Responses of plant-parasitic nematodeMeloidogyne incognita to carbon dioxide determined by video camera-computer tracking

A computer tracking system was used to quantify the responses of infective second-stage juveniles of the plant-parasitic nematodeMeloidogyne incognita to carbon dioxide. A sudden increase in concentration caused an increase in the rate of locomotion and a decrease in the frequency of changes of direction. The threshold was about 0.01 % vol CO₂/vol gas when the baseline concentration was very low and 0.05% CO₂ when the baseline concentration was 1% CO₂. The latter value represents a relative change of 5%. Concentrations above 10% CO₂ caused a general decrease in movement. In a second type of experiment, a constant concentration gradient of CO₂ was established, and the net movement of the nematodes along the gradient was determined. At low concentrations, the threshold was about 0.02% CO₂/cm. At higher concentrations, the threshold gradient was below 0.01% CO₂/cm or a relative gradient of less than 1% change/cm. At all concentrations to which nematodes responded they were attracted. The degree of orientation was estimated to be approximately 10% under most conditions. The rate of migration under the most favorable conditions was about 0.7 cm/hr. Three possible functions of the response are discussed: attraction to roots, movement toward optimal depth in soil, and as a collimating stimulus.