Uninfected Aphids Research Articles

Interactions between Two Aphid Natural Enemies, the Entomopathogenic FungusErynia neoaphidisRemaudière & Hennebert (Zygomycetes: Entomophthorales) and the Predatory BeetleCoccinella septempunctataL. (Coleoptera: Coccinellidae)

Interactions between the pea aphid,Acyrthosiphon pisum,and two of its natural enemies, the ladybirdCoccinella septempunctataand the fungusErynia neoaphidis,were examined in the laboratory. AdultC. septempunctatastarved for 48 or 24 hr or nonstarved were presented with aphid prey items at different stages of infection withE. neoaphidisand their feeding behaviors monitored. This demonstrated that, under laboratory conditions,C. septempunctataadults, particularly when starved for 48 hr, perceivedA. pisuminfected withE. neoaphidisas acceptable food sources. This is a potentially antagonistic interaction as inoculum necessary for further transmission and the development of epizootics would be removed from aphid populations. However, less time was spent feeding, except by 48-hr-starved individuals, on infected than on uninfected aphids and the number of “feeding” versus “ignore” mouthpart encounters was greater for uninfected than infected aphids, thus limiting the severity of this antagonism. In addition, under laboratory conditions,C. septempunctataadults were able to passively vector infective conidia ofE. neoaphidisto susceptible aphids and initiate infection in 10–11% of the population. Ladybird foraging onE. neoaphidis-infected aphids and their ability to vector infective conidia to susceptible aphid populations is discussed in relation to the epizootiology of the fungus and its implications for the manipulation of natural enemies in integrated management strategies.

A Simulation Model of the Epidemiology of Barley Yellow Dwarf Virus in Winter Sown Cereals and its Application to Forecasting

1. A computer model is described which simulates the spread of barley yellow dwarf virus (BYDV) by aphids in winter cereals in SW England. The model has as a theoretical base the dynamic interaction between two temperature-dependent rate processes: (i) the frequency of virus transmission to uninfected plants by infected aphids, and (ii) the frequency of virus acquisition by uninfected aphids from infected plants. A distinction is made between virus spread into crops by alate migrant vectors (primary transmission), and more localized spread within crops mainly by apterous vectors (secondary transmission). Both types of spread may occur concurrently. Probabilities of multiple infection, and effects of this on the rate processes in the model, are described by a generalized frequency distribution. Vector dispersal and the latent periods, following transmission and acquisition of virus, are represented as constant functions of thermal time. 2. Simulations used field data of plant and aphid populations sampled every few weeks in crops of winter barley and wheat at Long Ashton near Bristol during 1978-89. The infectivity of autumn migrant aphids caught each year in a suction trap was measured by feeding transmission tests. Daily max-min screen temperatures used to estimate thermal time were obtained from records at Long Ashton. Crop infections were assessed from visible symptoms or by enzyme-linked immunosorbent assay (ELISA). 3. The model was successfully validated for 61 cereal crops with 0-80% of the crop area affected. There was no significant divergence between field observations and incidence of BYDV as predicted by the model (barley, r = 0-98; wheat, r = 0.94). Curves of epidemic development obtained by measuring virus incidence at frequent intervals during crop growth were usually similar to those generated by the model. Prospects for practical and more reliable forecasting of BYDV epidemics, and of the need for control measures, are discussed.