Ok, I don't know anything about hive beetle larvae and their life cycle, but if I understand your question right then you want to get these lavae while they are in the soil. I am afraid here you come up against the limitations of neem oil.
One of the reasons that neem oil is so very safe for the environment is that it is not stable at all. It breaks down very quickly once mixed with water or when exposed to UV light.
And that means it's not really possible to get to the larvae in the soil. Spraying the surface so it gets them when they emerge is a waste of time, because the active ingredient in neem oil is not stable enough.
Whether mixing the soil with neem cake would make any difference I don't know.
Neem cake is the residue left behind after crushing and pressing neem kernels for neem oil. In countries where neem is grown for commercial neem oil production neem cake is used extensively for soil improvement as it stimulates beneficial soil organisms and combats pathogens. But if it would have any effect on the larvae I don't know. I also doubt the results would justify the effort and cost involved.
Neem oil on Hive Beetle larvae by: Bruce
I had actually gone out and bought some neem oil locally and upon opening the directions taped to the side of the container, read these words: "Bee Hazard: This product is toxic to bees expoused to direct treatment. Do not apply this product while bees are actively visiting the treatment area." The Hive Beetle, as I understand the pest, flies into the hive and lays its eggs. When the larvae hatches out, they crawl out of the hive and into the soil where they pupate. So, this information (from your response) was very helpful if the Neem oil does not actually get into the soil to kill the larvae. Other beekeepers are looking at beneficial nematodes which live in the soil and attack the larvae.
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