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Entomophthorales on cereal aphids

3. Natural occurrence of entomophthoralean fungi infecting aphids

Natural occurrence and dispersal of Entomophthorales is an important part of epizootiology (Tanada & Kaya, 1993). Emphasis thus far has been on the natural occurrence on pest insects. Only limited knowledge concerning the effects of other insects, soil, plants and air on dispersal and early development of epizootics of Entomophthorales is available. The aim of this chapter is to review the natural occurrence of Entomophthorales in different habitats such as aphids, soil and air. In chapter 4 this information will be used to discuss overwintering of the fungi, and chapter 7 will use this information to review the dispersal and development of epizootics of P. neoaphidis.

3.1 Occurrence on aphids

A number of publications document the natural occurrence of entomophthoralean fungi infecting aphids in Europe (Dean & Wilding, 1971; Veronina, 1971; Remaudière et al., 1981; Keller & Suter, 1980, Feng et al. 1990, 1991; Ba»azy, 1993; Si ev, 1992; Steenberg & Eilenberg; 1995), in USA (Feng et al. 1991; Steinkraus et al. 1995) and in Australia (Milner et al., 1980). Altogether sixteen species of Entomophthorales have been described to infect aphids (table 3.1).

Worldwide, P. neoaphidis, Entomophthora planchoniana Cornu and in a few cases also C. obscurus are the most dominant pathogens on aphids feeding on annual crops with prevalences up to 80% (Dean & Wilding, 1971; Veronina, 1971; Remaudière et al., 1981; Feng et al. 1990; Si ÿ ev, 1995; Steenberg & Eilenberg; 1995). In aphid populations feeding on perennial crops however, Neozygites fresenii (Nowakowski) Remaudière & Keller is often the dominating fungus species (Steinkraus et al. 1995; Nielsen et al., in press).

3.2 Occurrence on cereal aphids

In Denmark aphids were surveyed during three years for the natural occurrence of Entomophthorales. Cereal aphids were sampled from both their primary and secondary hosts. Four entomophthoralean fungi were documented on cereal aphids when feeding on cereals: P. neoaphidis, E. planchoniana, C. obscurus and Conidiobolus thromboides Drechsler (table 3.2). Only P. neoaphidis and E. planchoniana caused epizootics. Prevalences of up to 50% were measured (Steenberg, Eilenberg & Nielsen, unpub.) This is in accordance with observations for cereal aphids in the United Kingdom, France and USA where these two fungi have also caused epizootics (Dean & Wilding, 1971; Remaudière et al., 1981; Feng et al. 1990; Si ev, 1995; Steenberg & Eilenberg; 1995).

Table 3.1 Look here!
Natural occurrence of Entomophthorales on aphids (Aphididae).

For aphids sampled on their primary hosts no infection could be documented during spring despite very intensive sampling from particularly bird cherry, the primary host for R. padi (table 3.2). However, an infection experiment showed that R. padi sampled from bird cherry in spring could easily be infected with P. neoaphidis under laboratory conditions (lethal time four to six days at 17^oC). This means that R. padi feeding on bird cherry does not come into contact with inoculum in spring. In autumn all of the four species mentioned above were found on aphids sampled from primary hosts. Moreover N. fresenii and Zoophthora phalloides Batko were also found on R. padi (table 3.2) (Steenberg, Eilenberg & Nielsen, unpub.).

Table 3.2 Look here!

3.3 Occurrence in soil

Latteur (1977) documented the presence of active inoculum of P. neoaphidis and C. obscurus in soil immediately following an epizootic, while Corremans-Pelseneer et al. (1983) additionally documented Condiobolus coronatus (Costantin) Batko and Conidiobolus sp. Other than these studies, no information was found in the literature.

In this study we investigated the occurrence of Entomophthorales infecting aphids by baiting soil samples with S. avenae. Infection of S. avenae by P. neoaphidis, C. obscurus and Conidiobolus sp. from soil was documented.

3.4 Occurrence in air

Hamilton (1959) was the first to estimate the aerial concentration of entomophthoralean fungi. It was found that the concentration of Entomophthorales conidia was highest between July and September. Diurnal periodicity was also measured and the maximum conidial number occurred between 5:00 a.m and 1:00 p.m. However the results were based on a small number of entomophthoralean conidia and were never determined at the species or even genus level. Wilding (1970) also measured the diurnal periodicity of the occurrence of Entomophthorales and found the largest concentration of Entomophthorales conidia in the air typically between 5:00 a.m. to 7:00 a.m. and furthermore that the concentration was correlated with humidity and sunrise. The smallest concentration occurred between 2:00 p.m. and 5:00 p.m., when the air was usually driest. Steinkraus et al. (1996) found generally found the same diurnal periodicity for primary conidia of

N. fresenii in the air over a cotton field during epizootics in the cotton aphid Aphis gossypii Glover.

3.5 Conclusions

This chapter highligted current knowledge from our studies and from the literature on the natural occurrence of Entomophthorales infecting aphids with emphasis on cereal aphids in particular. With respect to basic observations of such occurrences, much information is available in the literature, however only limited detailed data are found.

Based on our studies in particular, we may conclude:

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