By Sarah Jandricic (OMAFA) and Judy Colley (Plant Products)
With the arrival of mandevilla, dipladenia and hibiscus cuttings, now is the time to start planning your management program for Thrips parvispinus. Unfortunately, it looks like this pest is here to stay in North America, and in the tropical plant industry at large.
In this post, we’ll focus on pesticide rotations to manage pesticide resistance, as well as a few tips and tricks to make sure your pesticides stay working.
Pesticide Resistance Management
With pests like MED/Q Bemisia whitefly and Thrips parvispinus demonstrating their ability to become resistant to pesticides after repeated exposure, there are some well-researched tactics to help avoid this.
These include:
- Basing sprays on pest monitoring. Although reliable thresholds for Thrips parvispinus have not yet been developed, a good rule of thumb is the presence 1 thrips per 10 plants, or visible signs of damage on the growing points of 2 out of 10 plants scouted.
- Spray ONLY those plant species/varieties showing thrips presence and/or damage. Although spraying the whole range might make sense on a labour front, our work with this pest shows this will just push you into resistance faster. Leaving a “refugia” of unsprayed plants means there will be a few susceptible thrips in the population that will breed with resistant thrips, bringing down the overall resistance level.
- Avoid using too low or too high pesticide application rates: Blasting thrips with exceptionally high rates is how we quickly develop resistant lines in the lab. Similarly, spraying insects with repeated low levels of chemicals also can lead to resistance over time, as this leaves a high proportion of surviving individuals that have been exposed to chemicals. Generally, this pest responds well to label rates, which are similar between the U.S. and Canada. You can find that information in this scientific article.
- Rotating between chemical classes. This means only spraying a chemical in an certain IRAC group ONCE a generation of the insect. For Thrips parvispinus, this means not repeating the same chemical class for at least 15 days at 25 C. At lower temps, this time will be longer (e.g. 21 days at 19 C).
Recommended Pesticide Rotations (Handout)
Although this will be a constant work in progress, the chart below has been based on the collective 6 years of experience both Judy and I have working with Parvispinus in Ontario greenhouses. This chart is downloadable and can be printed on 8×11 paper (landscape view).
For our readers in the U.S., Dr. J.C. Chong and Sepro Chemicals has already made a great pesticide rotation chart for this pest that can be found here:
Other Tips For Managing Thrips parvispinus:
- Always dip your cuttings if you can to reduce incoming adults and larvae. We have also had good success with a second dip of rooted cuttings at potting, which reduces any adults/larvae that have hatched out of the cuttings during propagation.
- To avoid problems in other potentially susceptible crops (I’m looking at you, Poinsettia!!!), make sure to clean out your greenhouse between summer tropical crops and fall crops. This generally means leaving the greenhouse empty for at least a week at high temperatures, removing any plant debris, and potentially using trap plants to “suck” out any remaining Parvispinus.
Nice article! By the way, which Mandevilla variety is showing less susceptibility to T. parvispinous damage? Thanks in advance.
Hi! In general, the reds and pinks seem less susceptible, while white varieties are the most susceptible, and seem to attract or breed the most thrips. It would be worth doing some variety testing of certain whites to see which is the best – something we plan on doing in the near future.