I have a forsythia, just planted last fall and I discovered today that there are hundreds of large black ants in the ground under it. Is this bad? I don't want to kill the ants unless they are going to harm the shrub. They seem to have quite an extensive mansion going on under there. The shrub so far looks ok.?

I do not know about the ants in your area, but here, ants making nests around and under plants, usually wind up killing the plant. They make cavities around roots creating air pockets that cause roots to die back and deprive the plant of water and nutrients.

I would suggest a call to your local County agent for more info.

http://extension.missouri.edu/...

That is what I was afraid of. the ants themselves are harmless, we don't fire ants or anything like that. But I still thought the plant might not like it. Thank you for the link also, I may give them a call later today.

I read the post again. I missed the word 'large' the first time. Are these carpenter ants?

well they are just big black ants. Not the tiny grease ants we get in the house sometimes. I have no idea, we just always called them black ants. I do know what termites look like for sure and I know they are not those.

You are very welcome.
I read what Greene Co Co-op said about carpenter ants, that there are 3 species of them in your area, but couldn't find anything on them nesting under shrubs...just living in rotting wood/limbs etc. I know they can be very damaging to homes. My concern would be the inadvertent damage to the plants.

Carpenter ants usually create tunnels in good wood, not rotting wood. They create chambers in which to live and breed. The photo looks like the ants are living in the ground so probably not carpenter ants.
I saw something they call 'Field Ants' that live in your area; they can be almost any color including black. Perhaps these are only 'field ants' and not to worry too much.

These are living in the ground. They have little mounds of soil and tunnels everywhere.
We cut some trees in the area two years ago so they could be after the old roots? well either way, I think I will just persuade them to move, because I am worried about my bush.

That was what I thought also Greene, but according to the Univ. of Missouri link, they prefer moist and rotting for nesting...

"Carpenter ants normally construct their nests in hollow trees, logs, posts and landscaping timbers. They prefer to nest in wood that is moist and rotting or that has been hollowed out by decay or by other wood-destroying organisms. Carpenter ants remove wood in the form of a coarse sawdust-like material, which they push from the nest. This often results in a cone-shaped pile accumulating just below the nest entrance hole. This pile may include other debris from the nest, including bits of soil, dead ants, insect parts and other food remnants. They cut galleries along the grain of the wood, preferring the softer spring grain. They leave the harder summer grain, which serves as walls separating the tunnels. They cut openings in these walls to allow access between tunnels. Carpenter ants keep their galleries clean; the tunnels look smooth as if sanded. "

http://extension.missouri.edu/...

so they are bad for my bush?

My guess would be that they are the field ants. They are probably eating the 'honeydew' produced by aphids. Check to see if there are aphids nearby and use the garden hose to knock the aphids off any plants in your garden. The ants don't pose an immediate threat to the Forsythia but as @Moonhowl said, over time the cavities/holes in the soil will have a negative impact of the shrub. I would send the ants an invitation to live elsewhere.

alright guys, I will move out the ants! thank you !

You are most welcome. I think the shrub will be healthier without the ants.

That’s really bad!! My plants are quite safe from the pests as I use regular services of Pest control Port Macquarie company. They provide organic services so the plants really don’t have any harmful effects.

@Walter5 Welcome to ATP.

I thought the pest control name looked familiar... Afraid NSW is a bit far away to be of help to Frillylily, but getting rid of the ants organically is a good idea.

I thought it was an advertisement for the pest control company. Yep, long way to travel to kill an ant.

Benefit o'the doubt, Greene and I agree, Port Macquarie is on the coast of New South Wales, Australia....

One year, the first year I grew lavatera, the plants were heavily covered with ants after they flowered. I haven't noticed many ants on the plants since then.

Maybe there were a lot of aphids that year, and the ants were chasing the honeydew.

Welcome to All Things Plants, Walter5 !