Agriculture Reference
In-Depth Information
Planting was staggered to mimic growers‟ practice. In contrast to the
lower altitude area, pea aphid numbers per plant increased as planting was
delayed (Wale, 2002).
At the cooler location, a total of 8 thousand aphids were recorded in 1995,
5 thousand in 1994 and 4 thousand in 1996. At the warmer location, between
10 thousand and 12 thousand aphids were counted during the same period.
This indicates that aphids always need warm temperature to speed up their
reproduction (Wale, 2002).
At the cooler location, the peaks of weekly aphid numbers were high in
August; and the seasonal totals of aphid populations increased as planting time
was delayed. At the warmer location, it was difficult to discern peaks because
they varied with season. Peaks were observed in July, early and mid
September depending on season.
Number of natural enemies (predators such as coccinellids and parasitoids
such as
Aphidius
spp.) varied with season (year); they were more in 1996 than
other years (Wale et al., 2002).
Coccinellids can prevent overpopulation of aphids and their harmful
effects; however, the population of aphids is reduced only after damage is
inflicted to the crop (Hodek, 1967). Without natural enemies, aphid numbers
quickly reach very high proportions. Parasitoids/predators recorded so far on
pea aphid in Ethiopia include
Aphidius (?) urticae
,
Cheilomenes lunata
and
C.
sulphurea
(Abate, 1991). The different events in the life history of aphids are
shown on Figure 3.
Generally weekly aphid populations showed positive correlation with
ambient maximum temperature, but negative correlation with ambient
minimum temperature and relative humidity. Increasing maximum
temperature promoted the buildup of aphid populations while increased
minimum temperature, increased rainfall and relative humidity suppressed it.
As planting was delayed, grain yield increased at the cooler location; it
decreased at the warmer place. This happened because early planted plots
suffered from severe storm damage while the late one did not. From these
results, it is recommended that planting of field peas should take place from
mid- to late June at the cooler and during the first week of June at the warmer
place to obtain higher yields; by doing so a mean of 0.32 t/ha more grain yield
at the cooler and 0.5 t/ha at the warmer place were obtained.
In another population dynamics study carried out at Wondata (close to the
Blue Nile River) and at Woreta (east of Lake Tana) on grass pea between 2009
and 2011, aphid density exponentially increased between 60 and 120 days
after crop emergence and then declined starting from 135 days until harvest
