The Value of Christmas Trees

"...there is no reason why the joy associated with the Christmas evergreen may not be a means of arousing in the minds of children an appreciation of the beauty and usefulness of trees; and keen appreciation of the beauty and usefulness of trees is a long stop toward the will to plant and care for them (Arthur Sowder, US Forest Service, 1949)."

Friday, January 22, 2010

Fall Treatment for Twig Aphids

Captions of photos from left to right: Hatched twig aphid egg. Dead twig aphid next to hatched egg. Normal twig aphid egg.

SUMMARY: Research I've conducted the past 4 years has helped determine that twig aphids are controlled when pesticides such as the synthetic pyrethroids are applied the fall before. Changes in recommendations for twig aphid control are found at
Research conducted in 2008-2009 involved careful twig aphid egg observations to determine why this works. Observations determine that Talstar-treated eggs were developing into aphids, but these aphids were dieing upon hatching. It is still not known why this is happening.

Another project that's been going on for several years now is the fall treatment of twig aphids.

This really started because of rust mites. Several of the materials that people were using for balsam woolly adelgid (BWA) control including Asana and Talstar were creating problems with rust mites when applied in the spring. But Doug Hundley, the IPM extension technician in Avery County, observed that when these materials were applied in the fall, there were far fewer problems with rust mites.

Why are rust mites affected by these materials? We really don't know. One would assume it has to do with these broad spectrum synthetic pyrethroids killing off natural predators. However, we really don't see a lot of predators feeding on rust mites. So though I don't really know the why, I do know that this observation has been made again and again and does appear to be real. Rust mites are of course a spring time pest, active primarily in March and April, and would be affected when BWA/twig aphid sprays are applied during this time frame.

In any case, this observation of Doug's caused many growers to start applying BWA controls in the fall. The result? A lot of folks began suspecting that they were having fewer problems with twig aphids the following year. So I decided in 2006 to determine if this was something that was really happening or not.

Honestly, I didn't think it would work. After all, in the fall twig aphids are in an egg stage that is not anywhere near hatching. These eggs won't start hatching until mid-March. Therefore they aren't respiring much. A pesticide shouldn't affect them. Also, the materials that were being used shouldn't last until spring to finally kill the aphid that hatches out. Research has determined these materials will last 4-6 weeks depending on the weather. Therefore, based on what I knew about twig aphids, pesticides and how they work, treating trees in the fall shouldn't work.

By the spring of 2007 when observing the trees Bryan Davis and I treated the fall before, it was quite obvious that it was working. The synthetic pyrethroids and especially Talstar, resulted in virtually no live twig aphids and no twig aphid curl.

Numerous studies and farmer's observations since then have confirmed this. Like one farmer told me last spring, he had the best twig aphid control he'd ever had by treating the fall before.

So how does it work? Frankly, I still don't know. That's one thing I tried to figure out in 2009. Doug Hundley, Jerry Moody and I set up a field study in fall of 2008 where we sprayed a block of trees every month for several months. Treatments included the following:
  • August 18, 2008 (applied Talstar @ 40 oz/A)
  • September 19, 2008 (applied either Talstar or Mavrik @ 5 oz/100 gallons)
  • October 31, 2008 (applied either Talstar, Talstar + Envidor for rust mites control @ 24 oz/A, Mavrik, or Thionex @ 24 oz/100 gallons)
  • March 20, 2008 ( applied either Talstar, Talstar + Envidor, Mavrik, Thionex)
There were untreated blocks for each of the treatment dates. That resulted in 16 different blocks of about 80-100 trees each and lots of pretty flagging everywhere. I don't think anyone but myself could figure out what was going on, and sometimes I wasn't too sure myself!!!
The spring of 2009 starting in February, I started evaluating twig aphid eggs from each of these plots. I collected 30 shoots from each of the 16 blocks and looked at each for the presence of twig aphid eggs. And beleive me, it was sooooo scientific. I took a pin and poked each one to determine if it looked normal or not. :-)
Data I took included if the egg was full, flat (and presumably non-viable), hatched, blackened inside (definitely not normal), or if I was pushing out of the egg an aphid that was almost ready to hatch. I did this on February 1, March 9, March 24, and April 4. I looked at the eggs the same day I collected them. And of course, I also made note of any rust mites that were present.
Of course when I was doing this, I still didn't know if any of these treatments actually controlled twig aphids. It ended up that all the treatments controlled twig aphids as compared to the untreated checks. Date of treatment with Talstar and Mavrik (another synthetic pyrethroid) didn't matter. The Envidor didn't help as rust mites didn't develop in any plot except when Talstar was applied in August when we didn't also include a plot with the Envidor.
So what did I learn from observing the twig aphid eggs? In all, I looked at 1,368 eggs. But with all this work, I didn't answer my questions.
On the earliest observation date (February 1), there were essentially no differences between any of the plots -- treated or untreated. There were a lot of eggs both treated and untreated that looked flat, in other words like they were no longer viable. But healthy looking eggs were all full of clear liquid. They looked like little balloons that were popping.
By March 24, there were live aphids in the untreated plots and there were some differences between Talstar treated plots and untreated plots. There were 14% of the eggs that looked black inside instead of clear with Talstar treated trees as opposed to just 2% of untreated trees. But there were still 22% of the eggs that when poked, a tiny aphid was apparent inside the eggs ready to hatch (11% of eggs in untreated plots).
Apparently, these fall Tastar treated eggs are in many cases starting to develop into an aphid just as if they weren't treated. But these aphids don't survive. They die after hatching. Why I don't know. Are these chemicals lasting until spring to kill the aphid? Are they associated somehow with the egg? I just don't know.
There were some dead aphids right next to Talstar treated eggs. But there were a few dead aphids found in untreated plots too. Perhaps they die from cold or other causes.
I have another site set up the fall of 2009 where I plan this spring to follow the condition of the twig aphid eggs to see if I will make similar observations.

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