Isaria fumosorosea, first described as Paecilomyces fumosoroseus by M. Wize in 1904, is an interesting fungal entomopathogen. It was first isolated from an immature sugar beet weevil in the Ukraine, but has since then been reported to have a relatively wide host range and a nearly worldwide distribution (Zimmermann, 2008). Species within the genus Isaria are placed in the phylum Ascomycota and class Sordariomycetes, a group characterized by the production of asci within perithecias, or flask shaped fruiting bodies (Humber et al 2012). I. fumosoroseus and many other invertebrate pathogens belong in the order Hypocrales (Yokoyama et al 2006) and family Cordycipitaceae (Humber et al 2012).
Isaria fumosorosea is commonly found in the soil (Cantone and Vandenberg, 1998), but has been reported on plants, in water, and less commonly in air on every continent except for Antarctica (Zimmerman, 2008). It has been isolated from over 40 species of arthropods, representing 10 orders. Some of the more commonly known succeptible organisms include weevils, ground beetles, plant beetles, aphids, whiteflies, psyllids, wasps, termites, thrips, and a wide variety of butterflies and moths (Smith, 1993; Dunlop et al. 2007; Hoy et al. 2010). Researchers have struggled to study Isaria fumosorosea in the laboratory because of the variation in morphological characteristics and behavior (Cantone and Vandenberg, 1998), but it has been described as a rapidly growing fungus, with white mycelial colonies which can change to a purple or pink color and produce pink synnemata. The phialides are flask-shaped and conidia are cylindrical to fusiform (Zimmerman, 2008). A sexual cycle has not been observed, but mitotic recombination by means of a parasexual cycle has been reported under laboratory conditions (Cantone and Vandenberg, 1998). This fungus is sometimes hyperparasitized by Syspastospora parasitica, another ascomycete (Posada et al. 2004).
Because of its wide arthropod host range, Isaria fumosorosea has received significant attention as a possible biological control agent for several economically important insect pests of agricultural crops (Kim et al 2010). Much of the research has been focused on controlling the whitefly, Bemisia tabaci; but it has recently been evaluated as a control agent of Diaphorina citri, the Asian citrus psyllid, which spreads the bacterial pathogen that causes the devastating citrus greening disease (Dalleau-Clouet, 2005; Hoy et al 2010). It is currently used in 7 different mycoinsecticides and mycoacaricides worldwide (Faria and Wraight, 2007). These are considered safe and non-toxic to humans (Dalleau-Clouet et al. 2005), and seem to have little effect on most off target organisms and beneficial insects when the appropriate formulation is used (Zimmerman, 2008). Isaria fumosorosea does not appear to significantly affect common beneficial parasitoid wasps and some generalist predators (Copping, 2004; Osborne and Landa, 2002). Heightened interest in this fungus has led to a series of studies, including genetic and molecular studies that have shed some light on taxonomy and classification of this fungus.
Isaria fumosorosea was known as Paecilomyces fumosoroseus until the genus was determined to be polyphyletic across three different orders (Luangsa-ard et al 2009). At that point, the previously sunk genus Isaria was conserved, and now includes several fungal insect pathogens (Gams et al 2005). Recently, numerous molecular studies have demonstrated high variability within the species. Gauthier et al. (2007) described at least two major lineages within Isaria fumosorosea, one of which was closely associated with the whitefly Bemisia tabaci, and mostly found within the Americas, and the other was distributed across Asia in several clusters. Fargues et al. 2002 recognized three different monophyletic groups based on ITS sequence data. As such, it is therefore more appropriate to refer to Isaria fumosorosea as a species complex rather than a single species, and there will likely be taxonomic revisions of this group in the future (Zimmermann, 2008).
Like most entomopathogenic fungi, Isaria fumosorosea, infects its host by breaching the cuticle (Hajek and Leger, 1994). Various metabolites allow the pathogen to physically penetrate the host as well as inhibit its regulatory system. For Isaria fumosorosea, these include proteases, chitinases, chitosanase, and lipase (Ali et al. 2010). These enzymes allow the fungus to breach the insect cuticle and disperse through the hemocoel. Isaria fumosorosea and other species within the genus also produce beavericin (Luangsa-ard et al. 2009), a compound that appears to paralyze host cells (Hajek and Leger, 1994). Succeptible insects exposed to blastospores and conidia of Isaria fumosorosea show declined growth and high levels of mortality (Dunlap et al. 2007).
Several factors, both biotic and abiotic, influence growth, stability, and pathogenicity of Isaria fumosorosea. These include temperature, relative humidity, radiation, and host plant of the target insect (Zimmerman 2008). I. fumosorosea has been reported to grow at temperatures ranging from 5°C to32°C, with an optimum temperature around 25°C (Fargues, J. and Bon, M., 2004). However, since I. fumosorosea is a species complex, there is high variability in the effect of temperature on growth from different isolates around the world (Zimmerman 2008).
Exposure to sunlight can have serious effects on survival of I. fumosorosea (Zimmerman 2008). Studies demonstrate that UV radiation, particularly wavelengths in the UV-A and UV-B region are the most detrimental. The conidia of Isaria fumosorosea are highly succeptible to solar radiation and high heat (Fargues et al 1996). In fact, low thermotolerance is one of the major impediments to storage and application of fungal biopesticides, including I. fumosorosea in an agricultural setting (Kim et al. 2010).
Efficacy as an insecticide varies with different isolates within the Isaria fumosorosea species complex. Some are associated with a specific insect, such as the whitefly, Bemisia tabaci, while others appear to be capable of infecting a variety of insects and mites (Zimmerman, 2008). Infectious ability also seems to vary with the type of propagule. For example, Lacey and Mercadier (1998) reported that blastospores were more efficient at infecting whitefly nymphs than the conidia of the same isolates. Other studies have demonstrated differences in insecticidal activity based on spore length and germination speed (Altre et al 1999).
Insect host plants can also affect viability of Isaria fumosorosea because of the production of allelochemicals, which can inhibit growth of the fungus (Zimmerman, 2008). Some substances, most notably tannic acid, solanine, camptothecin, xanthotoxin, and tomatine reduce germination of I. fumosorosea blastospores and conidia. The last three mentioned above also inhibit mycelial growth (Lacey and Mercadier, 1998). Since the fungus does not enter the plant, these chemicals do not play a role if they are isolated within the plant’s vascular system, but can be detrimental if present at the surface or are sequestered into the tissue of the arthropod host (Lacey and Mercadier, 1998).
Isaria fumosorosea, formerly Paecilomyces fumosoroseus, is an interesting insect entomopathogen with a large host range. Widely used as a biological control agent, this species complex is found worldwide and is the subject of much recent research. Many questions remain unanswered about the ecology of this organism, such as those regarding specific pathways the fungus uses to infect its host and obtain nutrients. Additionally, little is known about its lifestyle outside arthropod hosts, and how it survives in the soil. These questions will likely be addressed in the future as Isaria fumosorosea gains attention due to its qualities which make it an attractive myco-insecticide.
Isaria fumosorosea infecting a cocoon.
D. citri nymph and Isaria fumosorosea
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When a conidium or blastospore of Isaria fumosorosea lands on a suitable host, it produces enzymes to penetrate the insect's cuticle. A germ tube then grows into the haemocoel and the fungus proliferates inside the insect’s body. The fungus can also enter through the spiracles, the mouth or the anal opening. The mycelia spread in the haemolymph and tissues, eventually emerging from the insect and producing conidia. Mortality of the insect has been ascribed to the drainage of its nutrients, the destruction of its tissues and the release of toxins.
This fungus has a wide host range that includes insects in over twenty five different families and many species of mite. Agricultural pest insects which are susceptible to infection include the diamondback moth (Plutella xyllostella), the Russian wheat aphid (Diuraphis noxia) and the silverleaf whitefly (Bemisia argentifolii). Among mites, susceptible species include the spotted spider mite (Tetranychus urticae), the European red mite (Panonychus ulmi), the brown mite (Byrobia rubrioculus) and the apple rust mite (Aculus schlectendali).
Isaria fumosorosea has been used to control insect pests of plants grown for the production of cut flowers, ornamentals growing in greenhouses and nurseries, vegetable and cole crops, cotton, maize, rice and plantation crops.
A comparison made between several entomopathogenic hyphomycetes showed that Isaria fumosorosea (as Paecilomyces fumosoroseus) provided more effective control of the cabbage-heart caterpillar, Crocidolomia binotalis, than did either Beauveria bassiana or Metarhizium anisopliae.
Research at the USDA-ARS Bioactive Agents Research Unit in Peoria showed that blastospores start germinating at a faster rate on the cuticle of silverleaf whiteflies than do conidia. This suggests that the use of blastospores rather than conidia for the development of formulations would be advantageous.
The fungus neither grows nor develops at temperatures above 32 °C and is not thought to be pathogenic to humans. It has not been found to be toxic to rats in laboratory experiments and is not considered to be harmful to birds, honey bees, bumblebees or a wide range of non-target arthropods.