It turns out thrips and I have something very important in common – we both like to change up our look depending on the season!
Although you (probably) have no trouble recognizing me with a change of hair colour, different colour morphs within certain thrips species can throw growers for a loop when it comes to identification.
Read on for which thrips like to shake it up seasonally, so you don’t get fooled.
This post was written by Dr. S. Jandricic (OMAFRA) and A. Summerfield (Vineland).
The Issue:
Several types of thrips can have variable body colours within their population – from very pale, to very dark – but still all be the same species.
We call these “colour morphs” in the insect world, and they are quite common among many types of insects, from moths to grasshoppers.
While there are likely genetic factors at play, environmental effects can also influence insect colour. It’s been shown that cool temperatures during thrips pupal development can induce a change from light to dark in some species.
Whether this provides the thrips any advantage in the winter is still unclear.
However, this colour change often leads to concerned growers sending us samples of “unusual dark thrips” suddenly invading their greenhouses in the winter months.
Given the recent occurrence of new, darker-bodied invasive thrips such as Thrips parvispinus (tropical tobacco thrips) and Thrips setosus (Japanese flower thrips) in North American greenhouse, this concern is understandable.
But unless you have imported in new plant material, or are growing tropical crops or hydrangeas, the dark thrips you are suddenly seeing are most likely just your run-of-the-mill species, but a darker morph. As they say, when you hear hoofbeats, assume horses, not zebras.
Which Thrips Change Colour:
Given that they are our two most common thrips pest, it probably won’t surprise you that most of the “unusual dark thrips” we identify this time of year are either onion thrips (Thrips tabaci) or western flower thrips (Frankliniella occidentalis).
For western flower thrips, we know of 4 colour morphs:
- Light (yellow with very few dark markings)
- Bicolour (yellow with dark markings on abdominal segments)
- Intermediate (light brown with dark markings)
- Very dark (dark brown with dark markings). This is primarily found in lineages found in its native range in California, mostly at higher elevations (O’Donnell 2007)
Observations of western flower thrips in pepper greenhouses in other parts of the world have found that darker forms of western flower thrips are much more common in winter (i.e. temperatures below 20°C). The light form can actually even be completely absent when daily mean temperatures are below 15°C (Elimem et al 2011).
Here in Ontario, we’ve received darker forms of western flower thrips for identification from pepper and strawberry plants in winter, as well as from specialty cut flowers, which are typically grown cooler.
The story is very similar with onion thrips. Lab trials have shown that cool temperatures (15°C) during pupal development induce onion thrips to develop into the dark form (Murai & Toda 2001). Regionally, we’ve consistently received very dark onion thrips from cool temperature crops, such as cyclamen.
Further throwing off growers and IPM consultants, these darker thrips are often a bit larger in size. So, this time of year, they may be more easily confused with Echinothrips (Echinothrips americanus) at first glance.
Determining Who’s Who:
If you have an extra-dark population of thrips that’s confusing you, and you have access to a microscope, your first step is to consult OMAFRA/Vineland’s simple Thrips Key for Growers. Since it’s inception in 2015, we’ve added steps that account for light and dark morphs of the thrips we’ve encountered in Ontario.
If you’ve looked at the key, and still aren’t certain, you can always drop or ship samples to Vineland Station to be identified by myself (sarah.jandricic@ontario.ca) or Ashley Summerfield (Senior Research Technician, Vineland Research and Innovation Centre; ashley.summerfield@vinelandresearch.com).
References:
Elimem, Mohammed & Harbi, Ahlem & Brahim, Chermiti. (2011). Evaluation of Frankliniella occidentalis different body colours and their development in a pepper crop greenhouse in the Region of Moknine in Tunisia. Bulletin of Insectology. 64. 9-13.
O’Donnell, C. A. (2007). Color morphology of the western flower thrips of California and virus-vector relationships of members in the Terebrantia: Thysanoptera. University of California, Davis.