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

Abstract

SYNOPSISThe 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 by the fungus S. burmensis.

Population dynamics and foraging behaviour ofDiaeretus leucopterus(Hymenoptera: Braconidae), and its potential for the biological control of pine damagingEulachnusspp. (Homoptera: Aphididae)

“…rileyi in outbreak situations is to be reduced to below damaging levels, additional mortality factors are likely to be required (Hughes, 1989). For example, various general insect predators are known to feed on these aphids in Europe (Murphy et al, 1994;Volkl, unpublished data) and similar species have been recorded from E. rileyi in Zimbabwe (Katerere, 1984). Thus, the extent to which these general predators might complement the action of D. leucopterus should be ascertained should the parasitoid be considered as a biological agent for introduction.…”

“…In Zambia and Zimbabwe, the aphid has been recorded from a total of ten pines although some of these vary in their susceptibility; Pinus patula, a common industrial species, is particularly susceptible to attack (Odendaal, 1980;Mazodze, 1992). In Zimbabwe, peaks of abundance of the aphid occur in July and in November (Katerere, 1984). Studies on P. patula in this country have shown that aphid densities of approximately two per 0.1 m of twig are frequently found and at this or higher densities, needles turn yellow and may be cast prematurely (Katerere, 1982).…”

Biological control of forest aphid pests in Africa

“…The aphid normally occurs as apterous adults but alates can also be produced. Preliminary investigations from Zimbabwe (Katerere, 1984) suggest that peaks of aphid abundance occur in both July and November and that winter populations are greatest on P. patula and summer populations are higher on P. taeda. Thus, while P. patula escapes the attentions of the adelgid Pineus pini, it appears to be more susceptible to E. rileyi.…”

“…rileyi is an elongate, active, grey aphid, about 2.5 mm in length in the adult stage, that feeds on the underside of needles of Pinus species. In Zambia, it has been found on P. caribaea, P. elliottii, P. kesiya, P. merkusii, P. michoacana, P. oocarpa and P. patula (Loyttyniemi, 1979), and in Zimbabwe in addition to these pines, P. chiapensis, P. roxburghii and P. taeda are also attacked (Katerere, 1984). Observations from these regions indicate that the aphid has a reduced life cycle reproducing continuously throughout the year without production of the sexual forms.…”

“…In many cases the predators are habitat specific rather than host specific and in Europe a number of species are associated with conifer forests. Several more generalist predators of aphids have already been recorded attacking the lachnids in Africa (Barnes et al, 1976;Katerere, 1984), having moved into the new habitat from the surroundings and accepted this new source of prey into their diet. There may be some value in the introduction of some of the more conifer-specific predators from Europe but in general it is the parasitoids of the lachnids that have the greatest potential in biological control.…”