Look familiar? I've been deluged with letters and emails from panicked readers inquiring about small, white, fuzzy "things" all over their maple leaves. Some fear it's a disease, others think it's pollen. Actually, what you're looking at is an insect -- cottony maple leaf scale -- and it's a huge problem on Long Island this year.
The scale wreaks havoc on trees and can lead to a premature loss of foliage as well as dieback of some branches. The insects also excrete a lot of honeydew (droppings), upon which the black sooty mold grows. The mold eventually blackens leaves and maybe branches, and then everything under the tree.
Treatment is dependent on growing degree days, a formulaic system used to determine precisely when different insects reach different stages of maturity. Because different stages of different pests are vulnerable to different control methods, this is important to know.
Here's how it works: On March 1, the average daily temperature is recorded, and for each degree over 50, one point is assigned. On each subsequent day throughout the season, the number of degrees over 50 is added to a running tally. If the temperature is exactly 50 degrees or below 50, then the day is assigned a score of zero. So, for instance, if on March 1, the temperature was 53, the GDD on that day would be 3. If it was 60 on March 2, the GDD would be 3 plus 10, or 13. Let's say that on March 3, the temperature was 49. That would add nothing, and our accumilated growing degree days would remain at 13. Get it?
So, the first thing you need to know is the approriate time window to treat for cottony maply leaf scale. I can help you with this one: Treatments for this pest only will be effective if applied from 802-1265 growing degree days.
The next thing you need to do is figure out what the GDD is today. What? You weren't keeping track? No worries. I can help you with this one, too. Actually, the Cornell Cooperative Extension of Suffolk County can help. They post an updated accumilated GDD report every day. You can find it here: http://ccesuffolk.org/assets/GDD-Reports/2010-Daily-Weather-Email.pdf. Find the town closest to yours and go with that score. Right now, it's 802 in Bridgehampton and 1206 in Central Park. All the other towns listed fall somewhere in between.)
Right now, every town listed on the chart is in the scale treatment zone of 802-1265, but this can change quickly with a few hot days. If the weather hits 90 today, as predicted, that would add another 40 points to the GDD. So you can see how quickly things can change.
Next, get your hands on some horticultural oil and apply it at the summer rate (read the package for instructions and follow them precisely).
Hold off on the chemicals for now, but if you find you need to escalate after the horticultural oil, as a last resort, chemical pesticides that are available to homeowners to treat this scale are acephate (brand name: Orthene; 9.4% emulsifiable concentrate, malathion (25% wettable powder) and carbaryl (Sevin), applied when the insects are at their most vulnerable, during their crawling stage. To determine when to treat, wrap a bit of sticky tape around a few branches and check it daily. When you see tiny specks on the tape (crawling scale), you’ll know it’s time.