Ortho Brush-B-Gone will work better than Roundup. It's best to do it just as the leaves start turning color, but not after. The sap will will start flowing down and hopefully carry the chemical with it down to the roots.

I'll second the vote for Brush-B-Gon or Pathway/Tordon - and recommend doing a frill or girdling cut all the way around the trunk, then applying the herbicide. Now is the time to do your application, as trees are 'moving' reserves to the root system for next year's spring flush of growth.
With most any, if not all of these herbicides, you need to apply to the cambium layer - drilling a hole and filling it is a waste, as application to the (non-living, lignified) heartwood does nothing. Only application to the functional, active cambium tissues will result in translocation of the herbicide to the roots.

Honeylocust doesn't tend to sucker.

Well, around here they do. My parents locust up in SE Iowa suckers like mad as well. I did a quick search on this forum and found others have this problem as well.

Thanks everyone for the help. I was planning on making my drill holes almost parallel with the bark (not to deep, just into the cambium). That way the tree can soak it up over a longer period of time. Will let you know if it works - should know by next spring. Can't wait to get rid of those things. I'm surprised you don't hear about more injuries from these trees.

Here is a link that might be useful: suckers

Yeah, they sucker around here, too.

Heck, roundup doesn't even barely kill grass and weeds. Look for the ingredient "TRICLOPYR", usually in stump killers etc.

I was planning on using the roundup brush killer which has some triclopyr, but will definatley look to see if the other products mentioned have a higher concentration of it. Thanks.

Yes, I remember that link. The discussion was about a different species that is also called locust, Robinia pseudoacacia.

lkz5ia- You may not have witnessed honey locust suckering, but I can assure you honey locusts do sucker - it appears they are variable and not every tree suckers. The majority of them I have seen do- some extremely bad. My parent's thornless honey locust cultiver is extremely bad. Dozens of suckers every year. However, the thorney suckers provide the most headaches. They can't be pulled as they are full of thorns shorty after they sprout.

Here are a few more links where these suckers are referred to.

http://tn.essortment.com/honeylocusttre_rjyq.htm

http://davesgarden.com/guides/pf/go/38373/

http://forums2.gardenweb.com/forums/load/lawns/msg0819310320470.html

Here is a link that might be useful: HELP! honey locust tree taking over my lawn!

I've seen plenty of suckers coming from the roots of Gleditsia. And the herbicides mentioned will work, but glyphosate (Roundup) will as well, and is considerably safer for the applicator. We've also used Garlon as a cut stump treatment, such as when we're eradicating buckthorn.

Roundup barely killing weeds and grass?...........how dead do you need it to be? I must have sprayed thousands of gallons by now, and I can assure you, the end result in every case was the death of the target vegetation.

+oM