Aphids are among the most widespread pests affecting the agricultural industry around the world. While we may not think we need to compete against these tiny insects for the potatoes on our plate, if not monitored, the effects of aphids may present a serious problem for potato production.

There are stringent requirements in place regarding the quality of tubers, or seed potatoes, based on the South African Seed Potato Certification Scheme 2010. Aphids are efficient transmitters of plant viruses that threaten the quality of seed potatoes. Two of these viruses that have devastating potential in the potato industry globally are potato virus Y (PVY) and potato leafroll virus (PLRV). Potato plants are susceptible to infection with these viruses by aphids feeding on them, acting as efficient vectors. Aphids feed on infected plants and are able to retain the viruses for several hours, in the case of PVY, or for their lifetime in the case of PLRV. They feed on the sap of infected plants after which they move on and transmit the viruses to healthy plants.

Prof Kerstin Krüger, of the Department of Zoology and Entomology in the Faculty of Natural and Agricultural Sciences at the University of Pretoria (UP), has been monitoring the flight patterns of different aphid species and researched their efficiency in transmitting viruses that threaten the South African seed potato industry. This research ultimately aims to provide an early warning system for potato growers who are at risk of yield losses and, consequently, huge financial losses resulting from plant viruses transmitted by aphids. This early warning system, which emulates systems used in Europe, relies on the use of a nation-wide network of suction traps that sample flying or wind-borne aphids.

Of the 4 700 aphid species known world-wide, 50 are vectors of PVY and at least 13 are vectors of PLRV. While aphid species vary in their efficiency of transmitting PVY, for example, in South Africa, Krüger and her colleagues have already found 21 that can transmit PVY and 11 that can transmit PLRV. For this reason, it is important to have reliable species identification and estimates of aphid numbers, as well as to know the transmission efficiency of the different aphid species, in order to predict the risk of transmissions. The mostly asexual reproduction of aphids facilitates rapid increase in their numbers and the associated spread of the viruses.

In order to reliably estimate aphid numbers and the risk of virus spread, Krüger and her research team collect data from the suction traps. The evaluation of the data informs them which aphid species, most capable of vectoring PVY and PLRV, occur in the various potato-growing regions. Growers are notified via the nation-wide network and are thus able to keep track of aphid numbers and vector pressures by receiving weekly SMS notifications and regular news bulletins.

These data provide very important information for the agricultural industry on specific species and the potential threat they pose in specific regions. The data also provide information on the commencement of aphid flight and peak periods, which can be used to time control measures such as insecticide application. Not every individual of a vector species carries plant viruses. Current research involves improving risk forecasting by determining the number of individuals in a given vector species population that are carrying viruses.

The national aphid-monitoring suction trap network comprises thirteen 12,2m Rothamsted-type suction traps situated in the major seed potato and wheat growing regions of South Africa. The height of the traps was determined in experiments carried out by the Rothamsted Experimental Station in the United Kingdom. At this height, samples provide a standardised collection of migrating winged aphids. ‘They operate at a standardised air volume per hour, and the aphids caught therefore provide standardised counts allowing for comparison not only between species but also of samples from different traps,’ Prof Krüger explains. These traps can provide a representation of aphids in an 80km radius. She says it is also planned to extend the use of existing traps to monitor vectors of medical and veterinary importance, such as mosquitoes.

UP’s findings have identified the aphid species that are able to cause catastrophic effects. This study is also the first of its kind in South Africa to generate data on transmission efficiency for South African aphid biotypes (aphids that share a genotype).

The national aphid monitoring programme coordinated by Krüger’s research group forms part of a joint initiative by Potatoes South Africa, UP, the Department of Agriculture: Western Cape, KwaZulu-Natal Department of Agriculture, the Agricultural Research Council (ARC) – Small Grain Institute, the Winter Cereal Trust, the ARC – Plant Protection Research Institute, and regional laboratories of Potato Laboratory Services. This research is the first in South Africa to monitor aphid movements using a nation-wide network of suction traps.

Prof Krüger’s research on aphids is now also looking at aphids as indicators of climate change. Aphids react quickly to changes in climatic conditions due to their short generation time and the large number of offspring they produce. Aphid suction trap data are therefore ideal for determining the effect of changes in climatic conditions on aphid abundance and are making an important contribution to our understanding thereof.