Why are birch leaves a sticky mess?

That's the pupal skin of a lady beetle stuck to the birch leaf along with lots of honeydew -- evidence of aphids, ants and lady beetles all at work. (Photo: Debbie Arrington)

Overhead aphid invasion follows ants up a tree

By Debbie Arrington

It's a sticky mess.

This fall, the huge birch tree that drapes over our back fence from a neighbor's yard dripped a gooey syrup over rose bushes, camellias and anything else beneath its branches.

A first, it was a mystery. What's all over my plants? There were no signs of aphids on the roses. Then, I realized that sticky stuff was dripping from above.

Now, thousands of yellowed birch leaves are raining into our yard. Covered with that sticky stuff, those leaves become glued to everything -- shoes, clothes, dog paws, rake, plants, you name it. Many of those leaves get tracked indoors to stick to carpet and furniture. There was nothing sweet about this honeydew.

I asked other gardeners if they had similar experiences during an unusually warm October and November. Yes, big trees seemed to be dripping goo more than normal, particularly birches and elms.

That syrup is known as "honeydew," a secretion from aphids. But how did these wingless critters get up in that 40-foot birch? And why were there so many of them?

Ants. They are the most likely explanation. Those little aphid wranglers "herd" them onto plants (including up trees) to feed on tender leaves and growth, then "milk" them for their honeydew to feed their colony.

Besides making a sticky mess, the honeydew also attracts sooty mold, another messy plant problem (particularly on honeydew-drenched plants in the splat zone).

Got a birch tree in or near your garden? You might
 want to check it for ants. (Photo: Kathy Morrison)

How did aphids cover such a big tree? They multiply at an outrageous rate, particularly in warm weather. Besides promoting late-season growth, these balmy days created a perfect environment for an aphid stampede.

According to the UC Cooperative Extension master gardeners, aphids reproduce asexually at maturity, which takes about eight days. Each female aphid can produce a dozen live offspring a day. As long as the weather and food supply hold out, they keep reproducing. Although their individual life cycles are short, that's a lot of aphids.

Such an infestation will cause leaves to turn yellow more quickly than normal and may stunt growth, but usually won't harm the tree. To keep it from happening again, dissuade ants from going up the trunk and starting another aphid ranch. A band of Tanglefoot or other sticky substance usually does the trick. 

While attempting to rake the sticky leaves, I discovered reinforcements were on the job. Lady beetles, who love to feast on aphids, were breeding in the tree, too. I hope they're hungry.

For more information on aphids in trees: ipm.ucdavis.edu or https://bit.ly/2c673pU