Nursery Notes: Increase your spray water to kill scale
The white peach tree scale that is attacking ash and lilac so bad in the nurseries is being harder to kill than I imagined. While oil is effective and air blast sprayers are generally great, I think the primary limiting factor has to be too little spray water being used per acre. Not making a second application a month or two later in the winter may be another missed opportunity.
Most air blast sprayers apply about 20 to 30 gallons of water per acre. That seems to be fine for foliar insects and leafspot diseases. Air is a major carrier with the air blast sprayers, in addition to water.
But oil kills by suffocating insects that can’t crawl out of it. The oil also melts away the waxy covering of scale insects and allows them to dry out. Absolute total coverage is essential for effective control. Insects not covered will not be killed. Generally, scale is more difficult to control on the trunks. The tree canopy catches the spray. Oddly, one nursery is controlling scale on the trunks, but not the canopy of his larger ash trees.
It may be necessary to stand back and watch where your air blast sprayer is putting the spray. If needed, make one trip to cover the lower trunks and an second trip to cover the canopies.
It appears that the oil must be applied in 80 to 100 gallons of water per acre; that the bark must be wet, but not necessarily dripping. Use thumb tacks to hang pieces of toilet paper in the canopy and on the lower trunks. Water sensitive paper can be purchased from Spraying Systems that make TeeJet spray tips. Two by 3 inch pieces of white paper turn blue when wet.
The easiest way to increase water volume is usually by changing tip size. When spray tip size can not be increased, reducing tractor speed and increasing pressure are the only other options.
One producer did not get control after making 4 trips around his ash blocks. I assumed he would have. The dead scale dried out and rubbed off easily with the hand, like dust. I found living adults under the dead ones. You can choose to battle them in the winter or summer or both. You can spray in the winter or summer. Dormant oil is recommended during the fall and winter. Dormant use rates are normally 1 to 2 gallons per 100 gallons per acre.
Summer oil is recommended when foliage is present. Summer use rates is usually about half of the dormant rate. Only use a product in the summer that explains on the label how to use it in the summer. Never use oil on maples in the summer and be sure they are totally dormant when dormant oil is applied.
White peach tree scale is also being found on weeping mulberry, privet, birch, willow and pear. It is not a new pest, but it has become widespread here rapidly. The population explodes rapidly with 3 generations a year.
Summer applications can be timed by using double stick tape as an indicator of when the crawler stage is present. Wrap double stick tape around a few trunks and branches. After hatching, the babies crawl up or out on the branches to the more tender parts. They will get trapped on the tape and can be seen. A hand lens helps me.
Crawlers are more easily killed than adults. Adults only produce one set of babies and then die. The babies mature rapidly in time to produce babies that will mature and produce babies in the same season. Three generations per summer.
If you choose a summer attack, then spray and kill the first generation of crawlers. We never kill 100 percent of anything. Some crawlers are likely to escape and produce the second generation.
Time a second summer spray to kill the smaller second generation of crawlers by watching the tape. Fresh tape will be required occasionally. I would expect killing two consecutive generations will solve the problem until more move in. But, it might be good to watch tape in case there is a third generation.