Features Production Research
Onion thrips in Ontario

In Ontario, a variety of
alliums are grown, including cooking, bunching, pickling, set, red and
Spanish onions, leeks, shallots and garlic.

In Ontario, a variety of alliums are grown, including cooking, bunching, pickling, set, red and Spanish onions, leeks, shallots and garlic. While the majority of onions are directly seeded, other options include the planting of onion sets or transplanting bare root or plug seedlings.

Onion growers face many challenges. The two major insect pests, onion maggot, Delia antique (Meagan), and onion thrips (OT), Thrips tabaci Lindeman, are found in all major onion-growing regions in Ontario. In this province, OT control has often been secondary to onion maggot control. More recently, however, the use of soil insecticides targeting the larval lifestage of the onion maggot, coupled with effective adult monitoring strategies, has reduced the number of foliar sprays required for maggot control resulting in an increase in OT numbers, especially during dry, hot growing seasons.

The OT lifecycle has several stages: egg, two feeding larval instars, two non-feeding larval instars and adult. Larvae range from 0.6 mm to 0.8 mm in length and may be translucent or pale yellow in colour. The two feeding larval instars, the predominant life stage found on onions, tend to congregate in the leaf axils of onion plants. Adults are approximately 1.0 mm in length and, depending on the time of season, may vary from brown to yellow in colour.

Onion thrips cause economic damage by piercing onion leaves and sucking out the chlorophyll. This feeding behaviour results in white or silvery patches on affected leaves. Onion fields heavily infested with thrips can take on a metallic silver appearance. In addition to this direct damage, infested plants are more susceptible to secondary infections by plant pathogens such as purple blotch disease, Alternaria porri (Ellis), which can further reduce plant yield.

Researchers in the United States and Canada have reported that if OT populations reach or exceed a threshold of 40 thrips per plant, yield losses will result. In 2001, 2002 and 2003, on-plant OT numbers neared or exceeded this threshold in two commercial onion fields in the Thedford-Grand Bend Marsh, near Grand Bend, Ontario. The current Ontario Ministry of Agriculture and Food (OMAF) recommended threshold for OT management is three OT/leaf for dry yellow cooking onions. Many growers feel that if they wait until this threshold is reached, OT populations will be unmanageable.

While plant counts allow growers and scouts to gauge the number of OT present at a moment in time, a more practical approach to determining OT numbers is the use of sticky traps. Sticky traps can provide growers with information on when and where adult OT enter and fly within their fields. During the course of our research, white sticky traps were installed in two commercial onion fields (Site 1 and 2) in the Thedford-Grand Bend Marsh. In each field, a transect of seven traps was established approximately 40 m into the field from the headlands. Traps were separated by 10 m in a line perpendicular to the field-edge. Each trap (9 x 14 cm pasteboard rectangle) was painted with Tremclad® glossy white paint, positioned at the top of the plant canopy, stapled to a wooden stake and coated with Tanglefoot®. Traps were changed each week throughout the growing season.

Figure 1 (a)

Figure 1 (b)

Figure 2 (a)

Figure 2 (b)

Adult OT were detected on sticky traps at both sites as early as mid-May in all years (Figs. 1a and 2a). Meanwhile, OT were not recorded on plants in these fields until late June and early July (Figs. 1b and 2b).

By mid-August 2001, at Site 1, OT numbers on sticky traps peaked at 950 OT/trap (Fig. 1a) even though the number of thrips on plants in the field had not reached the plant threshold of 3 OT/leaf (Fig. 1b). At Site 2, the plant threshold was reached by mid-August (Fig. 2b, p. 20) when OT numbers were already at 457 OT/trap (Fig. 2a, p. 20). In 2002 at Site 1, OT numbers did not exceed the plant threshold (Fig. 1b) until early September when OT numbers peaked at 1000 OT/trap (Fig. 1a). At Site 2, OT numbers peaked at 695 OT/trap (Fig. 2a, p. 20) in mid-August, the same time the plant threshold was reached (Fig. 2b, p. 20). Although overall OT numbers were lower during the 2003 field season, sticky traps effectively provided an earlier indication of OT presence in onion fields compared to in-field plant assessments.

In 2003, based on our results, 100 OT/trap was identified as a threshold when using white sticky traps. Using this threshold, an insecticide was applied the second week of July, resulting in a reduction in OT numbers. When this threshold was exceeded again in early August, a second insecticide was applied, reducing the field population of OT once more.

Differences between the length of time required for OT numbers to reach three OT/leaf and the high numbers of OT captured on sticky traps, demonstrate that sticky traps can provide an early warning of the arrival of OT into onion fields. This early warning will allow growers to target early OT populations and reduce both the physical damage to plants and the overall OT numbers. In 2003, 100 OT/trap proved to be a more usable threshold than the current recommendation of three OT/leaf.

If growers and scouts find that the use of white sticky traps is not feasible, the threshold of three OT/leaf can still be used. The practice of dividing the total number of OT by the total number of leaves on the plant to determine thrips/leaf (i.e. 10 OT found on inner three leaves only divided by 10 leaves on the plant = one OT/leaf) should however, be modified. This is especially true later in the season. As mentioned earlier, first and second larval instars, the predominant life-stages found on onion plants, congregate on new growth. Including older leaves later in the season essentially lowers the numbers of OT/leaf below the current threshold. Therefore growers and scouts should count OT only on the inner three or four leaves and divide by the total number of leaves monitored (10 OT on three inner leaves divided by three leaves monitored = 3.3 OT/leaf).

Once sticky trap or plant thresholds are reached, management programs should be initiated. There are currently four insecticides recommended for OT control in Ontario. Three of the four are pyrethroids, including CYMBUSH® 250EC (cypermethrin [Syngenta Crop Protection Canada Inc.]), DECIS® 5EC (deltamethrin [Bayer CropScience Canada]) and MATADOR® 120EC (cyhalothrin [lambda – Syngenta Crop Protection Canada Inc.]). The fourth insecticide, DIBROM® EC (naled [Amvac Chemical Corporation]), is an organophosphorus (OP) insecticide. DIAZINON 500E (diazinon [United Agri Products]), also an OP insecticide, while not currently recommended for OT in Ontario, is legally registered for control of this pest. Over the past few years, OT control failures in their fields have led Ontario growers to question if resistance had developed to these insecticides.

Insecticide resistance screening of OT populations from a number of commercial fields across southwestern Ontario has confirmed the presence of resistance to DECIS, MATADOR and DIAZINON. In 2003, resistance-screening bioassays using a dose of each insecticide that killed 99 per cent of a susceptible lab strain were performed. This lab strain was collected from a commercial onion field in September 2001, and continuously reared under laboratory conditions. Sixteen commercial OT populations were tested. A population with 20 per cent or higher survival at the dose tested was considered resistant. Fifteen of 16 populations were resistant to lambda-cyhalothrin; 14 were resistant to deltamethrin. These results confirm that widespread resistance to pyrethroids is occurring across southwestern Ontario. Seven of the 16 populations were resistant to diazinon. Although fewer populations were resistant to diazinon, it is important to note that these populations were collected from three of the growing regions, Thedford-Grand Bend Marsh, Bradford-Holland Marsh and Kent County. Therefore, OP resistance, like pyrethroid resistance, is not limited to one growing area and should concern onion growers across Ontario.

In research field studies, foliar application of SUCCESS® 480F (spinosad [Dow AgroSciences Canada Inc.]), ORTHENE® 75SP (acephate [Arvesta Canada]), RIMON® 0.83EC (novaluron [Makhteshim-Agan of North America Inc.]) or KNACK® 0.86EC (pyriproxifen [Valent Agricultural Products]) significantly reduced OT populations.

Figure 3

In addition to foliar sprays, application of imidacloprid as a seed treatment (GAUCHO® 480FL [Gustafson Partnership]), planting-water or tray-drench (ADMIRE® 240F [Bayer CropScience Canada]) treatment delayed development of OT populations. Although OT tend to cause economic damage later in the season, early exposure to imidacloprid reduced initial numbers, ultimately delaying and reducing the overall population pressure observed later in the season.

While any one of these control products would benefit growers by helping to slow and minimize insecticide resistance while affording effective OT control, none are currently registered for application to onions in Canada.

Based on our monitoring and insecticide resistance research we suggest the following program to maximize OT-control while minimizing the development of insecticide resistance (Fig. 3). Using trap or plant counts, growers should apply their first insecticide application once the threshold (100 OT/trap or three OT/leaf) is reached. At this time, growers should apply one of the three available pyrethroids.

Following application, monitoring should continue and once the trap or plant threshold is again reached a second insecticide application should be made. At this time, growers should switch insecticide classes and apply an OP insecticide, such as DIBROM or DIAZINON. Again, monitoring should continue following application and, in the event that the threshold is reached again, a third application should be made. At this time, due to limited options, the third application will have to be a pyrethroid. The key is to choose one of the
two remaining pyrethroids. To minimize the development of insecticide resistance it is important that growers do not re-apply the same the pyrethroid used in the first application.

Until onion-growers have more tools in their toolbox to manage OT populations, conscientious monitoring and wise insecticide choices must continue.

Partial financial support for this research was provided by: Ontario Fruit and Vegetable Growers’ Association; Thedford-Grand Bend Vegetable Growers Association; the Matching Investment Initiatives Program of Agriculture and Agri-Food Canada; Food Systems 2002 of the Ontario Ministry of Agriculture and Food (OMAF); and, the University of Guelph-OMAF Plants Program. Special thanks are extended to the summer students and AAFC technicians who provided invaluable assistance and to Kristen Callow for her review of this manuscript.

Jennifer MacIntyre Allen, Cynthia Scott-Dupree and Ron Harris are with the Department of Environmental Biology at the University of Guelph, Guelph, Ontario. Jeff Tolman is with the Southern Crop Protection and Food Research Centre Agriculture and Agri-Food Canada, London, Ontario.