A new report points out the sensitivity of two important aphid virus vectors to pyrethroids. In this article, Sue Cowgill, AHDB Crop Protection Senior Scientist (Pest), studied the implications of the results for potato growers.
Nowadays, growers have fewer and fewer ways to control insect pests. The “Draft National Action Plan on the Sustainable Use of Pesticides” recognizes that such concerns will encourage people to develop resistance. Although this may ultimately provide a comprehensive strategy for pesticide resistance management; in the short term, we must use the information and pesticides that are now available.
In terms of management, it is important to clearly consider the virus to be considered. They differ in the speed at which they are picked up and spread by aphids. In turn, this will affect the effectiveness of the insecticide and the harm of the target aphids. In potatoes, the commercially significant viruses are divided into two categories.
In the UK, the potato leaf roll virus (PLRV) is mainly transmitted by the peach-potato aphids, but other settled aphids, such as the potato aphids, may also be involved.
Aphids feed and absorb PLRV, but it takes several hours before they can spread it. However, the infected aphids can continue to spread the virus throughout their lives (this is a “persistent” virus).
Due to the time lag, it can be reasonably expected that pesticides will help interrupt the transmission cycle. Therefore, the state of resistance is crucial for PLRV management.
Non-persistent potato viruses, such as potato virus Y (PVY), are the most problematic in GB potato production.
When the aphids protrude out of the leaves, the virus particles are picked up at the tips of their mouthparts. These can be delivered in minutes, if not a few seconds. Even if potatoes are not the traditional host of aphids, they can still be infected by random aphids detection.
The speed of spread means that pesticides are often difficult to break this cycle. In addition to increasing reliance on non-chemical control, more aphid species need to be considered for these viruses.
According to the researchers, peach-potato aphids, grain aphids, cherry-cherry-oat aphids and willow-carrot aphids are key species related to PVY in Scottish seed potatoes.
Because of its key role in the spread of PLRV and PVY, it is necessary to understand the resistance status of the aphid. Unfortunately, it turned out to be proficient in producing resistance-about 80% of British samples showed resistance to pyrethroids-in two forms:
There are reports of neonicotinoid resistance in peach-potato aphids abroad. A limited number of on-site samples are screened in GB each year to monitor their reduced sensitivity to acetamide, fluniamide and spirotetramine. So far, there is no evidence of reduced sensitivity to these active substances.
The initial concern about the resistance of cereal aphids to pyrethroids can be traced back to 2011. Compared with the fully susceptible cereal aphid, the presence of the kdr mutation was confirmed and it was shown that approximately 40 times more activity was required to kill resistance.
A technique was developed to screen for kdr mutations in aphids (from the national water-trapping network). In 2019, samples were tested from five traps, and as many as 30% of aphids have this mutation.
However, this type of test cannot provide information about other forms of resistance. As a result, by 2020, a small number (5) of live grain aphids samples have also been collected from grain fields and tested in laboratory bioassays. Since 2011, this indicates that the resistance strength has not increased, and there may still be only kdr resistance in grain aphids.
In fact, applying pyrethroid sprays at the maximum recommended amount should control grain aphids. However, their impact on PVY transmission is more susceptible to the flight time and frequency of grain aphids than the resistance status of aphids.
Although there are reports that a cherry oat aphids from Ireland have reduced sensitivity to pyrethroids, bioassays on GB samples starting in 2020 (21) have not shown evidence of this problem.
At present, pyrethroids should be able to control bird cherry oat aphids. This is good news for grain growers who are concerned about BYDV. BYDV is a persistent virus that is easier to control through the use of pesticides than PVY.
The picture of willow carrot aphids is not clear. In particular, the researchers have no historical data on the susceptibility of pests to pyrethroids. Without data on the fully sensitive form of aphids, it is impossible to calculate the resistance factor (as the grain aphids do). Another method is to use an equivalent field frequency to test aphids. So far, only six samples have been tested in this way, and the kill rate is between 30% and 70%. More samples are needed to have a more comprehensive understanding of this pest.
The AHDB yellow catchment network provides local information about GB flights. The 2020 results highlight the variability in the number and species of aphids.
The Aphid and Virus page provides overview information including resistance status and spraying program information.
Ultimately, the industry needs to move to an integrated approach. This includes long-term measures, such as the management of virus inoculation sources. However, this also means using other alternative methods, such as the use of intercropping, mulch and mineral oil. These are being investigated in AHDB’s SPot farm network, and it is hoped that the trials and results will be available in 2021 (depending on the progress of controlling a completely different virus).