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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

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.


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Dig In: Garden Checklist

For week of March 26:

Sacramento can expect another inch of rain from this latest storm. Leave the sprinklers off at least another week. Temps will dip down into the low 30s early in the week, so avoid planting tender seedlings (such as tomatoes). Concentrate on these tasks before or after this week’s rain:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Knock off aphids with a strong blast of water or some bug soap as soon as they appear.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to help corral blossom blight.

* Feed citrus trees, which are now in bloom and setting fruit.

To prevent sunburn and borer problems on young trees, paint the exposed portion of the trunk with diluted white latex (water-based) interior paint. Dilute the paint with an equal amount of cold water before application.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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