New strategies for managing San Jose scale
by Sally Colby
Each year, growers are challenged with managing disease and insect pests, both of which are the focus of scouting programs. Emily Pochubay, tree fruit IPM educator at the Northwest Michigan Horticulture Research Center, discussed an important and potentially devastating orchard pest.
“San Jose scale is a sucking insect and is a pest of many crops,” said Pochubay. “We talk about it in the context of sweet cherries and other tree fruits, but it can attack a lot of different crops, and it can cause both direct and indirect damage.”
Pochubay said in sweet cherries, the concern is direct damage to the tree. “In high populations, the insects are sucking sap out of the trees and cause tree health decline and limb or branch die-back, which ultimately results in loss of fruiting wood in orchards,” she said.
The life cycle of San Jose scale starts with overwintering on the woody tissue of the host as immature scales. “They produce a waxy covering over their bodies, and in spring, when temperatures reach 51º, the scales begin to suck on flowing sap,” she said. “They grow and molt into females or males. If they’re males, they come out from under the waxy covering and home in on a pheromone that’s emitted by the female to locate and mate with them.” A few weeks after mating, the females give live birth to crawlers – up to 400 or more during their lifetime.
The first flight of males usually occurs around petal fall in sweet cherries and can last into June. About two weeks after the males start to fly, active crawlers are present, and they remain active for several weeks in summer.
Some regions see two generations of the pest each season. “The second generation is occurring post-harvest,” said Pochubay. “We see the second flight of males that starts in August and a few weeks later, the crawlers will be active in late August and into early fall.”
Pochubay said there are several theories as to why San Jose scale is becoming more of a problem for growers. One is that the insect is tiny and difficult to detect. “Growers who are not intensively monitoring for it will miss it,” she said. “It tends to show up in clustered or hot spot areas, so if you aren’t looking in the right spot you may miss it. We tend to only notice it when populations are high and we see limb die-back or death.”
One of the management challenges is timing spray applications. Pochubay said the first opportunity to manage San Jose scale is in early spring with a dormant oil application. “We’re always trying to dodge frost/freeze events for early spray timing,” she said, “so it can be difficult to put spray on and avoid potential bud damage.”
The next management opportunity is targeting crawlers. “It can be difficult to find crawlers in the orchard,” said Pochubay. “We rely on growing degree day accumulations to estimate the best timing, but if you aren’t right on the mark, you might miss the opportunity and get a buildup in the orchard over time.”
The increased regulation of traditional and effective insecticide products also presents a challenge. Newer, less lethal compounds may not effectively kill scale, resulting in a higher population over time.
Many of the management challenges are related to what Pochubay referred to as insecticide-dependent spray management programs. Alternative management strategies may help minimize some of the challenges that come with insecticide dependent programs.
Mating disruption studies for San Jose scale began with researching three pheromone components. “What they found is that each of the components is equally attractive to the scale,” said Pochubay, “which is good news in terms of developing mating disruption strategy that will be effective and in terms of producing the pheromone dispenser to keep the cost low.”
Michigan researchers performed dosage response experiments to determine pheromone mechanisms and to gain a better idea of optimum field deployment density for dispensers. The goal is to keep pheromone strategies low cost but effective to ensure higher likelihood of grower adoption of the method.
Pochubay explained that Michigan research involved traps and monitoring captured males. “We can take the data and plot the catch over dispenser density – how many males they are catching in each of the plots,” she said. “They can look at the data using previously developed mating disruption theory to get an idea of the mechanism of disruption. Is it non-competitive, competitive or possibly a combination? It looks like a hybrid situation.”
After assessing last season’s work, Pochubay says 2020 data look similar to previously published work. “At some point, in the higher dispenser densities, we are achieving non-competitive disruption,” she said. “While some of the males may be taken out of the game, others are responding to the pheromone. Adding more dispensers to the plots didn’t give much more benefits in terms of disruption. With 100 dispensers per acre, we’re getting pretty good disruption in those plots. We think we might be able to half the rate of dispenser density and still get good disruption.”
In a sweet cherry study, researchers worked with five commercial orchards. “In 2019, we looked at mating disruption treated versus no disruption plots,” said Pochubay. “We monitored male catch over time.” Researchers concluded that mating disruption had an effect on how the males were responding in the disruptive plots.
In 2020, the plots were divided in half and only one half of the original block had disruption. The goal was to determine whether mating disruption could be established in alternate years to minimize labor and expense. “We found that for the first flight, in any of the plots that received mating disruption, it didn’t matter if it was in 2019 only, in 2020 only or every year,” said Pochubay. “What happened in the second flight [post-harvest] was we started to see the populations rebound in plots where disruption was removed.”
Pochubay said there’s been good progress in just two seasons on how to best address the pest. “We’re finding that over the past few years, we’ve achieved disruption with 200 dispensers per acre,” she said. “That’s good news, particularly in sweet cherries when that second generation occurs post-harvest. That helps minimize insecticide applications or the need to rely on those for this pest.”
Seeing slower scale reinfestation likely means alternate year disruption strategy will be effective, which will save growers money. Pochubay said an unintended result from the study showed that mating disruption is effective in small plots. “In 2020, small plots were half to one acre,” she said. “That led us to the potential for spot-treating areas. If you know where that pest is, you could spot treat those areas again to minimize labor.” While the finding was positive, more research is needed to confirm whether spot-treating is a viable option.
Plans for the 2021 growing season include repeating the alternate year strategy and reducing dispenser density rate to determine whether there is still adequate control.