Entomopathogenic nematodes – used to control fungus gnats, shoreflies and thrips – are often a “gateway bio” into biocontrol use in greenhouses. This is because not only are they effective and easy to use, but they’re generally compatible with insecticide use. Readily applied with regular spray equipment or through drip lines, nematodes can even be tanked mixed with pesticides to save on labour costs.
In this post, I’ll share some of my research at NC State, looking at which commonly used pesticides in Canadian and U.S. greenhouses are safe to use with nematodes.
Effective biocontrol programs for western flower thrips often usemultiple natural enemies. These includepredatory mites like N. cucumeris or A. swirskii, but also generalist predators like Atheta and Orius, which can feed on mite eggs and nymphs.
The spring bedding crop season is over, so now’s the time to reflect on what worked, and what didn’t, for foxglove aphid control. This way we can prepare for their re-appearance in the fall.
Foxglove aphid feeding on pansy. Note the characteristic dark-green blotches on the abdomen, and the dark joints of both the legs and antennae. Photo by S. Jandricic, OMAFRA.
To recap, foxglove aphid (Aulacorthum solani) is a “cool weather pest”. It prefers temperatures between 15-25 C, and can’t survive in the greenhouse in summer (1). Unlike other aphid pests, foxglove aphid tends to feed in hidden locations – primarily the lowest leaves of plants – making it difficult to detect and treat.
And, unlike green peach and melon aphid, biological control of foxglove aphid is definitely a challenge.
Some growers and consultants have been trying both Aphidius ervi and Aphidius matricariae for foxglove aphid. But, as demonstrated by the Buitenhuis Lab here at Vineland, foxglove aphid is a terrible host for A. matricariae, and this wasp will barely parasitize it (see graph). Further, using A. matricariae for foxglove aphid can actually end up spreading this pest, and it’s damage (2). This is because the wasps simply pesters the aphids to the point where they drop of the plant and go find a quieter place to eat. So it’s pretty clear that releasing A. matricariae is simply a waste of money and effort if you’ve got foxglove aphid.
Parasitism rates of foxglove aphid by different Aphidius species. Wasps were offered 50-60 2nd instar aphids. Tests were done in small plastic containers.
But what about A. ervi? Although parasitism rates were high in the lab (73%), results were not as good in practice. Tested in the greenhouse, A. ervi was able to offer about 50% control of foxglove aphid after 1 release.Repeated releases may offer greater control, but this is still not reassuring when you’re talking about a pest where populations can explode quickly.
Why A. ervi does a great job of parasitizing foxglove aphid the lab, but not the greenhouse, is something we’re currently investigating. But until we have an answer, it seems that pesticides may currently be the best option for control of foxglove aphid in floriculture IPM programs. With the current limitations on neonicotinoids, growers will want to turn to Beleaf or Endeavor. (But, since these two chemicals have similar modes of action, and it usually takes several sprays of either to provide complete control, you may want to consider rotating these chemicals with Enstar II to prevent resistance).
If you’ve had infestations of foxglove aphid in your greenhouse, and have anything to say about it’s control, feel free to leave me a comment!
(1) S.E. Jandricic, S.P. Wraight, K.C. Bennett, and J.P. Sanderson. 2010. Developmental times and life table statistics for the aphid Aulacorthum solani (Hemiptera: Aphididae) at six constant temperatures, with recommendations on the application of temperature-dependent development models. Environmental Entomology 39(5): 1631-1642.
(2) L. M. Henry, J.A. Bannerman, D.R. Gillespie2, and B.D. Roitberg. 2010. Predator identity and the nature and strength of food web interactions. Journal of Animal Ecology doi: 10.1111/j.1365-2656.2010.01723.x.
Well, now that I’ve gotten your attention, let’s be clear that I’m talking about bugs, here, people. Specifically, Western flower thrips and predatory mites such as N. cucumerisandA. swirskii.
Second instar (L2) western flower thrips next to a predatory mite egg (left) and 2 adult mites (right). Photo credit: Sarah Jandricic.
We all know that predatory mites only kill and eat the small, first larval stage of thrips (referred to as L1’s). Larger life stages (i.e. L2’s and adult thrips) are simply too large for the mites to kill.
But, that doesn’t mean they don’t help control them.
Predatory mites will repeatedly attempt to kill L2 thrips – attacking them up to 40 times an hour (1). The L2 thrips are able to fend off these attacks by either running away, or “slapping” mites in the face with their abdomens. You can watch the hilarity that ensues when mites attempt to attack too-large thrips larvae here.
But this “harassment” by mites takes it’s toll on thrips. Because thrips spend more time fending off mites, they spend 30% less time feeding (2). Over time, this translates to 40% less damage on plants with predatory mites, compared to no mites (2). And, the presence of mites can reduce survival of L2 thrips by up to 78% (1), probably because eating less means the thrips lack the nutritional reserves to complete development.
And all of this is accomplished just through “intimidation” of thrips by mites – not through consumption. Scientists term these “non-consumptive” effects, and we are just starting to learn the importance of these effects in biological control. Research is now suggesting that non-consumptive effects like “harassment” may actually account for 50% of the pest control we see in greenhouses (3).
This makes sense with our thrips example. Not only do mites reduce the feeding and survival of larval thrips, but ongoing research from Cornell University shows that the presence of mites reduces the number of eggs laid by adult thrips, and shortens adult thrips lifespan (4). Even the presence of predatory mite eggs on a plant has been shown to “scare” L1 thrips into eating less, according to research from Austria (5).
And, it would make sense that the more mites you have, the higher the number of “scary” encounters thrips will have with them, improving control.
So, lets give a hand to the hard working, harassing, predatory mite, who’s doing more than we ever thought in floriculture IPM. Stay creepy, little guys. Stay creepy.
References: (1) Jandricic, S.E., Schmidt, D., Bryant, G., and Frank, S.P, NC State University. Unpublished data. (2) Jandricic, S.E. and Frank, S.P. 2014. Too scared to eat: non-consumptive effects of predatory mites. IOBC/wprs Bulletin 102: 111-115; (3) Preisser, E. L, Bolnick, D. I., & Benard, M. F. 2005: Scared to death? The effects of intimidation and consumption in predator-prey interactions. Ecology 86: 501-509. (4) Loughner, R., and Nyrop, J. Cornell University. Unpublished data. (5) Walzer, A., & Schausberger, P. 2009: Non-consumptive effects of predator mites on thrips and its host plant. Oikos 118: 934-940.
Spraying nematodes in your greenhouse for fungus gnats, shoreflies, or thrips? Then check out this article published in the January 2015 issue of Hort Matters (OMAFRA’s Horticultural newsletter):
Researchers realized that nematodes can “stick” to the inside of spray tanks, reducing the number of nematodes coming out of the nozzle as you spray. This is unlikely to be a problem using backpack sprayers (since the volume is so small), but could be an issue if you use a towed sprayer on wheels.
What’s the solution? Agitation, agitation, agitation.
To keep your nematode numbers consistent throughout the application, do your best to keep nematodes in suspension. And, if you have any concerns about your application technique, it’s easy enough to assess if you have a microscope (or have a friendly neighbourhood OMAFRA agent with one).
Nematodes (S. feltiae) preparing to enter an adult western flower thrips. Photo by R. Buitenhuis (Vineland).
Nematodes are around 0.5 mm long, and can be easily seen using the low magnification setting on a scope. A black background makes them easier to see. Live nematodes are usually serpentine or “J” shaped, and often wiggle slightly. Dead nematodes are stick straight.
By counting the number of live nematodes in a small volume of your original spray solution (1 mL should do it), you can compare this to the number of nematodes in the same volume (1mL) from the nozzle dribble at the end of your application. Seeing a huge reduction here? Then you may have a problem.
To ensure nematode health, also follow these other tips:
Air temperature should be less than 30 C at application time
Apply during low light levels since nematodes are UV sensitive
Nematodes can be stored in a refrigerator (4C) but should be used within 4 weeks of receipt
Do not apply nematodes though sprayers that exceed 300 psi
