To what extent is immature eggs in insects (beetles) a good approximation for maximum fecundity?
There are a number of things to clarify here; Fecundity is the number of offspring that is produced by an individual, and this can be separated into potential fecundity (maximum reproductive capacity) and realized fecundity (number of offspring actually produced). Realized fecundity is very similar to fertility, which is defined as the number of viable offspring that are produced.

Potential fecundity can sometimes be a good proxy for future fertility, and this is especially true for species where all eggs are present after metamorphosis of the adult insect (Awmack &amp; Leather, 2002). However, in some species the formation of eggs is a continuous process in adult individuals, and this is relatively common in beetles. Therefore, the number of immature eggs might be a poor proxy of both potential and realized fecundity in these species (Awmack &amp; Leather, 2002), and this is more likely in relatively long-lived species that feed extensively as adults. In beetle species that do not feed as adults (often lacking functional mouthparts), immature eggs is probably a better approximation of potential fecundity.

It should also be mentioned that the quality of host plants will often affect the maturation of eggs (Sterns &amp; Smith, 1960; Hopkins &amp; Ekbom, 1999), and will therefore also determine to what extent the number of immature eggs is a good indication of potential fecundity. When feeding on poor quality hosts eggs may e.g. be reabsorbed by females to use for other purposes (see references in Awmack &amp; Leather, 2002 &amp; Hopkins &amp; Ekbom, 1999 for further information).

The review paper by Awmack &amp; Leather, 2002 is very comprehensive, and contains a wealth of useful references on this topic.