CURRENT SEASON – 2009

Leafminers in Sugarbeet – July 22, 2009.

There have been isolated reports of leafminers in sugarbeets in the northern Valley. Adult leafminers are small flies with brown or grayish brown bodies, hairy spines on their backs and clear wings. They look like small house flies. Females lay eggs directly on the plant’s leaves. Eggs hatch into small whitish maggots which tunnel into the leaf and begin to feed on the plant material inside the leaf, decreasing the photosynthetic material. This feeding results in small, serpentine tunnels (mines) in the leaves. These tunnels get larger to accommodate growth of the larvae, the newest tunneling is larger than earlier feeding. Larvae can be found at the terminal end of the tunnel, unless they have already dropped out of the leaf to pupate in the soil. There are 2-3 generations per year in the RRV, with the end of the 1st or beginning of the 2nd generation late June – early July, I suspect what we’re seeing up north is 1st generation given how late the season is this year. Previous research indicates leafminers rarely cause economic damage when feeding on more established, mature plants with larger leaves. By this time, beets are undergoing such rapid vegetative growth, they are usually capable of outstripping any damage caused by the tunneling larvae.

 

  
Here are some photos Kelly Sharpe took back in 2003 when we had leafminers around Grand Forks (Click on any of the photos for a close-up).  Note the leaf in the lower left photo and how the mines increase in size, indicating the growing larvae - you can tell where the larvae entered the leaf.

 

Aphids in Small Grains – June 29, 2009

Text Box: There have been some reports of bird cherry-oat aphids in small grains in NW over the last week.  The populations I’ve seen are at very low numbers.  Added to this, the recent rainy weekend will likely have had a significant impact on those aphid populations, but it’s still a good idea to scout for aphids in small grains.  The most damaging aphid populations are ones that reach threshold around flag leaf stage, if populations are at or near threshold at this time then delaying treatment until heading may cost you yield (see below). 

Remember, the action threshold is when a field reaches 80% - 85% of stems with at least one aphid per stem.  Previous thresholds have been set at 12-15 aphids per stem, but as aphids are often difficult to count (and often messy…), a modified scouting method can be used; if 80% - 85% of stems in a field have aphids, then the average across the field is probably at least 12/stem and maybe more.  Sample 100 stems from a field, ensuring you get good representation of the entire field, and calculate the percent infested.  The best time to scout and treat cereal aphids is at the flag leaf stage, but nature doesn’t always cooperate and we can get later season infestations.  Most research indicates that treating for aphids at and after heading doesn’t usually provide an economic return.  Exceptions have occurred when aphid populations are very high, but in those years, greater returns were seen with earlier treatments (i.e. flag leaf stage).

There are always questions about waiting to treat the aphids until fungicides are applied.  This depends largely on how long the wait is….  If it’s a later-planted fields (i.e. still in 6-leaf stage) that are up to 2 weeks from fungicide treatment, then there might be a greater potential for yield loss if aphid numbers are already/near threshold.  Aphids damage plants by sucking sap, so yield loss depends not only to how many aphids are on the plant, but how long they’ve been there.  Entomologists use the concept of cumulative aphid days (CAD) – 20 aphids/stem for 1 day = 20 CAD, 20 aphids/stem for 5 days = 100 CAD, and so on.  The concept was borrowed from heat unit calculations.  Yield loss in cereals from aphid feeding has been estimated at approximately 0.6 bu/ac/100CAD.  Potential minimum yield loss can then easily be calculated by calculating the average number of aphids per stem and multiplying by the length of the wait.  I say the potential minimum yield loss because if 6-leaf plants are already at threshold, the aphid populations will likely increase over the next 2 weeks. 

Total CAD isn’t the whole story, however.  The rate of yield loss decreases as the plant matures.  There are a number of reasons: physiological changes in the plant and maturation of the grain make it less susceptible to aphid damage, after heading the plant starts to become less suitable as a host, aphid populations start to decline, and natural mortality factors, such as predators, start to impact the population so that aphid populations generally start to decline within 2 weeks after heading.  From heading on, there usually isn’t enough time to accumulate sufficient aphid days to cause the amount of yield loss that would economically justify an insecticide application.  

Data suggests that the way CAD accumulate also influences the amount of resulting yield loss.  Lower populations of bird-cherry oat aphids that fed over a longer period caused greater yield loss than did higher populations feeding for a short period even though the CAD were about the same.  So, generally speaking: the longer they feed, the more damage they do…

The Bottom Line – Keep an eye on the populations for now and see what happens, don’t treat before threshold, and don’t wait for fungicide application at heading if you’re at threshold now.

 

 

Wireworms in Small Grains – June 15, 2009

Text Box: I’ve received reports of wireworms in small grains this season - not surprising this year given they tend to be more active in cooler conditions.  There are several species of wireworms in the Red River Valley and although they’re usually neither a frequent nor wide-spread problem in the RRV, when they do occur, damage can be quite significant even leading to a total field loss.  The larval form of click beetles (put an adult on its back, it’ll flip right-side-up with a ‘click’ sound) wireworms are ½” to 1 ½” long, smooth, slender and have a relatively hard, yellow to brownish shell; if you hold them between thumb and forefinger and ‘roll’ them, they feel hard and, you guessed it, wiry.  They overwinter in the soil, burrowing down 9”-24” in the fall.  In the spring when soil temps rise above 50F, they become active and move near the surface where the larvae feed on roots and planted seeds. As temperatures increase, the larvae will move deeper in the soil and become less of a problem.  Feeding damage appears as gouging and furrowing in the root.

Wireworms are difficult to control and rescue treatments are ineffective, if not impossible.  Seed treatments and at-plant insecticides can be used as a preventative, but there’s little that can be done after symptoms become apparent.  Because wireworms live as larvae in the soil for 3-4 years, wireworm populations tend to build over time if not treated.  Consequently, areas that were in non-rotated or non-tilled cropping systems tend to have higher wireworm populations (e.g. CRP or any field in which grasses were growing).  Preventive measures should be taken if there was evidence of wireworm populations in a field the previous year or if the field is coming out of an at-risk system (e.g. CRP).  If re-planting a damaged field, seed treatments or at-plant insecticides should definitely be considered.

Selected Past IPM Updates

Summary of 2008 season

Summary of 2007 Season

August 3, 2006 - Tank Mixing Insecticides & Fungicides in Sugarbeet

July 25, 2006 - Two-Spotted Spider Mite Feeding Damage

July 19, 2006 - Soybean Aphid regional Update

June 16, 2006 - Orange Wheat Blossom Midge in Southern MN?  Let’s Find Out!

May 30, 2006 - Soybean Aphid Makes A VERY Early Arrival

May 19, 2006 - Cutworms in Sugarbeet

June 16, 2006 - Orange Wheat Blossom Midge in Southern MN?  Let’s Find Out!

May 30, 2006 - Soybean Aphid Makes A VERY Early Arrival

May 19, 2006 - Cutworms in Sugarbeet

July 05, 2005 – Armyworm Flight, Crookston

June 06, 2005 – Soybean Aphids – they’re back

May 23, 2005 – Cutworms in Sugarbeets

July 2003 - Leafminer in Sugarbeets

July 2002 - Armyworms in Small Grains
July 2002 - Grasshoppers in The Red River Valley
May 2002 - Cutworms in Sugarbeets
Fall, 2001 - Soybean Aphid in NW Minnesota
August, 2001 - Lygus in Sugarbeets
July/Aug, 2000 - Armyworm Outbreak in RRV
Fall, 2000 - Soybean Aphid, A New Insect Pest in Minnesota Soybeans
June 20, 1999 - Potato Leafhopper, Grasshoppers, Cereal Insects
Sept 12, 1999 - Impact of Aster Yellows on Canola May be Greater Than You Think
Aug 31, 1998 - Lygus in Sugarbeets

Publications Available for Download

Insects

Grasshoppers -
Minnesota Grasshopper Management - 2002 with color graphics
Minnesota Grasshopper Management Brochure with scouting and control information (available as a PDF file designed to be a tri-fold brochure).

Scouting -
Scouting for Insects in Wheat, Alfalfa, and Soybeans - a manual prepared for the University of Minnesota Extension Service Field School, held at the Northwest Experiment Station, Crookston, MN, July 7-8, 1998.
Early Season Scouting for Soybean Aphids - A guide to scouting early season soybeans for soybean aphid.

Questions? Comments? E-mail us at:

imacrae@umn.edu