FORAGE LOSSES CAUSED BY GRASSHOPPER CHORTHIPPUS DUBIUS (ZUB.1 ON GRASSLAND IN INNER MONGOLIA OF CHINA

Abstract To quantitatively assess the losses of forage, we conducted experiments to determine the average amount of forage destroyed (forage consumed plus forage cut-off and dropped to the ground) per day by the grasshopper, Chorthippus dubius (Zub.), at its different life stages in laboratory and then extrapolated estimates of losses caused by the field grasshopper population from these results and the population densities of this grasshopper. The main results are as follows:

• 1 Ch. dubius is a late-occurring species of grasshoppers in Inner Mongolian grassland. It occurred from June to October. The duration for each instar and the longevity of the adult under natural conditions ranged from 9. 09–9. 81 fdays) and 17. 65–19. 441days) respectively.
• 2 Both consumption and forage defoliation rates by Ch. dubius increase with its developemnt. The consumption rates had significantly linear relationships with the grasshopper's body weights.
• 3 Average losses caused by one grasshopper in its whole lifespan were 487. 55–784. 4 mg (dry weight) and the average amount of forage consumed and wasted per day was 7. 64–11. 45 mg.
• 4 Total losses caused by Ch. dubius population in 1990 ranged from 0. 762–1. 647 g/m² in the three types of vegetation, and the largest value was found in restorative type. The seasonal courses of cumulative forage losses would fit logistic equations in the three vegetation types.
• 5 Population densities (18. 5, 194., 11. 8/m², 4th instar nymph predominantly) were suggested as the levels at which control should be initiated on the three types of pastures respectively.