New Zealand Journal of Crop and Horticultural Science
abstracts
A new bioassay for determining the susceptibility of onion (Allium
cepa) bulbs
to onion thrips, Thrips tabaci (Thysanoptera: Thripidae)
N. A. Martin
P. J. Workman
New Zealand Institute for Crop and Food Research Limited
Mount Albert Research Centre
Private Bag 92 169
Auckland, New Zealand
email: Martinn@crop.cri.nz
Abstract A series of experiments was conducted to develop a bioassay for
determining the susceptibility of Allium plants to adult female onion thrips.
Onion thrips (Thrips tabaci (Thysanoptera: Thripidae)) are the main insect
pest on onions in New Zealand. This research aimed to develop better methods
for determining the susceptibility of Allium species leaves and onion bulbs
to onion thrips. Discs, 10 mm diam., cut from leek (Allium porrum) and onion
(Allium cepa) leaves or onion bulbs, were put singly into small plastic Petri
dishes with 1-day-old adult female thrips, and kept at 25°C in 16 h light
: 8 h dark. After 3 days the discs were stained in acid fuchsin and the eggs
counted using a stereo microscope with transmitted light. Adult thrips survived
equally well on leek leaves and onion bulb scale, but laid more eggs per day
on onion leaves than on leek leaves and least on onion bulbs. Thrips laid fewer
eggs per day when there was more than one thrips per dish. Changing discs daily
or leaving them for 3 days had no effect on the numbers of eggs laid by thrips
each day. Four cultivars of onions were grown with 50, 100, 150, and 200 kg
nitrogen (N) fertiliser per ha. The susceptibility of bulbs to onion thrips
feeding and damage from the highest and lowest N treatments was compared. Analysis
of the proportion of discs with more than one egg showed that cultivar ‘Kiwigold’ (early
brown) was more susceptible than ‘Meteor’ (early red), and ‘M&R
Regular’ (main crop brown) was more susceptible than ‘Red Star’ (main
crop red) to onion thrips. The proportion of discs with more than three eggs
was higher for brown onions in the high N treatment than in the low N treatment,
suggesting that brown onions from the high N treatment were more susceptible.
After 5 months in storage, red onions were softer and had more sprouts than
brown onions, and their innermost skins and the outermost scales had more thrips
damage. This apparent increase in susceptibility of red onions may be related
to changes in the physiology of the outer scale as it shrinks to become a skin.
The bioassays confirmed that both genetic (cultivar) and agronomic (N fertiliser)
factors affect the susceptibility of onion bulbs to onion thrips.
Keywords plant resistance; onion thrips; Thrips
tabaci; bioassays
New Zealand Journal of Crop and Horticultural Science, 2006, Vol. 34: 85–92
0014–0671/06/3401–0085 © The Royal Society of New Zealand
2006
H05091; Online publication date 13 March 2006. Received 11 August 2005;
accepted 16 January 2006
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