Megastigmus aculeatus

David, if you google Megastigmus aculeatus, you’ll see ssp./var. nigroflavus. Megastigmus aculeatus was investigated as a biocontrol agent for Rosa multiflora and Rosa rugosa. Sheesh. First Rose Rosette Disease and now this. There is good information about this pest.

My question is whether it is difficult to know if rose hips are infested with this pest.


is difficult to know if rose hips are infested with this pest

I think ‘infested’ is too strong a description. I come across larvae all the time when extracting embryos but never at a level that is alarming. Almost always only a few seeds are affected so it looks like the critters don’t lay massive numbers of eggs at any one time.

The largest numbers of larvae I have encountered were in seeds from spins and species roses from Canada, West Virginia and DC. These were at the level of about 10 - 30% of seeds in the worst cases iirc.

In seeds from my own garden and from Elizabeth Park they turn up regularly but never in alarming numbers.

No holes like in pictures above.

Werner, the holes are made when the larvae chew their way out of the seeds.

The variety that was touted for R. multiflora control is becoming widespread in the midAtlantic states.

One thing worth knowing about it: it’s small. It works against multiflora because its ovipositor can reach far enough inside the very small hips of multiflora and it can oviposit in the ovules there, all the ovules, because it can reach them.

It’s active in rose gardens, but many of the roses we’re interested in have much, much larger hips. Mme. Megastigmus can only reach the uppermost seeds on the big hips. So at least some of the fertilized ovules should make it through. (I’d expect seeds that become exogenous to be especially affected.)

I live in Jersey so that is Mid Atlantic. I have had many exogenous seeds this year, and in times past I would toss them but this year decided to keep them. I think I may regret that now.

I just want to mention also that the about seeds in the pictures I posted that the tiny black dots on the seeds are not holes… they are in fact the snail like grubs.

I think Don is surely right.

I did

Jeanie, the little black dots look a lot like something that I regularly see on rose seeds. I think it is probably a fungus, or could be a bacterial colony. Are they hard little bumps that you can rub off? Have you ever seen them move? If not, they are probably not slugs.

I haven’t opened seeds that have these on their surfaces, so haven’t confirmed whether or not they are detrimental to the embryos.

Jim Sproul

the little black dots look a lot like something that I regularly see on rose seeds.

I’ve been meaning to have a look under a microscope, but just looking with a jeweler’s loop I’m pretty sure these are dried-up anthers.

Hi Jim,

When I look at these under a magnifying glass they seem to have a soft body and the underside of them resembles a snails body. But they are very small… it is really hard to tell and like I said when I pick them off the seed they leave an indentation in the seed as though they were trying to eat through it. I tell you though I have had absolutely no mold on my seeds this year, maybe these little guys are keeping them clean for me.