Baute Bug Blog

My summer students are going to be frustrated when they find this out, especially with all of the scouting they have done looking for WBC egg masses and larvae and finding nothing to show for it.  But today Adam Pfeffer of Monsanto brought in the first WBC larvae found here in Ontario.  The larvae was from a corn field in Rodney Ontario.  This tells us that there are probably other sites with some larvae  in them as well, so I do encourage you to be out scouting your fields too.  Larvae would most likely be in the ear of the corn.  But we are also still catching moths in our traps so some egg laying could still be taking place.

Purdue University put a WBC scouting video within their newsletter this week which helps explain how to look for larvae in the corn ear:  http://extension.entm.purdue.edu/pestcrop/2009/issue21/index.html

And Ron Hammond from Ohio State University put together a video from our trip to visit Chris DiFonzo from Michigan State University in one of her heavily infested corn field last year: http://www.youtube.com/watch?v=u5EsPWySxEI

If you do find ear feeding, larvae or egg masses, please let me know!

I have had a few calls from some reps and growers who have been amazed and delighted and some who have been frustrated and frazzled this week.  But I’m in it with you, believe me.  Soybean aphids seem to be very active right now, developing winged generations and have been moving from one field to another for the last two to three weeks.  I have had sites that were approaching 200 or more and then the very next week there were barely 20 per plant with a lot more natural enemies present too.   In part this drop is due to the natural enemies that are moving in to make the kill.  But a stronger influence could be the cooler temps we are starting to experience at night and the desire of the aphids to keep finding later planted fields to finish off in before heading to buckthorn.  We tend to still see aphids active in fields until at least the second week of September but this year is a bit of a guessing game.  On one hand, the soybean growth stages are behind and have quite a few more weeks to go before maturity, on the other, the temps are starting to make it feel like fall already.

Either way, it does mean that it really pays off for you to at least hold off a couple days before spraying at threshold just to make sure that the aphids are truly there to stay.  If you are lucky like some, they may decide to leave and the ones that are left can easily be taken care of by the natural enemies.

Patience is key when it comes to these aphids.

The current threshold of 250 aphids per plant and increasing should be followed to the end of the R5 stage of soybeans. 

The R6 stage aka “Full Seed” has been reached when the upper 4 pods of the plants are full of plump green seeds.  Past research has indicated that at least 1000 aphids per plant are required before there is a yield response.  This would only apply for the very start of the R6 stage.  Once the plants reach the middle to end of R6,  the plants no longer respond as they are shutting down for harvest.  Also you need to worry about the preharvest intervals of the foliar insecticides registered which are 21 days to harvest for Matador or Silencer and 30 days to harvest for Cygon or Lagon.

 The majority of the soybeans we scouted this week were in the R4 stage.  So unfortunately we have a few more weeks of scouting to do before we can walk away and be done with the aphids.

Some counties in Michigan have been given the recommendation to spray their dry beans due to the high moth counts and observations of pod feeding at some locations.  Montcalm County (central Michigan) in particular is at the highest risk with some trap sites capturing over 400-500 moths in the last few weeks.  Isabella, Gratiot and Mecosta counties are also considered at risk.  Current Michigan WBC Trap Map:  http://www.msuent.com/files/WBCmapAug15.pdf

Thankfully Ontario has not yet experienced these kind of levels yet.  Current Ontario WBC Trap Map: http://www.cornpest.ca/?LinkServID=848D2ECA-188B-3AFA-083F4B11E7B95388&showMeta=0

Only one trap site near Appin has accumulated more than 50 moths so far in Ontario.  Others have been ranging from no catches to about 30 moths so far.  We have been scouting for egg masses and feeding damage at most sites that have captured moths but have not found anything yet in corn or dry beans.  However, given what Michigan is dealing with this year, I don’t think we have many more growing seasons to go before we are see levels high enough that management will be needed in at least some fields.

I have been getting more calls this week with people wanting to spray the aphids just to take care of them..even if they are not at threshold.  Most realize that the natural enemies are going to get killed but they also assume 100% kill of the aphids and struggle to understand how the aphids can build back up so quickly in that same field. 

The problem is, you don’t get 100% kill of the aphids.  Even if you get 90% control, which is pretty good control, that leaves 10% of the aphids surviving on the plant.  They are baby making machines and can build up their numbers back up to threshold in no time…especially now that you killed their natural enemies too, which are not baby making machines and take about 2-3 weeks to recover.  Unfortunately the natural enemies need to find mates, produce eggs again, wait for about a week before hatching, then be larvae for another week or two (and hopefully the larvae feed on aphids) before finally becoming the hungry adults.  Aphids are different.  They are born pregnant and don’t need to mate.  They pop out hungry nymphs that can suck on the plants  immediately, just like the adults.

Here is a simplistic example to help get this point across.  Let’s say a field is sprayed when there is only 100 aphids per plant.  If you got 90% kill, that would leave 10 aphids per plant left surviving, and not many natural enemies left to eat these 10 aphids per plant.  Aphids can double their population in 1.5 to 2 days, depending on the temperature.  So if you started at day 0 with 10 aphids per plant, Day 2 would have 20, Day 4 would have 40, Day 6 would have 80, Day 8 would have 160 and Day 10 would have 360 aphids per plant.  You’d essentially be back up above threshold in less than two weeks time..not saving you any money or time for your effort in spraying too early.

At least wait until you are confident that the aphid population is rising above 250 aphids per plant.  That is truly the only way to know whether the natural enemies are not doing their job and that a spray is finally necessary. 

Spraying before the threshold, grinds all of your biological control to a complete halt, when they may have actually been the ones making a difference.   Otherwise, wouldn’t every single soybean field in Ontario need to be sprayed every single year?