As much as we all hate thrips, there are, frankly, worse problems to have. And it’s name is Leafminer. These flies cause just about the ugliest damage we see in floriculture (Fig. 1), and they have incredible pesticide-resistance capabilities. Outbreaks seem to go in cycles, and I’ve had quite a few gerbera and mum crops come across my desk with leafminer this past 2 weeks.
This post covers chemical options (BawHawHawHa!!! Oh… Sorry… I’ll get myself under control now) and non-chemical options for leafminer, as well as how their control fits into the big picture in greenhouse IPM programs.
Several species of leafminers can attack greenhouse crops (both vegetable and ornamental), but all species are similar in terms of biology, damage, and control.
To detect leafminer on cards or tape, look for small black flies (2-3 mm; a little smaller than a fungus gnat, but “huskier”) with a yellow markings (Figs. 2&3). Their antennae are barely visible.
However, you’re much more likely to notice leafminer damage first, before you ever see them on cards. Damage includes stippling (i.e. female flies ovipositing in, or feeding from leaves; Fig. 4) and mines created by the larvae feeding between the leaf epidermis layers (Fig. 1).
In late summer, the leafminer life cycle is quick: at 30C, it’s only 14 days from egg to adult, meaning there will be multiple, overlapping generations, and multiple life stages present (making it harder to manage).
Additionally, at warm temperatures, larvae can go through all 3 instars in 1 week – making bigger and longer mines on leaves as they go. This means you can go from a clean crop to a heavily damaged one very quickly. For more detailed information on leafminer biology, click here.
Time to sit down and grab a stiff drink if you’ve got leafminer already, because the news on the chemical front ain’t good.
Currently registered pesticides – including Pounce/Ambush (permethrin) for adults and Avid (abamectin), Citation (cyromazine) and Orthene (acephate) for larvae – are ineffective, most likely due to resistance. (This is from first-hand experience in several operations this summer). Even pesticides for other insects that you’d think might work tangentially for leafminer adults don’t (I’m looking at you, DDVP).
Greenhouse vegetable growers have it slightly easier, since products containing relatively new systemic insecticides (i.e. chlorantraniliprole or cyantraniliprole) still show decent larval control. But, given leafminer’s resistance history, these are probably only going last for 2-3 sprays per operation. Even our U.S. friends, with what seems like their plethora of pesticides, don’t really have any good products (see the latest IR4 report here) – which means we don’t have any good pesticides coming down the pipeline, either. (Insert audible sigh here).
If you’re adamant about trying pesticides for leafminer control, then lots of yellow sticky cards for monitoring are a must. Without a microscope, it’s difficult to distinguish old, empty mines from new ones. But sticky cards put up after a spray allow you to see if adult leafminer are still emerging from treated plants (see Fig. 5).
Other Control Tactics:
So if you can’t spray the heck out of it, what CAN you do in the face of a leaminer infestation? Here’s the management plan as I see it.
- STEP 1: Sticky tape, sticky tape, sticky tape: Adult leafminer are very attracted to yellow, so put as much tape up as you can, as close to the crop as possible, to remove a good chunk of the adults. I recently compared patterned sticky tape to plain yellow tape to see if we could get improved catches, but the two tape types were comparable.
- STEP 2: Foliar applications of nematodes. In resistant leafminer populations where a bunch of pesticides have already applied, residues will affect parasitic wasps (see below). However, nematodes (i.e. Steinernema feltiae – the same species used for fungus gnat control) will help suppress leafminer populations until parasitoids can be introduced. And, they can continue to be used with parasitic wasps. For details on application, click here.
STEP 3: Release Parasitic Wasps. Diglyphus isaea really IS your front line defense against leafminer. It’s a very effective biocontrol agent, able to parasitize learminer larvae through the leaf.
The only caveat is that Diglyphus is EXTREMELY sensitive to pesticide residues. Meaning you can’t release it until about 6-8 weeks after your last pesticide application.
- STEP 4: Sanitation is especially important for cut flower growers, since leafminer pupate in the soil. After harvest, clean up all leaf material, and consider placing a tarp on the soil to stop new flies emerging. If possible, steam the soil between crops to kill pupae. Also think about where your compost pile/bin is: if you throw out infested plants, are these excellent fliers going to just fly right back in? Consider bagging discarded plant material.
The Big Picture:
Regular releases of Diglyphus are the most successful strategy we have for controlling ongoing populations of leafminer (e.g. in gerbera and cut mums, where the crop continues year round). However, Diglyphus can be easily knocked out by chemicals, and pesticide residues make it difficult to easily re-introduce. Therefore, pest management decisions for ALL OTHER PESTS should be made with this in mind. Before you spray for a a sudden thrips or whitefly outbreak, consider the short-and long-term repercussions on your leafminer program, so you don’t end up with a bigger problem on your hands in a few weeks that has NO pesticide control option.