Do you have any pilot data on the toxicity of these oils in general? It might be best to do a small scale feeding assay where you have added the oils at a range of 3-5 concentrations to the diet of your weevils.

Try 10 microlitres per litre for a start since that is what gives 100% mortality for some herbal extracts (Article Fumigant Toxicity of EO against 4 Major Stored-Product Insects

Having tested a lot of essential oils for this specific purpose, I can ensure you that different essential oils give very different results. There is also a possibility that fumigation doesn't kill the adults but stops the developements of the eggs inside the grains.

Way of applying the product (fumigation, direct spraying...) also gives different results, so there is no easy direct answer to your question

it depends on the way of applying essential oils on the weevils as Dr. Antoine Trigaux said. Generally, large volumes kill any insects on contact like 100ul, so try small volumes of 25ul, 20ul, 15ul, 10ul in my opinion

It is recommended to consult with a qualified professional, such as an entomologist, agronomist, or pest control expert, to determine the appropriate dosage for fumigating wheat grains with essential oils to minimize weevil infestation. They can take into account the specific conditions and needs of your crop and provide tailored advice. It is also important to follow all safety guidelines and regulations related to pesticide usage to ensure the health and safety of both the crop and humans involved in the process.

It would depend on various factors such as the type of essential oil used, the concentration of the oil, the volume of the grains, and the severity of the weevil infestation.