Notonecta glauca- how to beat them without chemicals?

Hello!
I had a number of casualties in Macropodus opercularis and Gambusia affinis due to Notonecta predation. Removed all the surviving Macropodus, put in the small pond lotus only, and after few weeks, a Gambusia trio caught from a lake nearby. Male dissapeared, one female showed a dark discoloration on the posterior part and the body was found a week later in decomposition . Before letting them in the Death Pond, I obtained about 20 youngs, and after finding the body of female nr.2, let the little ones to challenge the Beast.
Seven weeks later the little ones are young adults and the initial female looks like "the Mother of all Fishes"- big, fat monster loaded with youngs. They were new fry from her, but cannot appreciate the level of predation from Notonecta or the fish themselves. It seems good till now, but...
The pond is full of small 5mm Notonecta- I wonder what will happen later, Gambusia carnage maybe ? I caught about 50 adults before placing Gambusia, but not possible anymore because the plants. Can you tell me how to get rid of them without chemicals? Will they decline in a natural way eventually?
Thanks!

That's a tough one; I've never heard of anyone wanting to kill off backswimmers before. A broad-spectrum insecticide would work, but would probably kill off your zooplankton and other desirable critters; in any case, the pond would probably be swiftly recolonized. I can assure you that Gambusia affinis in their native habitat reach very high abundance in waters with large populations of notonectids and other predatory invertebrates.

Likely the adult Gambusia themselves will help control the notonectid population once they reach high numbers. Until then, you might try rearing the young fish in a tank until they are large enough to resist predation.

Good luck!

Thank you, Newt!
In fact, I was hoping that next move will belong to Gambusia eating fresh hatched Notonecta larvae, but is not the Gambusia I am affraid for. These are funny and relaxing to watch- they give a feeling of satisfaction as they are so durable. Yes, I saw how many they can be in the 5-10cm shallows, almost completely covering the bottom!
But that is in the wild and we talk about Damnbusia, and here I have a 1,5 cubic meter foil pond destinated primarily to captive Macropodus ocellatus- I am sure that in Chinese rice paddies/lakes there are many invertebrate predators as well, and the ocellatus thrive in the same conditions as Gambusia. I just not have yet the "critical mass" of ocellatus to let them earn their "land" by fight. I don't know if the Chinese can be much of a challenge for Gambusia in exploiting new opportunities and tough habitats, although inhabiting the same niche.
Other problem is Notonecta entering guppy and other more tender fish tanks... but easier to handle, since most of them are in 90l tubs. 2 killers took about 10 guppy lives in 2 days until being predated by me.
So, I should let the nature work- you gave me hope about that way of solving the problem!
Cheers,
Fabian

How 'bout copper?
I have no first hand experience but have read that copper kills inverts while mostly leaving fish unaffected. Don't take my word for it, but may be worth looking into.

In your place, I'd leave nature to take it's course. As long as you have no valuable, rare or fragile fish, wait and see what happens. My experience seems to suggest healthy fish populations keep the backswimmers under control. I see the heaviest concentrations of backswimmers in vernal pools without fish. Obviously your species may be different from ours.
I'd guess you'd need a screen cover to keep them out completely.

Thanks, Mikez!
I think copper won't work on air breathers (?), but on other invertebrate types- worms, molluscs etc. They kill backswimmers in fish farms with a little petroleum subproduct or an oil, which makes a thin film on the surface and suffocate them. I find weird the first method, but it seems it works.

I see the heaviest concentrations of backswimmers in vernal pools without fish

If you say so, it's clear that they feed on other inverts better and decline in the presence of fish . I will definitely use a cover next year to prevent reinfestation.
Cheers!