| Front page | | Contents | | Previous | | Next |

Entomophthorales on cereal aphids

7. Dynamics of Pandora neoaphidis epizootics in Sitobion avenae populations

7.1 Definitions of some model terms

An understanding of the dynamics of fungal diseases of insects is critical to the development of epizootiological theories as well as potential prediction of infection levels for pest management purposes (Hajek et al., 1993). A typical integrated pest management (IPM) system model consist of three parts: (1) a biological-conceptual model developed from the literature, (2) a mathematical representation of that framework, and (3) a computer program implementing the mathematics (Brown, 1987).

However, the dynamics of fungal epizootics are poorly understood, and factors necessary for the development of epizootics have only been identified for very few host-pathogen systems (Hajek, & St. Leger, 1994). As far as we know, only one IPM model takes entomopathogenic fungi into account, namely a system consisting of the cotton aphid (A. gossypii) and N. fresenii in Arkansas, USA (Steinkraus, 1998) which is a system quite different from the cereal system in Denmark.

7.2 A biological conceptual model for the dynamics of Pandora neoaphidis in Sitobion avenae populations

The dynamics of P. neoaphidis in populations of S. avenae are illustrated using a flow diagram in figure 7.1. In the figure both the aphids and the fungus are divided into three ‘stage variables’ each according to the principle phases of the disease. These stage variables are connected with several ‘development rates’. The development rates are dependent on abiotic factors such as temperature, moisture and solar radiation (‘forcing variables’). Moreover the forcing variables may determine whether an infection can occur (Benz, 1987; Carruthers & Hural, 1990). Since most of the processes are occurring in a cereal field, the crop development as well as agricultural practices must be included as forcing variables in the model. Some pesticides have shown to have a negative impact on the development of epizootics (Zimmerman, 1978; Wilding & Brobyn, 1980). As discussed in chapter 4, soil is important as a reservoir of P. neoaphidis and thus for early development of epizootics with P. neoaphidis.

Figure 7.1 Look here!
Flow diagram of an epizootiological model of the dynamics of Pandora neoaphidis in a population of Sitobion avenae. Grey rectangles are stage variables, ovals are developmental rates and the two large rectangles are forcing variables.

7.2.1 Stage variable

Aphids are divided into the following categories: susceptible healthy aphids (S), infected aphids (D) and sporulating cadavers which spread the infective conidia (I). The sporulating cadavers only exist long enough to release the infectious units and will then disperse from the system. In other models the insect population has been divided into as many as five categories according to the development of the disease within the insect (eg. Schmitz et al., 1993; Ardisson et al., 1997). However, the categories described here are those which can usually be estimated from field data.

In this model it is assumed that all host stages and morphs are equally susceptible to aphid pathogens. This is however a simplification since results analysed in this project demonstrated a difference in prevalence between alate and apterous S. avenae within cereal fields (Appendix D). Bioassays were conducted and it was shown that this difference in prevalence was due to differences in susceptibility rather than differences in behaviour. To keep the model as simple as possible, however this is not taken into account and may not be important for the dynamics since most alate aphid probably emigrate before they die from the disease.

The pathogen is divided into survival structures (O), primary conidia (P) and secondary conidia (Q). The cadavers produce either conidia, or survival structures or both. The conidia are disharged actively from cadavers and cause secondary infections among the aphids. Whether the primary and / or the secondary conidium is the infective unit is still not known. However, for

E. muscae it has been proven that secondary conidia are about 200 times as infective as primary conidia (Bellini et al., 1992) and the same can be true in our system. As mentioned in chapter 4 the question of how P. neoaphidis survive is poorly understood. The fungus must however, overwinter in some stage and initiate primary infections in spring.

7.2.2 Effect of abiotic factors on development rates

Abiotic factors do not act independently but as a complex of processes. Therefore, analysis of single environmental factors on the epizootics of fungal diseases is not always successful. Moreover, true quantitative values are difficult to obtain in nature since the microenvironment of an insect and/or a pathogen may differ considerably from the average measurable conditions of the environment (Benz, 1987). The effects of abiotic factors on the dynamics of disease development are discussed in greater detail below.

In spring S. avenae migrate to cereal fields when accumulated day degrees reach 1150-1250D^o (above a threshold of 0^oC) and they begin to reproduce (Hansen, 1995). The reproduction rate is primarily temperature dependent. Because only horisontal transmission of fungus diseases occurs, both susceptible and infected hosts produce susceptible nymphs which moult through a normal sequence of nymph instars and finally become adults. The fecundity decreases for an infected aphid compared to a healthy one. The closer the aphid gets to death the larger the decline in fecundity (Schmitz et al., 1993).

It is assumed that the migration rate for infected aphids follows the migration rate for healthy aphids since information in the litterature concerning change in migration behaviour is found.

Before infection of an aphid can occur, contact is necessary between the infective unit and the aphid. Tanada & Kaya (1993) reported that the spread of disease depends on both densities of host and infective unit. An increase in one or both pools enhances the probability of contact between spores and aphids. In contrast, Wilding (1975) found that aphid density had only minor importance in the transmission of infection. Missionnier et al. (1970) similarly concluded that epizootics were independent of host density. An analysis of Danish data has shown that density of aphids only has an impact when the density is low and that a threshold value probably exists. In addition to contact between spores and aphids, suitable temperatures and relative humidities are necessary for germination of spores and for penetration of the aphid integument (Benz, 1987).

Conidia of P. neoaphidis require a humid environment to be able to germinate and penetrate the integument. At high temperatures, a shorter time with free water is required to ensure optimal germination and penetration (Milner & Bourne, 1983). Germination occurs at any temperature between 10 ^oC and 20^oC (Milner & Bourne, 1983), and outside of this interval no results have been found in the literature.

Our experiment showed that the lethal time increased as temperature decreased for aphids infected with P. neoaphidis. The relationship between lethal time and temperature can be expressed by the equation:

LT₅₀ = -6.05*ln (temp.) + 23.0

R² = 0.96

Dromph et al. (1998) demonstrated a positive correlation between dry weight of S. avenae cadavers and total production of P. neoaphidis primary conidia at 18^oC. The relationship can be expressed as:

Conidia produced = 3186*dry weight (in mg) + 11412

R² = 0.97

Climatic factors, particularly humidity and temperature act on conidia discharge (Benz, 1987). Dromph et al. (1998) demonstrated that temperature had a significant impact on sporulation, particularly on total production, which increased with temperature. At 18^oC most conidia were produced within the first twelve hours following death of the aphid. Wilding (1969) proved that

P. neoaphidis only sporulated when RH was at least 90 %. Within this range, the numbers of spores increased with increasing humidity. Light apparently does not affect the sporulation of P. neoaphidis (Milner, 1981).

It is assumed that climatic factors, particularly humidity and temperature act on the capacity of secondary conidia production in the same way as in primary conidia.

No quantitative data have been collected concerning the initiation of the survival structure. Temperature and nutritional conditions may play an important role (Latgé & Papierok, 1988) and probably the day length too.

For some entomophthoralean fungi a dormancy period is required prior to germination (Latgé & Papierok, 1988). Our data suggest that quiescence or dormancy for the survival structure of P. neoaphidis is broken a long time before the aphids arrive to the field. Infection can thus start as soon as the aphids arrive.

P. neoaphidis loses the infectivity at a rate dependent on humidity and temperature (Wilding, 1973; Brobyn et al., 1987). Inoculum on leaves near the base of the plants remained infective longer than on leaves near the top (Broby et al., 1985) probably as a result of less solar radiation.

7.3 Conclusions

In summary, we may conclude

| Front page | | Contents | | Previous | | Next | | Top |