Effect of mixed cropping on some insect pests of carrots and onions
1984
Uvah, I.I.I. | Coaker, T.H.
Mixed cropping carrots with onions reduced attacks by carrot fly, Psila rosae Fab., on carrots and Thrips tabaci Lind, on onions, compared with those on carrots and onions in monoculture. Increased plant density also reduced the numbers of carrot-willow aphid, Cavariella aegopodii Scop., Carabid and staphylinid predators of P. rosae eggs were trapped in higher numbers on the intercropped plots with higher onion:carrot plant density ratios (PDR) over both P. rosae generations but losses of exposed marked P. rosae eggs were not consistent with these differences. In addition, the PDR treatment effect was greatest when the carrots were intercropped with young onions and virtually ineffective once the onions began to bulb. Over 90% of adult P. rosae trapped entering carrot plots were female and the majority approached the plots upwind at a height of <40 cm above the ground possibly in response to the carrot odour. Row-intercropping with onions caused fewer flies to enter the plots. Mixed cropping of carrots with onions probably reduced P. rosae attack by the onion volatiles disturbing host-plant finding particularly when the onions were young. Mixed cropping with the French marigold, Tagetes patula L. was ineffective. C. aegopodii was not directly affected by intercropping as was T. tabaci, the latter being reduced in numbers by increased PDR possibly due to reduced 'apparancy' of the onion plants.
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