Dear Master Gardener,
Please shed some light on this conundrum: my aspen trees have broken out with a bad case of acne. Well, not all of my aspens, just a few. And not all of their branches, just here and there. Come to think of it, my aspens鈥� usual display of brilliant fall golds were kind of 鈥渙ff鈥� this year. Do you think this could have anything to do with the yellowish creepy crawlies that I鈥檝e spied on some of the trunks for the past couple of springs? Please help me, Master Gardener, before my aspens become deathly ill. -- Aspen Hugger
Dear Aspen Hugger,
Your aspens are likely infested with an insect called Oystershell Scale. If you examine each 1/8鈥� 鈥減imple鈥� closely, you鈥檒l notice that they鈥檙e comma shaped and resemble an oyster鈥檚 shell. It鈥檚 a good thing you鈥檝e asked for help because these pests may eventually cause die-off of twigs and branches in addition to weakening the tree and eventually spreading to your entire grove. Not only that, aspens infested by oystershell scale may be susceptible to a fungal pathogen, Cytospora, which forms orange fruiting bodies that seeps brown liquid on the bark.
Oystershell scale begins their life cycle in late spring or early summer by emerging from a clutch of eggs that have overwintered under the hardened 鈥渙yster鈥� shell of their mother. Equipped with legs, these nymphs (your creepy crawlies) immediately spread out in search of suitable bark on which they permanently attach using secretions that act like glue. Here they probe their piercing mouthparts deep into tissue and suck out the aspen鈥檚 cytoplasm. As they go through several molts, they lose their legs and become sac-like under the hardening shell. The adult lays eggs then shrinks to allow room for them to develop over the winter.
Achieving eradication of oystershell scale may be a bit out of reach, but if you adhere to the following multi-pronged approach, you should be able to attain a satisfactory degree of control:
First, make an assessment of the general appearance of your aspens. (You鈥檝e already taken a crucial step just by observing the pimply bark and lackluster leaf coloration.) Do your aspens grow in a thicket with crossing branches and thick undergrowth, or do they grow as an open grove? This is significant because these trees thrive with unimpeded air flow, ample light, and room to spread. You may need to remove crowded branches and brushy undergrowth.
Now that you have room to move about your aspens, you鈥檒l be better able to pinpoint the site of each scale outbreak. If a heavy infestation is on an easily accessible branch not offering much in the way of balance or beauty to the tree, you may opt to remove the branch. Otherwise you may accomplish some control of the scale by simply scrubbing it with a plastic dish scrubber. Put a tarp under the tree to catch falling eggs and insects. You can do this any time of the year.
If it鈥檚 wintertime, you鈥檒l want to spray your grove with horticultural 鈥渄ormant鈥� oil to suffocate some of the developing eggs.
Starting in May and continuing through June, be on the lookout for the nymphs. You鈥檒l have to look closely; they鈥檙e incredibly tiny! As soon as you detect them, spray with insecticidal soap, which when compared to regular insecticides, will have a limited impact on scale enemies like lady bugs. (However, some immature stages of beneficial insects may be killed.) Remember, you must spray directly on the scale crawlers, and you may need repeat applications.
When the nymphs settle down on permanent sites in early summer, you can zap them with horticultural 鈥渟ummer鈥� oil. Get them early, before their shells become too thick.
Finally, before applying any horticultural oils or soaps to aspen, consult your nursery about the proper choices and usages.
That鈥檚 about it! Good luck.