Protect your plants from pests
Even One Thrips is Too Many
By Bobby Bowen
We’ve made it to the dog days of summer – nights are warm, cicadas are buzzing from the treetops, and the summer annuals are hitting their stride. It’s also the prime time for insect pests to make their existence known. In the past few weeks we have received many inquiries from growers trying to identify what is going on with their crops.
Sometimes what may appear to be fungal or bacterial at first glance is, in fact, the dreaded sign of thrips!
Here are some hints and tips on how to identify thrips damage on your crops, and potential ways to deal with the pests at hand.
What Are Thrips?
First, you might be asking… what exactly are thrips? Thrips are small, oblong, typically straw-colored, black or brown insects. Thrips feed by piercing plant tissue with their mouthparts and sucking out the contents of the plant cells, which kills the cells, and the surrounding tissue.
There are a few different species that can infest your crops, but most prevalent are the western flower thrips. Other thrips you might find are eastern flower thrips, gladiolus thrips, onion thrips, and echinothrips. And yes, the singular name for thrips is still thrips – with an s!
Adult western flower thrips are only 1-2 mm in length and go through five stages during their development – egg, larval, pre-pupal, pupal and adult. The larvae and the adults cause the damage on the leaves and flowers, but the other life stages happen on different parts of the plant – this is important to know, because thrips control can be targeted.
Life Cycle
Adult thrips lay eggs in the soft tissue of the plant – this can be leaves, stems or flowers. Eggs hatch, and the larvae feed on these soft tissues.
When the larvae are ready to pupate, they typically drop to the ground, and the pre-pupal and pupal stages happen in the top layer of soil. Once the adults emerge, they head back up into the foliage to mate and lay eggs.
Adult thrips have wings and can fly short distances, from plant to plant. This is why they can infect an entire crop so quickly.
Thrips Damage
Thrips damage is typically seen as white or silvery gray patches on leaves and petals, stippled with black or brown (this is the frass, or poop).
When populations grow and damage is extreme, the patches merge together creating large areas of damage which will turn brown. These necrotic areas sometimes resemble fungal or bacterial diseases which may get confusing for newer growers trying to ID the issue.
Failed Buds
If you have buds that are failing to open, or only partially open with damaged petals and unsightly flowers (think Icelandic poppies, roses, gladiolus) – chances are it is thrips getting into the buds and causing damage to the flowers before they even have a chance to open.
Silver & White Patches
Silver or white patches on petals, especially around the margins of the petal, with brown or black stippling, is a prime example of thrips damage. Not only is it unsightly, it also dramatically shortens the life of the flower.
Brown Patches
On lighter colored flowers the damage from the piercing mouthparts can be brown, and the petals usually show some degree of distortion.
Leaf Damage
Damage on leaves appears similar to the damage you would see on flowers. Patches of silver, or light brown, with dark stippling. In severe cases, this progresses to large necrotic (dead) patches on leaves.
Necrosis
This type of damage is not only cosmetic, but also detrimental to the health of the plant as it reduces the chlorophyll and the plant’s ability to photosynthesize. Thrips can also carry certain viruses from infected plants to healthy ones, so their control is important.
How to Use Sticky Traps
You can monitor for thrips a few different ways. In a greenhouse you can use yellow or blue sticky traps and make it part of your routine to intentionally study the traps once or twice a week to see how many thrips are present. You probably don’t want to use these traps out in the field or you will be catching a plethora of other insects and pollinators.
You can always bend a plant down across a sheet of white paper and vigorously shake or tap it to see what insects fall off – or my favorite method – dissecting questionable flower heads and buds to try and find the culprits actively munching away in the folded petals. This requires 20/20 vision – or a magnifying glass!
Beneficial Insects
Biological pests are small insects or mites that feed on thrips – often the larval and sometimes the adult stages. They can be purchased from many horticulture companies across the US and usually come in sachets that can be hung on your plants, or in a bulk container that can be distributed across your plants.
These are most often used in greenhouses or hoop houses, but can be used in the open field as well. Amblyseius cucumeris or swirskii are two species of predatory mites most often used for thrips, but there are plenty of other candidates as well, such as Hypoaspis mites. To target the pupal stage of thrips (while they are in the soil) you can use beneficial nematodes which are mixed in with your irrigation water and applied to the soil around the roots of your plants.
There are even fungal pathogens available (like Beauveria bassiana) that kill thrips and other pests. These fungi are naturally occurring in most soils around the world, and that have no negative effects on pollinators in most open field settings.
You can use Google to find the closest company to you offering biological predators (like Koppert) and they will often work with you to create a plan specific to your growing location and needs.
Taking Action
Although the thought of thrips can be scary, and their population control might be a daunting feat, it’s something all growers will have to deal with eventually. We hope this brief summary is a good jumping off point for your own research, as there is no one cure and everyone will have to figure out what works for them in their particular growing situation.
If you think you have thrips on your plants and need a second set of eyes for confirmation, feel free to reach out to the Farmer Bailey team at info@farmerbaileycom with photos and we will try our best to help you out!