Hi,
I saw a fly 1/2 inches long, black head and legs and bright, yellow body. It was stationary as if planting larvae(?) or doing something else. I saw it several times on the roses. I wander how bad it is for the roses?
Thank you.

Dina

You may have been looking at a large rose sawfly. The behaviour you saw may have been egg-laying. If so, the larvae, when they hatch, resemble caterpillars.

Here's the adult:

http://www.galerie-insecte.org/galer...agana_800_.jpg


I have a pesticide-free policy in my garden, but I know that this isn't for everyone. Treat the larvae as any other caterpillar if you want to avoid damage to the foliage.

Thank you. This is the one I saw!
I don't like pesticides either but what is alternative? How do we treat caterpillars without using pesticides (apart from killing them by hands)?

Dina

I think you'll have no option but to pick them off by hand. (Although in my garden, we have so many actively feeding small birds that nothing survives for very long.)

If you let the larvae reach maturity, the adults that emerge will produce up to two more generations this year, depending on the weather.

Good luck.