The estimate is based on a series of five glasshouse and field trials where a regression relationship was developed between grain loss/head and larval numbers.

Measuring yield loss in sorghum trials by weighing grain from individual heads is subject to major sources of variation. While the damage relationship in these trials was highly significant (P<0.001), it explained only 11% of the variation.

The value of 2.4 grams/larva is within the range from previous published research (1.6 to 8.4 grams/larva).

Results of the field trials showed that when the grain number is reduced by corn earworm, the remaining grains partially compensate by increasing weight. Percentage compensation at 50% seed set ranged from about 20-40%, which is similar to the amount of compensation after damage by sorghum midge.

Another key point when pest densities are low is that natural enemies can greatly reduce survival of larvae. At this time of the year it is not uncommon to find 30-50% of small-medium larvae parasitised by the small wasp, Microplitis demolitor. At low pest densities, the activity of this biocontrol agent combined with other mortality factors e.g. natural NPV, may be enough to tip the balance against applying a treatment to control grubs.