May 29, 2013 | Other

“Oh no, my plant has scales!”  This declaration might sound strange to a person unfamiliar with plants and plant pests.  “No, your plant has leaves” might be their first response, but we are referring to the insect order commonly known as scales.  A few weeks ago while inspecting and removing tree shelters from our sugar maples, I noticed these interesting, but destructive creatures along with red ants all too happy to lap up the “honeydew” or sugar exudate produced by the scale infestation.

Scales are closely related to other “sucking” insects like aphids, mealybugs, and whiteflies.  The adult female scale affix themselves to one spot and feed there the rest of their lives.  The males adults are small, winged, and do not feed on plants. The juvenile “crawlers” can be seen with the aid of a magnifying glass and look like little moving disks on the plant.  The juveniles are hard to notice, but if you catch them young they are more susceptible to treatments and do little damage.  Like aphids, scale infestations can be triggered by plant stresses like disease, over fertilization, or over watering.  I am attributing our infestation to the wet spring that we have experienced in the Northeast.

Scale on Sugar Maple

These are soft scale that had already found their feeding spots and inserted their stylet (slender feeding tube) into the trees. A few scales by themselves won’t cause much damage while feeding, but they can expose the plant to viruses that may cause much more damage than their reservoir host. So, it is important monitor your plants for scale and treat as soon as possible.

Identifying Scale

There are two different types of scale, armored and soft scale.  This description might be counter intuitive since most soft scales produce a shell that is just as hard as their armored cousins.  Armor, however, is in reference to the waxy covering produced by the armored scale.  Armored scale typically don’t excrete a large amount of “honeydew” so few, if any, ants will be around the infestation.  Soft scale secrete a lot of “honeydew” and have semicircular profiles (they remind me of snail shell profiles) as adults.  However, the best way to determine which type of scale you have is by using a pocket knife to “open” the scale.  Slipping the knife underneath the shell you should be able to flip the armored scale open without dismembering it.  Soft scale shells will not remove from their bodies.  Knowing what type of scale you have will help you better determine the most effective treatment.

How To Treat For Scale?

In treating scale the most drastic approach (a broad spectrum pesticide) is probably the worst thing you can do in the long run.  Oddly enough pesticide use can trigger scale infestations.  Natural scale parasites and predators usually keep populations in check, but when the parasites become collateral damage of a chemical war waged on another insect, scale will thrive.  The method we used was a physical control, rubbing off the adult females we could reach by hand, and then allowing the natural population of parasites and predators to do the rest.  On inspection this week, only a couple scale remained.  Other methods include treating with insecticidal soaps or oils (very effective against the juveniles) and releasing biological controls.

Biological Controls Of Scale

Green lacewing larvae are predators of many pests including scale.  They are usually sold as eggs that hatch out hungry in a matter of days.   Mealybug destroyers will also feed on scale even though their preferred food are mealybugs.  There are also parasitic wasps available for release against armored scale.   These biologicals are available from a number of different suppliers including our friends at Beneficial Insectary.

Happy Gardening!