Each year, more growers in the province seem to be having increasing problems with San Jose scale (SJS). With such a small size, early infestations are often overlooked. In most cases, growers aren’t even aware there is a problem until it is too late. By the time they see obvious symptoms on the fruit (Figure 1) or signs of poor tree vigour (Figure 2), the scale population is well established and very difficult to eradicate. Intensive management is needed for a number of seasons to help bring population pressure down, only after which can a maintenance program be put in place.
Becoming A Bigger Problem?
There are many reasons for why growers may be seeing an increase in SJS pressure:
- General decline of dormant oil sprays – Finding a reasonable window in the early season suitable for an application of oil can be very difficult. As well, concerns over impact to tree and/or yield and increased cost of some oil products have resulted in fewer growers applying dormant oil.
- Re-evaluation decisions cancelling or limiting use of effective products – In some orchard blocks, increasing SJS populations began shortly after the loss or restrictions of organophosphate insecticide use, including Guthion, Diazinon and Imidan.
- Resistance? – Scale species are known to be prone to insecticide resistance development; however, no recent studies have looked at resistance status.
- Newer active ingredients not as lethal – While newer insecticides may be providing some control, the overall knock-down or length of residual toxicity may not be as effective as previous products used.
- Lack of active monitoring – Since SJS is so small, it is hard to know a population is present unless using pheromone traps, scouting for immobile scales and conducting harvest assessments.
- Insufficient spray volume / coverage – As Peter Shearer from Oregon State University says: how much water is enough? A lot more than you want to use!
San Jose scale overwinters as immatures, known as “black caps” (Figure 3) under a thin waxy covering on trunks and branches. The blackcap stage can tolerate temperatures as low as -23˚C. They begin maturing as soon as sap begins to flow in the spring. Males emerge around pink and continue activity over bloom.
While male SJS fly, they travel only short distances – such as a few trees – and often times, walk to females. This is why populations can often be overlooked as they begin in small pockets throughout orchard blocks.
The adult female SJS are immobile so release pheromones to attract males to the area. Each mature female can live for 6 weeks, producing approximately 10 crawlers (Figure 4) per day, or 150-500 crawlers per female per generation.
The first-generation crawlers emerge as live young generally beginning mid- to late June and can continue for 4-6 weeks. Crawlers are most active for the first few hours before finding a suitable place for development (bark or fruit). Feeding starts within 24 hours after emergence. Once feeding, the immature scale begins secreting wax, forming the protective shell.
There are two generations of crawlers in Ontario. As mentioned, the first generation begins mid- to late June. This emergence is typically synchronized, and peak emergence occurs 7-10 days following first crawler activity. The second-generation crawler activity tends to begin early August and can continue into harvest of early varieties.
Could Mating Disruption Work?
In 2019 and 2020, Dr. Larry Gut and Extension Educator, Emily Pochubay from Michigan State University studied the efficacy of mating disruption for SJS in apple and sweet cherry orchards. Experimental twist-tie dispensers (currently not commercially available) were applied to multiple <1 ac plots at the Trevor Nichols Research Station in Southwest Michigan, the Michigan Horticultural Research Center in Northwest Michigan as well as 1 commercial apple orchard and 5 commercial cherry orchards in the state. Blocks treated with mating disruption at ~200 dispensers / ac were compared to untreated blocks using a grower standard program. Population density in all blocks was measured using pheromone traps for male SJS flight and black electrical tape wrapped around tree branches for crawler activity.
In all sites, trap shutdown was observed in the mating disruption blocks in 2019 and 2020 (Figure 5). In some cases, blocks treated with mating disruption in 2019 but not 2020 still saw less SJS than the grower standard, suggesting carryover from the previous season may provide enough control for at least the early part of the following year. However, since there currently is not an economic threshold for SJS, it is unclear if alternative year use could maintain acceptable control for this pest. The Michigan research team will continue their trials to determine if the strategy is viable.
Gut’s research also looked further into the mechanism of mating disruption in SJS, finding it appears to be a hybrid between ‘competitive’, or creating a false trail away from females, and ‘non-competitive’, or desensitization of males so they’re unable to find females. In other words, some portion of males in the population are taken out non-competitively while others can still respond to the pheromone so compete between dispensers and females. This is positive news for the effectiveness of mating disruption for SJS, as non-competitive mating disruption can achieve close to 100% control and is often not considered density-dependent. This means lower dispenser densities may still be effective if pest populations are not too high, potentially reducing the cost of this kind of program.
Dormant Oil Still an Important Tool!
As with other mating disruption programs, control of SJS would be most effective under low pest pressure. One of the key management tools remains dormant oil and would have a great fit in any mating disruption program, offering a soft chemical alternative while still providing good control of the pest.
However, while a dormant oil application can be an excellent management tool, timing and thorough coverage are critical to achieve good control. Understanding how oil works, as well as the biology of the pests will help to optimize control applications and avoid tissue damage.
Oil can work by suffocation, interfering with mite egg development and/or prevent settling of scale crawlers. If applied properly, the use of dormant or delayed dormant oil can sufficiently reduce the amount of sprays that may be needed for both scale and mites later in the season. Early season oil sprays also fit well into IPM programs because the product is applied before predatory mites and other beneficial insects are present. Despite warnings by some in the northeast, there is no published scientific evidence that oil may reduce the overall health of the trees.
Unfortunately, conditions in early spring in Ontario are not always optimal for good coverage. Do not apply oil within 48 hours of freezing temperatures, before rain or within 14 days of captan. If multiple applications are needed, drop to a 1% solution when green tissue is present.
Gut, L. (2020, December 8-10). Insect Control [conference presentation]. Great Lakes Expo, Michigan, United States.
Pochubay, E., Gut, L. & Rothwell, N. (2020). Further investigation of hand-applied mating disruption for San Jose scale in sweet cherries. Northwest Michigan Horticulture Research Center Research Report: https://www.canr.msu.edu/nwmihort/research/field-efficacy-of-hand-applied-mating-disruption-for-san-jose-scale-in-sweet-cherries.