Impact on Predatory Wasps

Yellow sticky card placed in a Christmas tree that has EHS.

On January 5 of this year, I posted about the parasitic wasps I was finding on cut Christmas trees that were infested with elongate hemlock scales (EHS) (see Parasitic Wasps on Scale Infested Trees). If you will remember, Jerry Moody and I placed yellow sticky cards in cut trees to determine if scale crawlers were coming off of them. Yellow sticky cards are about 3 X 5 inches and coated with a sticky clear substance that traps insects once they step onto it. The yellow is particularly attractive to many insects. During our little study, there were only a few wasps per sticky card coming out once the trees were displayed in water in a warm room just like they would in someone's house.

These parasitic wasps are very important to control of the EHS. They are very, very small -- the size of a single scale. You'd never see them flying around, and in fact you have to use a microscope to see them on the yellow sticky cards. These wasps, Encarsia citrina, are available commercially and are often used for whitefly control in greenhouses. But they also love scales. Whenever you see a scale that has a round hole in it, you know that one of these wasps has developed inside it and killed it.(Think ALIEN with Sigourney Weaver!) I especially enjoy probing into scales this time of year and finding the immature wasp inside one. Sometimes the tiny little wasps even move their heads! (Nature is so cool!)

This scale has been parasitized. The arrow points to the exit hole the wasp made to get out
once it had fully formed.

The arrow points to a wasp that is still forming inside this scale. The eyes are to the left.

Over the last week, I've put out some yellow sticky cards in trees infested with scales in an Ashe County Christmas tree field (pictured up top). Travis Birdsell, the new county agent in Ashe County and I looked at these cards on Thursday after leaving them for 48 hours. I was amazed at how many wasps there were! We put two cards in one tree and there were 56 wasps on one of them and 34 on the other. In a second tree we had one card and it had 15 wasps. This was in a field where there were some scales, but they had never become really bad as they are in some fields.

I was surprised that there were so many wasps out now. So I became curious. Could I see a difference in the number of parasitic wasps in fields that had been treated with a synthetic pyrethroid compared to those that hadn't? I've thought for a long time that the increased use of the bifenthrin products in particular might be causing a lot of the increase problems in scales.

It just so happened that Jerry Moody and I had created the perfect place to test out this theory. We were working in a small field of trees in Avery County where few insecticides had been used and scales had become a problem. I already knew there were lots of parasitic wasps as I had seen them inside the scales. We had sprayed a block of these trees two weeks ago with Safari and a bifenthrin product, in this case, OnyxPro. We are treating a different block of trees every two weeks to see how late in the season we can get good scale control.

So on Friday, when we were making another treatment, I put a yellow sticky card in three untreated trees and three trees treated two weeks ago with Safari + OnyxPro. The two sets of trees were probably only about 75 feet apart. Today I looked at the cards (after 72 hours) and was amazed at the results.

There was an average of 45 wasps on the cards placed in untreated trees (79, 35, and 22) and an average of 5 on the trees that had been treated (10, 3, and 1). That is a 90% reduction in the wasps just two weeks after treatment! And though I didn't look at scale control, I am sure there is very little yet as Safari takes a long time to work.

I plan on looking more at this issue, trying to evaluate different chemical combinations put out at different times of year to see what the optimum time is to get control of scale without doing away with the parasitic wasp. But the early take home lesson is that the use of synthetic pyrethroids can have a profound affect on these important natural controls. Hopefully in the coming months we can determine when these materials can be used with the least negative impacts.