Observations on the feeding of adults of the red locust (Nomadacris septemfaciata (Serville))

A study has been made of the feeding behaviour of the Red Locust in the Rukwa Valley, Tanganyika.

The amount of food in the oesophagus was used as an indication of feeding. It was found that the rate at which the oesophagus emptied varied with temperature.

In March, October and November, feeding occurred in morning and evening peaks and was reduced in the middle part of the day. From April, when it continued all day, it steadily declined, and reached a minimum in September.

Analysis showed that feeding was greatest between 26°C. and 34°C. but the exact limits varied according to the temperature conditioning of the locusts, being higher when they were conditioned to high temperatures and lower when they were conditioned to low ones. This applied both during the course of a day and from season to season.

There was no evidence of feeding at night.

It was concluded that the seasonal variation in feeding must be attributed to physiological changes within the locusts, as must a marked increase in feeding in females as compared with males which occurs in December; only at the time when the eggs were developing were marked differences between males and females in this respect observed.

A rough key was constructed to enable the crop contents of locusts to be identified. The species and condition of the food eaten depended not only on the choice available but also on the displacements of the locusts relative to the food. Mostly short grasses were eaten except in March and April when food was also available in the roosts. Of 4,555 locusts examined 95 per cent. had fed on grass leaves.

In food preference experiments, Echinochloa, Cynodon and Cyperus were the most acceptable grasses. Analysis suggested that these were the grasses with softest leaves and highest moisture content.

Mortality experiments suggested that the survival rate was highest in Echinochloa.

The number of locusts that fed depended on the grass present even in the absence of choice. More fed on the soft, moist grasses than on the drier, harder ones. Thus, far more fed on Echinochloa than on other grasses. This was true even after long periods of starvation. An indication was obtained that the locusts differentiated similarly in the field.

Marking and recovery experiments suggested that activity was related to the vegetation present in the habitat. Locusts marked in Sporobolus moved farther in a shorter time than those marked in Echinochloa.

The distribution of the locusts was associated with the vegetation, and concentration into zones of Echinochloa was an active, directed movement.

Echinochloa is an important factor in Red Locust behaviour in the field. The evidence brought forward indicates that its importance may be associated with its value as food as well as its physical properties.