1IFFIVE-Instituto Nacional de Tecnologia Agropecuaria (INTA), Camino 60 cuadras (5119) Cordoba, Argentina; 2FCEF y N Universidad Nacional de Cordoba, Av. Velez Sarsfield 299, (5000) Cordoba, Argentina

Background and objetives
Biotic and abiotic factors of an agroecosystem influence the behavior of aphids acting as vectors of viruses affecting crops. Thus it is of great interest to understand the biology of these aphids, such as the flying activity of alate forms (in connection with the incidence of viruses transmitted in crops), taking into account not only weather variables but also population seasonality in alternative host plants (considering both aphids and viruses).

Materials and methods
A study was conducted in a 40 ha agro-ecosystem in the province of Cordoba (Argentina) for two years (1996-97). Mainly soybean, sunflower, sorghum and maize are currently sown in this field in different periods of the year. Yellow traps were used to determine flying activity of aphids. Sampling was carried out every fifteen days in a 200x50 m strip containing spontaneous vegetation in order to detect alternate host plants for aphids as well as weeds acting as reservoirs for the following viruses: AMV, SMV, MDMV and the sunflower chlorotic mottle virus belonging to the potyvirus genus. DAS-ELISA, NC-ELISA and electron microscopy were among the techniques used to detect viruses.

Results and conclusions
The colonization by aphids and the incidence of viruses mentioned above were recorded during the crop cycle. Twenty-four genera and forty species of aphids were captured with yellow traps. The continual flight of aphids was observed with capture probes due to the predominance of certain species. This may also be due to the selectivity of yellow traps to some aphid species [4, 5]. Weather conditions influenced aphid flight [6]. The most flying activity was observed during periods of lower humidity records (r: -0.38; P: 0.002). This global curve was mainly determined by Therioaphis trifolii showing a correlation to minimum humidity (r: -0.41; P: 0.001). The flying activity of Myzus persicae had a negative correlation in connection with shorter photoperiods (r: -0.49; P: 0.001) and minimum temperature (r: -0.45: P: 0.0004). A flight pattern for aphids along annual cycles was estimated by also considering former data recorded in the same agroecosystem [1-3]. A number of aphid species performing as virus vectors together with weeds acting as reservoirs for both aphids and viruses were detected. AMV and SMV were found in soybean with incidences of 3 and 5% respectively, while MDMV was found in maize (2%) and sorghum (32%) and Sorghum halepense. AMV was found in the following weeds: Sonchus oleraceus, Melilotus albus, Chenopodium album, Bidens pilosa, Leonorus sibiricus and Oxalis cordobensis; the latter three species are the first reported as reservoirs for this virus. The sunflower chlorotic mottle virus was not discovered in this agro-ecosystem. Forty-two weeds were recorded as aphid reservoirs, and colonization took place in different periods during the year. Aphid-virus-plant interrelationships in the agro-ecosystem under study were analyzed.

1. Alegrae AE et al., 1994. Agriscientia XI, 35-42.
2. Alegrae AE et al., 1996. Fitopatologia 3, 207-216.
3. Herrera PS et al., 1995. Agriscientia XI, 49-58.
4. Peez K, Robert Y, 1984. Agronomie 4, 161-169.
5. Robert Y, 1987. Aphids, Their Biology, Natural Enemies and Control. pp. 306-310.
6. Thacker JI et al., 1997. Journal of Applied Entomology 121, 137-145.