The grape IPM crew has been busy stripping bark and counting mealybugs and scales to monitor development in Niagara.
Current growth stage of Mealybug
In the majority of sites we’re monitoring, mealybugs are mostly adult or have already laid eggs. This is about normal relative to vine development with first instars starting to emerge in the next 7-10 days around lead shot to pea-sized berry growth stage of grapes. A post-bloom spray of Movento will be effective in reducing this summer generation of mealybugs. We are also finding young scales so the timing is about right for scales too.
If you do not know for sure that you have mealybugs or scales, this is an expensive spray to apply recreationally. If you applied Movento prebloom for phylloxera, you will still have activity vs mealybugs when they emerge.
How much mealybug is too much?
A threshold for mealybug has not been developed. However, a single crawler feeding on an infected vine and moving to a healthy vine to feed can successfully transmit grapevine leafroll virus. Each egg cluster contains about 50 eggs, each of which can hatch to produce a new crawler capable of transmitting leafroll virus. Also consider that we are looking at only 10 vines per week and the values in the graphs are mean values. For example, at one date and site, the number of first instars ranged from 81 on one vine to 0 on most vines. The bottom line is if you have mealybugs (or scales – not mentioned extensively in this post but also leafroll virus vectors) and leafroll in your vineyard, applying a spray at this time will at least reduce the spread of leafroll in your vineyard.
Research results – rate and timing of Movento spray
In a replicated trial conducted in 2015, Movento was applied postbloom only or postbloom with a followup spray 30 days later. The most effective treatments included Movento applied at 420-500 mL/ha at postbloom, followed by a second application 30 days later. A 500 mL/ha application at post bloom and a split application of 365 mL at each application date also significantly reduced the population. In the figure below, bars with the same letters are not significantly different.
Does Movento work? Is it required every year?
The graphs below show the changes in mealybug counts from 2018 to 2019 after 2 sprays of Movento at 460 mL/ha in blocks with different levels of infestation. In the first site (Cab franc, East Niagara) there was a significant decrease in the number of mealybugs. For this vineyard, at least one spray of Movento is recommended.
The population at the second site was considerably lower than the first site in 2018. Monitoring stopped after the second Movento spray in 2018. In hindsight, looking at the egg cluster counts in May 2019 we should have continued to monitor for the remainder of the 2018 season. A spray of Movento was applied July 14 in 2019. The population was very low, and mealybug was not detectable in September.
In the third site, Movento was applied only once. The population was lower but started to creep up later in the growing season in 2018. In the spring of 2019, the population was lower than in 2018 but started to increase in the second generation. The most recent count shows a spike in first instars. In hind sight, a second application of Movento in 2018 might have kept the population lower in spring 2019. Two Movento applications in 2019 appear to have reduced the population down to negligible.
These results suggest that once the population of mealybugs is significantly reduced, Movento sprays are not required annually. It is important to continue monitoring in subsequent years to ensure that the population doesn’t start to build up again and spread leafroll virus.