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Wednesday, December 11, 2013

Will this cold snap help keep insect pests down, next season?

It's a beautiful, spring-like day today in Corvallis ~ a welcome relief from our recent spate of freezing weather!  Being half-Filipino and half-Italian, I have a propensity for sunlight etched into my genetic code.  But, I took solace in this recent round of cold weather, by imagining freeze-induced die-offs of key insect pests, in epic proportions.  

How might this current cold snap be wreaking mass entomo-cide on some of my most challenging garden pests? It likely varies across insects.

Azalea Lace Bug (ALB):  I couldn't find any data on the cold tolerance of ALB, other than secondary citations that the bug is hardy to cold weather.  The original publication is Neil, JW. 1985.  Pest-free azaleas can be a reality. J. Azalea Soc. Am. 7: 25-29, but I can't get my hands on that publication.  From the title and the journal, it seems that it would be an anecdotal report that ALB can survive extended periods of cold weather.  This isn't a surprise, since the insect overwinters in the egg stage, the egg is layed INSIDE leaf tissue (offering the eggs protection from cold, predators and parasitoids), and that the egg is covered by an adhesive goo that further provides protection from cold and natural enemies.  It's unfortunate, but this cold snap is unlikely to put a huge dent in ALB populations that we might see next season.
Azalea lace bug adults are about 1/4 of an inch from head to tip.  Start scouting your rhododendrons and azaleas for the first generation of young nymphs, as early as mid-April.  The first clutch of eggs generally hatches in early May, but could be earlier or later, depending upon local weather and other environmental conditions.  Photo Credit:  Robin Rosetta.  OSU Department of Horticulture and North Willamette Research and Extension Center.
If you want to see whether or not your rhododendrons or azaleas have overwintering eggs in them, you can scout your plants anytime between now and the April.  Look for signs of egg-laying damage, on the underside of the leaf, along the mid-rib.  The eggs are super-tiny, but are covered with a tiny brown speck (the adhesive I mentioned, earlier).  Robin Rosetta's fact sheet on ALB has a magnified photo of the eggs.  If you see signs of eggs in your rhodies or your azaleas, mentally or physically make note of which shrubs carry ALB.  Make sure to monitor those shrubs for the first generation egg hatch, anytime from mid-April to mid-May.  One of the best things you can do for season long control is to knock back that first generation of nymphs, so that they don't become adults who will lay more eggs in your rhodies.

Chemical controls are available, but some people have tried to use strong jets of water to knock the nymphs off of the plants.  Whether you use an insecticide or jets of water,  it's important that your shrub canopy is 'open' enough so that your control measure actually reaches the nymphs.  The nymphs feed on the underside of leaves, and in dense rhodie canopies, you will have a harder time reaching the nymphs with an insecticide or a jet of water, compared to a more open canopy.

If that first generation gets past you, ALB will be much more difficult to control.  The bugs are long-lived (by insect standards), and as time passes, you'll start to see an overlap of generations (egg stage, nymphs, adults all occurring together on the same plant).  Control measures, such as chemical controls or streams of water, will help control the nymphs ~ but will be less effective against other life stages, particularly the egg stage.  Keep in mind that chemical controls will also negatively impact natural enemies, such as lacewings, predaceous bugs, spiders and parasitoids ~ all of which have been shown to help naturally suppress ALB populations.

For long term control of ALB, make sure your rhodies are part of a diverse landscape.  Tree canopy cover and other shrubs will help promote spiders ~ one of the main natural enemies of ALB.  Annual wildflowers will provide nectar for green lacewings, which have been shown to help control ALB at a rate of 10 lacewing larvae per shrub.


Brown Marmorated Stink Bug (BMSB):  BMSB are often reported as 'nuisance pests' because they can aggregate in large numbers in your house.  In fact, National Wildlife Federation Senior Scientist Doug Inkley tracked the movement of BMSB into his house, and up to the attic in 2011.  Over the course of 72 days, Dr. Inkley counted 20,116 BMSB in his house!!  You can see from the graph, below, that there was a progressive movement up and into the attic.  Because BMSB overwinters in protected structures ~ like your house ~ this cold snap is unlikely to result in massive die-offs of this insect.

Dr. Doug Inkley tracked the movement of BMSB into his house and up into his attic over the course of 72 days.  Image source: http://www.nwf.org/Wildlife/Threats-to-Wildlife/Invasive-Species/Stink-Bugs.aspx.
In addition to being 'nuisance pests' in your house, BMSB can also be a real problem for gardeners.  BMSB use their piercing-sucking mouthparts to probe fruits and seeds.  They can damage pea pods, berries and stone fruits in the garden.  Unfortunately, for home gardeners, there aren't a lot of control options.  Biological controls aren't generally successful, because BMSB have a noxious scent that make them unappetizing to many predators.  Chemical controls are available for this pest, but at best ~ these options provide 'temporary relief' rather than long term suppression.  From our friends back east, we've learned that you just kind of learn to live with BMSB.  It becomes a 'new normal' in the garden.
Brown Marmorated Stink Bug Adults survive the winter by hiding out in attics, barns and other protected structures.  Photo Source:  Brown Marmorated Stink Bug website, OSU Department of Horticulture
Spotted Wing Drosophila (SWD):  We're still not sure how SWD overwinters.  On a sunny, winter's day, you can catch SWD in monitoring traps.  This suggests that the fly overwinters as an adult ~ but it is also quite possible that other stages are present in the winter.  In any case, the fact that we can find SWD in the adult stage, in the winter, bodes well for a massive freeze-induced die off of this berry-feeding fly.
In fact, there was a recent paper published by Danny Dalton (OSU) and colleagues, on the cold-tolerance of SWD, and how adults and pupa react to a 7-day 'freeze' event (Dalton et al., 2011).  And, their data suggests that these generally low winter temperatures, and the recent Willamette Valley, OR 'freeze' event bode well for SWD control.

If SWD are kept in conditions below 50 degrees F for about 30 days, they are super-susceptible to a cold snap.  However, if SWD are kept in conditions above 50 degrees, there are at least some adults that may survive a harsh cold snap, such as the one we've just had.

To see if we've been above or below the 50 degree threshold, I plotted the weather history for Covallis, Oregon over the last month or so ~ and marked on the plot where the threshold is for the 50+ degree F 'safe zone' and the below XX degree F 'freeze zone'.   And it looks like we have plenty of days where the mean temperature was below the 50+ degree 'safe zone', preceding our 7 day 'freeze zone' temperatures.  All of this suggests that this recent weather will help gardeners and growers with Spotted Wing Drosophila populations next season ~ a welcome relief for berry growers and berry lovers across Oregon.


Of course, it's important to point out that the Dalton et al., (2011) experiment was conducted in laboratory growth chambers, with very little organic material that the flies could use as shelter from the cold.  In 'the real world', the flies probably have many more opportunities hunker down from the cold ~ and thus probably have a greater buffer from the cold than my graph might suggest. 

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