Entomophthora sphaerosperma Fresen. 1856

Entomophthora sphaerosperma have distinguishable characteristics of other species of Entomophthora genus. The conidia of Entomophthora sphaerosperma are true conidia and are not developed within a sporangium. Conidia are narrowly elliptical, with a slightly rounded base and surrounded by a slightly visible neck (“collar”), this structure is to assemble the conidia on the conidiophore. Conidia are hyaline, uninucleate and granular cytoplasm (Aruta et al., 1974). Conidia produced on artificial media and upon host are 22 X 7 micrometers. Conidia are enclosed on a one-layered membrane. A transparent gelatinous cap crowns the apex which aids in its attachment to the host, the cap is not a part of the spore membrane, and severe handling can displace it, but this gelatinous cap is not so evident as the spores of Entomophthora muscae. Nucleus is always present, are circular or oval, 3 X 4 micrometers in diameter and located in the center. Nucleus is only visible when stained. There is a variation in germination of conidia. In wet conditions the germ tube is formed, although, the tube can grow vertically and functions as conidiophore (capilliconidiophore) to form secondary conidia and the secondary conidia may develop third conidia. Spore germination does not occur at temperatures exceeding 26 ºC and at least 70 % of relative humidity is essential to germination (Sawyer, 1931). Conidiophores are digitately branched at the distal end; this is one of the main characteristics to distinguish species of Entomophthora genus. Finely granular protoplasm with occasional small vacuoles and numerous nuclei linearly arranged or placed in pairs. Each branch is segmented by one or more septum, generally near the tip. Conidiophores tend to aggregate in clumps in culture and on the host. The conidiophore changes their behavior after release of the spore; usually conidiophore and its terminal membrane lose stretch and only a conidiophore portion is removed. Conidiophores arise from the hyphal bodies or from unsegment hyphae (Sawyer, 1931). This fungus develops short cystidia and numerous rhizoids. Rhizoids are tubular structures that push through the ventral surface and attack the host tissue until most of the body disintegrates. Rhizoids play a role as anchor to the body insect. Rhizoids consist of many perpendicular and unbranched hyphae and may not be developed upon culture media (Sawyer, 1931). Resting spores are spherical, thick wall; smooth surface and granular internal structure and can easily grow in culture media and rarely found in the host tissue. Resting spores starts as tumor at the end of the hyphae, where the cytoplasm and nuclei migrate thus hyphae content is empty (Sawyer, 1931).

The mechanism of conidia discharge is very peculiar and some authors named “cannon-ball mechanism” (Keller, 2007). Firstly the nucleus passes into the spore and the conidial membrane thickens, then the portion of attachment between conidiophore and conidia matures and is differentiated as a collar shape. The protoplasm increase conidia size and the pressure of conidiophores increase until the attachment between conidiophore and conidia is aggressively released (Sawyer, 1931).

Entomophthora sphaerosperma is a parasite of insects and has a worldwide distribution and is a widespread species with a wide host range including Diptera, Coleoptera, Hemiptera, Homoptera, Hymenoptera, Lepidoptera, Orthoptera, Thysanoptera (Barta & Cargán, 2006).

Interest in entomopathogenic fungi is increasing as an alternative of the chemical pest control in agriculture and mainly in countries that support ecological agriculture. One of the main uses of Entomophthora sphaerosperma is the biological control of insects during their different stages that affect crops (Batko, 1974). Entomophthora sphaerosperma is one of the few species that attack insects from different orders, considered a polyphagous species (Batko, 1974). One of the important features of Entomophthora sphaerosperma is their capacity to cause epizootics (disease affecting a large number of animal during a period time over a large area) and decrease host populations within a short time and is considered a potential fungus of pests (Keller, 2007). Isolates adaptations from one country to another have been success and the fungus was able to produce resting spores to persist several years and spread on its own which highlights the relevance for humans in the agriculture as part of the Integrated Pest Management (Milner & Soper, 1981; Milner et al., 1982; Milner & Mahon, 1985).

The infection of Entomophthora sphaerosperma are always due to conidia adhering to the host tissue and usually leaves a triangular hole (Brobyn & Wilding, 1983). The penetration is due to biochemical activities as the combination of hydrolytic enzymes and mechanical forces as the penetration peg (Xu et al., 2009). A peculiar characteristic is the sporulation while the host is alive until the mycelium overspread through whole the body of the insect (Keller, 2007). Uncommon host behavior is caused by Entomophthora sphaerosperma; infected larvae feeding of diamond-back moth (Plutella xylostella) was not affected until the third day of infection. On the day of death no food was consumed and infected larvae ate 44 % less than healthy larvae, also the number eggs laid by infected females is reduced (Furlong et al., 1991). It also, inhibits the production of sex pheromone and disrupts the mating behavior (Reddy et al., 1998). Resting spores are produced in unfavorable conditions, particularly low temperature and high humidity, high inoculum density, host age or inappropriate hosts (Ben-Ze’ev and Uziel, 1979; Shimazu, 1979; Perry et al., 1982; McCabe et al., 1984; McGuire et al., 1987; Glare et al., 1989). Under laboratory conditions, resting spores may remain dormant for 4 months at 4 ºC and 100 % relative humidity. In laboratory studies, 12 h light at 23 ºC and 12 h dark at 16 ºC, conidia survived for 16 days in soil and foliage, otherwise, ultraviolet radiation is the most important factor in conidia mortality (Furlong & Pell, 1997).

According to Sawyer (1931) there are four synonyms for Entomophthora sphaerosperma: * Empusa radicans (Brefeld, 1870) * Tarichium sphaerospernum (Cohn, 1870) * Zoophthora radicans (Brefeld, 1871) * Entomophthora phytonomy (Arthur, 1886) The larval stage of a cabbage worm (Pieris brassicae) was the first host where Fresenius found this parasitic fungus (Brefeld, 1870). The first time that Entomophthora sphaerosperma was described with other name was during 1870 to 1881 by Brefeld (1870, 1871 and 1881) and was called Empusa radicans but until 1881 was named as Entomophthora sphaerosperma. In 1870, the genus of Tachirium appeared on a description based in resting spore features (Cohn, 1870). Another synonym was described in 1886 under the name of Entomophthora phytomoni and was the first report of this fungus in United States of America (Arthur, 1886). In 1888, was confirmed that this fungus was identical to Entomophthora sphaerosperma (Thaxter, 1888). Entomophthora species are known as pathogen of invertebrates (MacLeod, 1963) and Entomophthora sphaerosperma is described as insect destroying fungus (Sawyer, 1931). The first time that Entomophthora was isolated was in 1927 from the larvae of black-headed fireworm (Rhopobota vacciniana) that affect cranberry (Vaccinium macrocarpon) attacking foliage and fruit (Sawyer, 1931).

Entomophthora sphaerosperma is a fungus described by Fresenius in 1856 as an insect pathogen Zygomycete and has been observed parasitizing a wide variety of insects. It belongs to the following taxonomic classification (Mycobank): Kingdom: Fungi Phyllum: Zygomycota Order: Entomophthorales Family: Entomophthoraceae Genus: Entomophthora Species: sphaerosperma