Biology and Population Dynamics of the Pine Needle Aphid, Eulachnus rileyi (Williams) in Zimbabwe

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Abstract
The biology, ecology, and population dynamics of the pine needle aphid were studied in the pine plantations of Zimbabwe. The annual cycle is simplified by the suppression of the sexual cycle, parthenogenetic reproduction occurring throughout the year. The number of nymphs produced by each adult varied from three to five a day and the total number produced from 15 to 35. Numerical change was seen to result from the action and interaction of intrinsic and extrinsic factors. The aphid numbers were observed to increase substantially between June and July, and between September and November. Exposed colonies of aphids were killed and washed off the needles by rain. Multiple regression analysis was used to try and identify factors important in aphid population fluctuations. Altitude appeared to be the most important variable influencing aphid populations, but was highly correlated to sampling date. No significant statistical differences in aphid density were found between crown levels or crown aspects. In future E. rileyi studies the crown will be stratified and only the mid-crown sampled. The pine needle aphid was found to prefer feeding on year-old and old foliage. Both young and old leaves are likely to be particularly rich in translocated nutrients. Eight species of predatory insects feeding on E. rileyi were identified. Coccinellid beetles were the most abundant predators. Hoverfly larvae, although not as abundant as the coccinellids, were found to be fairly important. The fungus E. plachoniana was found to maintain effective natural control of the aphid in two separate areas. Ants tending the aphids for their honeydew were infected bythe fungus S. burmensis.