I would like to calculate total amount of food that beetle larvae consume during larval stage. Unfortunately, they are soil dwellers by feeding on organic-matter rich soils. Feces can’t be distinguished from the food (soil). So, I used indirect methods to estimate their feeding performances.

I found that food retention time in the gut had linear relation with larval weight as:

R = 3W

where R is retention time (h) and W is larval weight (g)

and gut load (amount of food in larval body at a time) had linear relation with larval weight as:

L = 0.25W

where L is gut load (g) and W is larval weight (g).

The problem is beetle larval weight is not constant. It gradually increases over time. The growth of beetle larvae was varied in each individual and non-linear. Now, I have data about beetle growth for many individuals (the figure below is an example of the growth from one beetle). How can I estimate the total amount of food (soil) that each beetle eats during larval stage from these data?  

Thank you in advance.