Comparative studies of three aphelinid parasitoids of Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) with emphasis on Eretmocerus eremicus Rose and Zolnerowich : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Protection (Entomology) at Massey University, Palmerston North, New Zealand
This thesis investigates the effectiveness of Eretmocerus eremicus Rose and
Zolnerowich (Hymenoptera: Aphelinidae) as a parasitoid of the greenhouse whitefly,
Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae). This investigation
was performed on glasshouse tomatoes, because this is most economically important
glasshouse crop in New Zealand. The Er. eremicus strain used in this study has been
recently identified in New Zealand and differs significantly from other strains of Er.
eremicus found in Europe and America.
The parasitism and host-feeding of Er. eremicus were investigated and compared to
two other whitefly parasitoids (Encarsia formosa and Encarsia pergandiella) to
determine which is the most effective parasitoid of the greenhouse whitefly. The
parasitism study was performed on tomato plant leaf cuttings and the host-feeding
study on tomato plant leaf disks. The leaf materials used in these studies were infested
with 2nd instar greenhouse whitefly nymphs. To replicate the range of temperatures
encountered in a glasshouse, these studies were performed within temperature
controlled rooms set to 15, 20, 25, and 30 degrees C.
The results indicated that at temperatures of 25 and 30 degrees C, both Er. eremicus and En.
formosa performed better in terms of parasitism and host-feeding than En.
pergandiella. En. formosa parasitised the highest average number of whitefly nymphs
(26 nymphs), which was 19% higher than Er. eremicus (21 nymphs) and 42% higher
than En. pergandiella (19 nymphs). En. formosa also killed a significantly higher
average number of whitefly nymphs through host-feeding (8 nymphs), which was 13%
greater than Er. eremicus (7 nymphs) and 25% greater than En. pergandiella (6
nymphs). Furthermore, En. formosa also had a significantly longer average longevity
(6 days), which was 17% greater than the Er. eremicus and En. pergandiella (both 5
days). At 30 and 20 degrees C, En. formosa had a higher parasitism than Er. eremicus and at
30 degrees C also a higher level of host-feeding. However the difference between these two
parasitoids was small overall. En. pergandiella only displayed a high level of
parasitisation at 15 and 20 degrees C, indicating it has adapted to cool temperatures, in New
Zealand, and is unlikely to be beneficial as a biological control agent in glasshouses -
except in winter.
Two further studies were performed on Er. eremicus to determine the effect of adult
parasitoid age on levels of parsatism and preference for specific greenhouse whitefly
nymph instars. These studies found that the highest levels of parasitism occur in the
first 5 days after the adult parasitoid emerges and a clear preference for early instar
nymphs (1st, 2nd, and 3rd instars).
These results of the studies presented in this thesis do not indicate any advantage in
developing Er. eremicus as a biological control agent of greenhouse whitefly in
tomato glasshouses in New Zealand. En. formosa had a significantly higher level of
parasitisation and host-feeding with a wider temperature tolerance and greater
longevity. Since En. formosa is already used as a biological control agent in New
Zealand tomato glasshouses, this study shows no benefit in replacing it with Er.