Aphids are generally seasonal, showing up in spring when all that new, tender growth sprouts like supreme salad bar. They are usually a recurring mainstay of gardening.
I don't know how much you paid the guy, but there are systemics (Bayer makes one) available at garden centers for about $30-40.
They may or may not visit your tree, but you should be on the lookout for them, esp. in spring. They're fairly easy to control with either a strong spray of water, neem oil, or insecticide of your choice. Using a systemic is just a worry-free insurance.

wanda

I live in Davis with a Hackberry in the front yard. We've been using the Bayer systemic for about 5 years now. We forgot one year and regretted it. The systemic takes about 3-4 weeks to get into the upper levels of the tree, so it's good to apply the Bayer early spring before the tree has leafed out. Our next door neighbor is a plant pathologist at UCD and suggested a half dose in early spring and the second half in June. We have found that a single dose tends not to last the whole season - we were getting some aphid activity as well as scale in September. The Bayer systemic is easy to apply and I'm sure cheaper than paying someone to do it.

I would not recommend spraying. The Hackberry is too big to reach effectively with a spray.

BettN

Thanks for the replies.

Yeah, after looking into this a bit, I see that U.C. Davis recommends Bayer Advanced Tree & Shrub Insect Control, available at Lowe's online for $20. I was paying between $100 and $200 for the guy to come out!!!

Two half-dose applications sounds interesting. The pest control guy would come out every January and do a single application, and it gave us pretty effective control over what was a huge mess before, but two half-doses makes more sense. I originally looked into a spray of some kind, like an insecticidal soap, but our Hackberry is gigantic and it simply wouldn't be feasible. I dislike the thought of using such a non-specific pesticide in my yard (I otherwise garden organically) but we simply couldn't use our backyard during the onslaught of aphid honeydew. :(

I wouldn't stop treatments yet. Five years ago we removed a huge vine that had a similar problem which also extended to the Pluot. If we don't spray the Pluot yearly the aphids are back, altho' fewer each year. There may be a smaller, unnoticed infestation nearby that might also need to be treated in order to totally eradicate the aphids.