Agriculture Reference
In-Depth Information
declined through to January and February. Vetch plants, on the other hand,
were found infested a bit earlier than clover, i.e, in September.
Pea aphid density varied significantly between the different legumes
(Wale et al., 2003a). Vetch plants carried more aphid load per plant than
clover in all survey areas (χ
2
=10.2, P=0.005, df=1). Furthermore, vetch plants
in the forage field were carrying more aphid load than vetch plants in the crop
field margin.
Literature indicates that low protein to sugar ratio of plants contributes for
immunity to pea aphids (Nielsen and Lehman, 1980). Pea aphids choose plants
with more protein levels. This may, among other factors such as biotype
specificity to specific varieties, have contributed for the absence of aphids on
alfalfa. In the present study, lupines were found immune to aphids; alkaloids in
lupine may have served as deterrents. Studies in Poland have shown it is the
alkaloids in the phloem sap of lupine plants that determine their resistance to
attack by pea aphid (Wegorek and Krzymanska, 1975). One other reason may
be that the aphid biotypes present in Ethiopia may have evolved specifically
adapted to the pulses grown in Ethiopia for a long time in the past.
5.
R
EPRODUCTION AND
S
URVIVAL
The effect of pea aphid density (2, 6, 10 and 14 nymphs/per plant) on
survival and reproduction was studied on four cool-season food legumes, i.e.,
on faba bean, field pea, lentil and grass pea in the greenhouse in northwestern
Ethiopia at ambient temperature and humidity. Aphid survival generally
tended to decline with increasing aphid density per plant particularly on faba
bean (Wale et al., 2003b). Decline was more consistent with grass pea, perhaps
because it was the source of the test aphids that were used in the study. Aphid
stock cultures were prepared by using aphids from grass peas widely grown in
the area. Aphids survived significantly longer on lentil when 2 aphids were
allowed to feed on a plant compared to when 6, 10 and 14 aphids per plant
were allowed (Wale et al., 2003b) (Figure 2).
Similarly, progeny production consistently declined with increasing aphid
density per plant on the four crops (Wale et al., 2003b). Low aphid density,
i.e., 2 aphids/ plant, gave the highest mean progeny production, i.e., 37.3
offspring/female, regardless of legume species. Again regardless of density,
the number of nymphs produced per aphid was significantly lower on faba
bean compared to the other host plants, indicating unsuitability of faba bean
for pea aphid survival and reproduction.
