Susceptibility of Larvae of Nun Moth, Lymantria
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ACTA ZOOLOGICA BULGARICA Applied Zoology Acta zool. bulg., 68 (1), 2016: 117-126 Research Article Susceptibility of Larvae of Nun Moth, Lymantria monacha (Linnaeus, 1758) (Lepidoptera), to the Entomopathogenic Fungus Entomophaga maimaiga Humber, Shimazu and Soper (Entomophthorales) under Laboratory and Field Conditions Daniela Pilarska1,8*, Ann E. Hajek2*, Melody Keena3, Andreas Linde4, Manana Kereselidze5,9, Georgi Georgiev6, Margarita Georgieva6, Plamen Mirchev6, Danail Takov1, Slavimira Draganova7 1 Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, 1, Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria 2 Department of Entomology, Cornell University, 6126 Comstock Hall, Ithaca, New York 14853-2601, U.S. 3 USDA Forest Service, Northern Research Station, 51 Mill Pond Rd., Hamden, Connecticut 06514-1703, U.S. 4 Eberswalde University for Sustainable Development, Alfred-Möller-Straße, 16225 Eberswalde, Germany 5 V. Gulisashvili Forest Institute of the Agricultural University of Georgia, 240 D. Aghmashenebeli Alley, 0159 Tbilisi, Georgia 6 Forest Research Institute, Bulgarian Academy of Sciences, 132, St. Kliment Ohridski Blvd., 1756 Sofia, Bulgaria 7 Institute of Soil Science, Agrotechnologies and Plant Protection, 7 Shosse Bankya Str., 1080 Sofia, Bulgaria 8 Faculty of Forestry and Wood Sciences, Czech University of Life Sciences, 1176 str. Kamycka, Prague CZ-165 21, Czech Republic 9 Scientific Research Center of Agriculture, 8 st. Chachava, 0159, Tbilisi, Georgia Abstract: Susceptibility of Lymantria monacha larvae to Entomophaga maimaiga was investigated under laboratory and field conditions, using larvae of the natural host,Lymantria dispar, as positive controls. In laboratory bioassays, L. monacha and L. dispar were injected with protoplasts of two isolates of E. maimaiga and mortality was monitored for 20 days. While virtually all injected L. dispar died, with ST50s (median sur- vival times for 50% of insects injected with the two isolates) of six – seven days, only 65.6-86.7% of the injected L. monacha died, with ST50s of 11-17 days. Both isolates produced conidia and resting spores more frequently within dead L. dispar than L. monacha. In more ecologically relevant host range assays, larvae of both species were exposed to germinating soil-borne E. maimaiga resting spores in the laborato- ry. More L. dispar than L. monacha larvae died after these exposures. However, while resting spores were formed within 100% of L. dispar larvae that died, significantly fewer (10%) dead L. monacha contained resting spores. When L. monacha larvae were collected during an E. maimaiga epizootic occurring in a sympatric L. dispar population, only 0.2 % of the L. monacha died and produced spores. These findings corroborate those of previous studies reporting a narrow host range for this fungal pathogen. Key words: host specificity, physiological host range, ecological host range, Lymantria monacha, Lymantria dispar, Entomophaga maimaiga Introduction Entomophaga maimaiga Humber, Shimazu and the invasive European gypsy moth, Lymantria dis- Soper (Entomophthorales: Entomophthoraceae) par (Linnaeus, 1758), as of 1989 (WESELOH 1998, is a fungal pathogen that was introduced to North HAJEK 1999). Entomophaga maimaiga appears to be America and was documented to be established in providing control of the gypsy moth in some areas *Corresponding author: These two authors have equal contribution. 117 Pilarska D., A. E. Hajek, M. Keena, A. Linde, M. Kereselidze, G. Georgiev, M. Georgieva, P. Mirchev, D. Takov, S. Draganova of north-eastern forests and is considered to be the the lepidopteran subfamily that includes the genus most important natural enemy of this introduced, Lymantria Hübner, 1819, although it was shown to defoliating pest in the U.S. (HAJEK 2007, FUESTER infect a number of other species at low levels (HAJEK et al. 2014). et al. 1995a). In field studies, only one individual During the period 1999-2001, E. maimaiga was of two species in the families Lasiocampidae and introduced to three localities in Bulgaria through in- Erebidae were found to be infected with E. maim- oculum from the U.S. (PILARSKA et al. 2000), and in aiga (HAJEK et al. 1996). Because high levels of E. 2005 strong epizootics caused by this fungus were re- maimaiga infection occur in L. dispar when late in- ported in four L. dispar outbreak populations located star larvae spend time in or under leaf litter (HAJEK 30-70 km from the introduction sites (PILARSKA et al. 2001), non-target Lepidoptera specimens from leaf 2006). From 2008 to 2011, six more introductions litter were evaluated. Only two non-target larvae of E. maimaiga were performed in outbreak popula- of the families Noctuidae and Gelechiidae collect- tions of L. dispar in Bulgaria where E. maimaiga did ed from leaf litter produced E. maimaiga spores not yet occur and as a result, all outbreaks of the pest (HAJEK et al. 2000). Finally, studies conducted were suppressed (GEOR G IEV et al. 2013; MIRCHEV et across five years of naturally occurring epizootics al. 2013). Today, this fungus is widespread in nearly in L. dispar found relatively lower levels of infec- all regions of Bulgaria in which L. dispar occurs tion in tussock moths of two genera, Dasychira and (GEOR G IEV et al. 2012a). Orgyia (Erebidae: Lymantriinae; HAJEK et al. 2004). After the introductions of E. maimaiga in Investigations in forest stands with E. maimaiga epi- Bulgaria, the pathogen has quickly spread on the zootics in Bulgaria found no alternative hosts among Balkan Peninsula and South-eastern Europe. In 1,499 non-target individuals belonging to 38 species 2005 it was found in Georgia, where it was thought from ten lepidopteran and one hymenopteran fami- to have spread (KERESELIDZE et al. 2011). In 2011, lies (GEOR G IEVA et al. 2014). E. maimaiga was found in the European part of The nun moth, Lymantria monacha (Linnaeus, Turkey (GEOR G IEV et al. 2012b) and in Serbia 1758), is an outbreak pest in Eurasian conifers that (Ta b a k o v i ć -To š i ć et al. 2012). In 2012, the fungus poses an ever-present threat of being accidentally in- was introduced into a L. dispar population on Avala, troduced to North America. It has high potential of a mountain overlooking Belgrade, and it was also being transported via commerce because, although introduced to or reported from numerous additional eggs are typically oviposited in bark crevices, they Serbian locations (Ta b a k o v i ć -To š i ć 2014a, b). In are potentially oviposited in crevices of wood that 2012 E. maimaiga expanded its range into Greece is used for containers, pallets, and ship surfaces, etc. and FYR Macedonia (GEOR G IEVA et al. 2013), and in Lymantria monacha feeds primarily on Picea, Pinus, 2013 it was found in Croatia (Hr a š o v e c et al. 2013), Abies, and Larix spp. but can also develop when Hungary (CSÓKA et al. 2014), Slovakia (ZÚBRIK et feeding on leaves of deciduous trees and shrubs. al. 2014) and Bosnia and Herzegovina (MILOTIC et European forest managers continue to seek natural al. 2015). control measures for L. monacha. Entomophaga maimaiga produces two types of We present results from studies of the suscep- spores, conidia and resting spores, on or in bodies tibility of L. monacha larvae to E. maimaiga. We of larvae that have died from infections. Conidia are tested L. monacha larvae under laboratory condi- formed externally on dead early season hosts and are tions using protoplast injections and next via ex- actively ejected, thus spreading infection within the posure to germinating resting spores. Lymantria spring larval population. Thick-walled azygospores, monacha is not normally known in Bulgaria as a or resting spores, are produced internally within serious pest and has not caused heavy damage to dead, late instar larvae. Larval bodies containing forest stands in the recent past. However, in 2014 resting spores are often located on tree trunks, rocks high densities of both L. dispar and L. monacha or vegetation above the soil but the bodies then fall were observed in the Kirkovo State Forestry region to the soil and decompose, leaving resting spores at and this change may have been due to a favourable or near the soil surface (HAJEK et al. 1998). Resting combination of natural beech and oak forests inter- spores overwinter in the soil and germinate in the mixed with conifer plantations. We therefore used spring to produce germ conidia that infect the new this opportunity to also investigate the potential for generation of L. dispar larvae. L. monacha to become infected with E. maimaiga Entomophaga maimaiga only infects larval under field conditions by collecting, rearing and hosts. Based on previous laboratory bioassays, E. diagnosing larvae during epizootics in several L. maimaiga appeared to be quite specific to hosts in dispar populations. 118 Susceptibility of Larvae of Nun Moth, Lymantria monacha to the Entomopathogenic Fungus Entomophaga... Materials and Methods MEIER analyses. Likelihood ratio tests were used to Laboratory studies determine significant effects (JMP Version 10; SAS Institute 2012). Percentages of larvae dying and Larval injection with protoplasts of E. maimaiga producing different types of spores or no spores Lymantria monacha larvae were reared on ar- were compared using Fisher’s exact probability tificial diet at the USDA Forest Service Quarantine tests with the BON F ERRONI correction for conduct- Facility in Ansonia, Connecticut (KEENA et al. ing multiple tests. 2010) for ten days post hatch and then transferred to Quercus velutina foliage. Lymantria dispar larvae Infection experiments with E. maimaiga resting were reared on high wheat germ artificial diet (BELL spores et al. 1981). Bioassays were conducted using two The susceptibility of L. monacha larvae to E. maim- isolates of E. maimaiga stored in the Agricultural aiga was further evaluated at Eberswalde University Research Service Collection of Entomopathogenic of Sustainable Development, Germany by conduct- Fungi (ARSEF) in Ithaca, NY.