During the 2013 Muck Crops Conference in Bradford, Ontario, Dr. Mary Ruth McDonald, University of Guelph, presented valuable information about thrips management strategies under Ontario growing conditions. Christy Hoepting from the Cornell Cooperative Extension Vegetable Program gave a very informative presentation on management and on-going research in New York State for onion thrips. The first part of this article will discuss why onions thrips are such a problem and how to scout for them, while the second part of the article will discuss management strategies.

Why are thrips such a problem?

Thrips are very small (<3mm), cream to light brown soft-bodied insects that feed on a wide variety of crops (Figure 1). Thrips feed by scratching on plant tissue with their mouthparts and sucking up the cell contents, which appears as silvering on the leaf tissue. Due to loss of cell contents, feeding by thrips reduces the ability of plant to photosynthesize and can thus reduce yield, especially when extensive feeding occurs during bulb formation of onions.

Figure 1. Onion thrips are very small insect pests of onions. They are the small yellowish-brown spots seen in the centre of this plant.

Feeding damage also predisposes plants to foliar diseases. Onion thrips can also vector plant viruses such as Iris Yellow Spot Virus (IYSV). IYSV hasn't been reported on onions over the last few years in Ontario, but it can pose a serious threat to onion yields if present. According to preliminary research from Cornell University in New York State, IYSV infected bulbs were 8 to 33% lighter when compared to uninfected bulbs.

If you have both transplanted and seeded onions, be sure to scout both. The use of transplants or seeded crops may influence how long protection is needed. Transplants may require less treatments compared to seeded crops; however, treatments may start earlier in the season for transplants compared to seeded crops.

Scouting for onion thrips

Start early! Early detection is key to controlling this pest, as populations can explode quickly under ideal conditions. Thrips can develop from eggs to adult in as little as 10 days, or up to 30 days, depending on the temperature, but typically it takes 2-3 weeks to complete development from egg to adult. Female onion thrips reproduce asexually (without mating). Females can lay between 25 and 100 eggs. Under conservative parameters, if a single female lays 25 eggs and 50% hatch, populations can explode up to ~1900 thrips in three generations. Under the worst case scenario, if one adult lays 100 eggs all of which hatch and develop into adults, in three generations there is the potential of 1,000,000 thrips! Clearly, this is why early detection and management of this pest is important.

Thrips overwinter on winter grains, clover and/or alfalfa. When these field and forage crops are harvested or start to dry down in late spring/early summer, thrips are likely to migrate into vegetable crops. Note adjacent fields that have these different crops and keep an eye on harvesting activities of these field and forage crops. Research from NY has shown that thrips are likely to overwinter in soils of previous onion fields, as well as on vegetation surrounding these fields. Volunteer onion plants are likely sites for early season colonization by thrips.

In NY, there could be 3 to 4 overlapping generations of onion thrips per year with infestation of onions starting in June, meaning that the onion crop, if planted in mid to late April and harvested early September, requires 6 to 8 weeks of protection. At the Muck Crops Research Station in the Holland Marsh, onion thrips didn't reach action thresholds for treatment until the 19th and 20th of July in 2011 and 2012 , respectively; however, this may be earlier or later depending on the weather and your location in Ontario. Thus, it is important to scout early, so you aren't playing catch up the rest of the season.

Thrips prefer to feed on the young, inner leaves of onion plants. Keeping this in mind, when scouting for thrips, pull the leaves apart to look for these small critters (Figure 2). Thrips may also be found on the older leaves that are folded over. Even though thrips are very small, they are very mobile and can move quickly, so try to count as fast as possible before they hide.

Figure 2. Carefully pull the onion leaves apart to count thrips.

Record the number of thrips (both nymphs and adults) per plant on 100 plants across the field, making sure to pull onion leaves apart to reveal the youngest leaf tissue. Determine the average true leaf stage and divide the number of thrips per plant to get number of thrips per leaf. The threshold on dry cooking onions, leeks and Spanish onions is 1 thrips per leaf. Thrips thrive in hot, dry weather, so you may need to shorten the time between scouting events. In cooler, wetter weather, thrips numbers are unlikely to rise as quickly over time, but it is still important to scout diligently. Regardless, it is important to communicate the counts to the growers, so that they can make an informed decision, even if the counts haven't reached the threshold. In some cases, they may not be able to make it to the field to spray until a few days after scouting, so it's beneficial for them to know if the counts are approaching the threshold.

For insect pest situations in Ontario, growers and stakeholders are encouraged to follow the crop update published by OMAFRA on ONvegetables.com. This crop update aims to summarize pest reports from different areas of the province when available, including updates from Muck Crops Research Station (MCRS). Weather, growth degree days and insect pest situation for the Holland/Bradford marsh is also available on the MCRS website, which is updated twice a week during a growing season.