A new bioassay for determining the susceptibility of onion (Allium cepa) bulbs to onion thrips, Thrips tabaci (Thysanoptera: Thripidae)

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, 10mm 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^oC in 16 hlight: 8h 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.

Key words:

• plant resistance
• onion thrips
• Thrips tabaci
• bioassays