Ningxing Zhou and Sean Prager, University of Saskatchewan and Tyler Wist, Agriculture and Agri-Food Canada – Saskatoon
Pea aphids are a pest of leguminous crops found anywhere pulses are grown in the world. The little green insect is tiny in size, ranging in length from one to 10 millimetres, depending on how old they are. The stylet (mouthparts of aphids) pierce into plants and extend to the phloem where they suck on the plant’s sugar-conducting fluids. Plants that are eaten by aphids suffer reduced vigour, with symptoms such as reduced productivity, leaf yellowing, plant stunting, reduced yield, wilt and death. Pea aphids prefer temperate regions with moderate climate fluctuations and constant temperature and humidity. This climatic preference suggests that aphids have a hard time overwintering in Canada. In Saskatoon, for example, aphids usually appear in early July. Due to their small size and tendency to develop wings when they want to leave plants, they travel from one area to another by flying and by using wind currents.
The common natural enemies of pea aphids include lady beetles, lacewings, parasitic wasps and syrphid flies. At low aphid densities, these predators may prevent economic damage from pea aphids to plants. However, these aphids can reach incredibly high densities very quickly, as they did in our Saskatoon field sites this past season (2019). If the population of pea aphids is huge, losing a few percentages of the population is not worth mentioning and will not reduce plant damage. Aphids have unusual reproduction where at different times of the year, reproduction can be either asexual (parthenogenesis) or sexual. Parthenogenesis means that mature female aphids can give birth to the next generation through cloning and without needing to mate. For one single adult aphid, this cloning results in around 20 to 30 nymphs in seven days, depending on plant nutrients. It takes about 15 days for pea aphids to become reproductive from nymphs. Thus, within 22 days, one single aphid can have 625 offspring. So, imagine if ten aphids migrate into one field, how many aphids there could be in 22 days or two months? If all of the offspring continue to reproduce, pea aphids can quickly cover an entire plant. Once the aphid populations start to crowd each other on plants, some aphids produce wings and migrate to other sites and plants.
Pea aphids can cause significant economic damage by reducing both seed quality and quantity. For example, we found last year that infestations of pea aphids in faba bean can lead to drastic yield losses (up to 100%). Besides the direct damage from feeding, aphids can also vector diseases and cause indirect damage through the transmission of viruses such as the Pea Seed-borne Mosaic Virus (PSbMV). PSbMV symptoms on pulses include mosaicked leaves, delayed or uneven crop maturation, and various forms of seed and pod deformation. The presence of PSbMV in fields adds great insult to injury.
The first step to successful aphid management is controlling their population density. While this can be accomplished through multiple means, knowledge of the economic thresholds is a crucial element in any management strategy. We don’t have good thresholds for lentils, and we have none for faba beans.
Economic Threshold/Economic Injury Level Research in Progress
The point when pest injury results in economic loss is called the economic injury level (EIL). The action threshold is defined as the point where the action (insecticide treatment) needs to be taken to prevent the aphid population from reaching the economic injury level with the economic threshold working in costs such as insecticide and labour. As we know, when we get a mosquito bite, we may feel itchy from a few seconds to a few hours, and we could easily recover from it, depending on your body’s immune system. Similar to the relationship between mosquitoes and humans, the plant immune system also has a defensive response to the aphid feeding. Low numbers of aphids sucking on plant phloem will not cause huge issues to the plant, while large numbers will overwhelm the plant’s ability to defend itself or compensate for damage. Insecticide resistance is becoming an increasingly important topic, especially concerning pea aphids in lentils and faba beans, which have limited registered insecticides available in the market. As a result, there is a need to use existing materials carefully. Instead of applying insecticides early in the season, and doing several treatments in a season, it is better to spray when aphids approach the economic threshold. Treating with insecticide too late, though, might result in irreversible yield loss like we demonstrated last season. Finding a balance point between economic loss and spray timing is essential for pea aphid control.
In the 2019 field season, we conducted experiments on both lentils and faba beans to determine economic injury levels and evaluate the efficacy of three insecticides. For lentils, we used plots of CDC Impulse, a small red variety, while for faba bean, we planted CDC Snowdrop (a non-tannin variety). Lentil fields were sampled using a sweep net before, two days and ten days after the application of insecticides. In lentils in Saskatoon, aphids appeared around the bud to the early flowering stage (most buds with a few fully opened flowers). Based on the 2019 season’s data, we found that controlling aphids at low density gives the highest yield return. However, lentil plants were sprayed twice in the lowest density to keep aphids at our predetermined aphid density. We also found that if aphid populations reached 1,000 aphids per sweep, it resulted in a very low yield (~ 2 tonnes/ha). After reaching 1,600 aphids per sweep, the pea aphid population started decreasing. The decreasing trend may result from the maximum capacity of the lentil plants to host the pea aphid as well as the senescence of the crop. Without any insecticidal management of pea aphids, our average yield return was 0.9 tonnes/ha (control plots).
In our faba bean experiment, pea aphids were counted using an individual plant cutting and shaking method since aphids usually hide under the leaves and cluster on areas of new growth. Under high aphid density, new growth started wilting and aphids covered the plants (Figure 1). Pea aphids arrived in faba beans around the early flowering stage (one calendar week later than lentil). The results of the first year of this study demonstrated that earlier spray timing with insecticides provides the highest yield. Preliminary data shows no significant difference between spraying at 50 aphids/plant and 120 aphids/plant. Aphid populations that reached 800 aphids/ plant before insecticidal treatment resulted in a loss of at least half compared to the plots kept at the lowest aphid density while waiting to apply insecticides until populations reached 1,500 aphids/plant resulted in complete yield loss. It took less than one week for aphid populations to go from 280 aphids/plant to 800 aphids/plant. In our plots, however, pea aphid populations may have been influenced by migration from neighbouring lentil and pea fields. Due to the aging of lentil and pea plants, the aphids may have migrated to the still-green faba bean fields. Therefore, monitoring pea aphids frequently from the end of July to mid-August is essential for preventing crop losses due to pea aphids.
In both pulse crops, given the high densities of pea aphids that are possible, it is important to monitor and control aphids, as unmanaged pea aphid populations can spread into untreated fields in bad aphid years. Our experiments using contact and systemic insecticides indicated that contact insecticides gave better control of pea aphids than the systemic insecticide that we tested. To limit the development of insecticide resistance, producers should rotate between different classes of contact insecticides when controlling pea aphids. Now that we have uncovered the devastating effects of high pea aphid populations, our work continues to pinpoint the optimal aphid density to use in control decisions.