 48 Gkounti et al. ve plants, Agave americana L., in the ur- α=0.05. The analysis included life stage (lar- ban landscape area of Ardittos hill (Athens, val and adult), species of fungi (B. bassiana, Greece). The infestation of the agave plants I. fumosorosea and M. anisopliae), and con- was spotted in the summer of 2012 and no centration of spore suspensions (106 and action for controlling the pest was taken pri- 107 spores/ml) as variables. Additionally, or to the collection of the weevils. The adults the average survival times were calculated were collected by hand at site, while whole and compared by the Kaplan-Meier surviv- plants were cut from the basis and taken to al analysis (Kaplan and Meier, 1958). All the the laboratory for the isolation of 4th instar GLM procedures and survival analysis were larvae. The adults were kept in polyester carried out in R (v. 2.1; R Foundation for Sta- cages (30 x 30 x 30cm) and the larvae in plas- tistical Computing, Vienna, AT). tic boxes (20 x 20 x 20cm), with an opening covered by mesh for ventilation. All insects were kept at controlled laboratory condi- Results tions (25±1°C, 65±5% R.H. and 14:10 L:D) and fed on a natural diet (agave slices; twice per The examined strains of entomopathogenic week) until they were used in the bioassays. fungi caused diff erent levels of mortality to S. acupunctatus. The mean mortality of wee- Bioassays vils diff ered signifi cantly at diff erent life stag- The application of the spore suspen- es, and fungus species, but it did not diff er sions was achieved through direct cuticle between diff erent concentrations of spore contact. Groups of fi ve, 4th instar larvae and suspensions (X2=22.939, df=1, P<0.0001; adults were immersed in the prepared aque- X2=9.5758, df=2, P<0.0001 and X2=2.5984, ous suspensions for 30 sec after which each df=1, P=0.1070, respectively).