Winter Soil Scouting of Western Corn Rootworm Larvae
 
Erich A. Fromm (efromm@aol.com)
Elisa J. Bernklau (bernklau@lamar.colostate.edu)
Louis B. Bjostad (lbjostad@lamar.colostate.edu)
BSPM Dept., Colorado State University, Fort Collins
 
 
Introduction

We have recently developed a technique for scouting western corn rootworm during the winter by bringing in soil from the field and warming it to room temperature.  This technique is based on our previously described method for scouting western corn rootworm in cornfields during the growing season (described in the article “Simple Technique for Scouting Corn Rootworm Larvae”).  Winter soil scouting can be used in the months prior to planting to make a map of rootworm pressure in a field, and also to estimate the timing of peak hatch.  Corn rootworm populations have now been reported with behavioral resistance to rotation and with extended diapause for two years or more, and the winter soil scouting technique is well-suited for fields in which rotation-resistant rootworms or extended diapause rootworms might be present.  Growers who are uncertain about the rootworm pressure in such fields can carry out winter soil scouting months before a planting decision has to be made, and also allowing plenty of time to plan control measures if needed.
 


Sampling Technique

Sampling was conducted by taking 4” diameter  X 6" deep cores of soil at the surface using a heavy duty bulb digger (3 for each sample).  This helped to standardize the samples.  Two samples from the same location were put into separate 4"X6"X10" polypropylene tubs with fitting lids (Home Essentials Shoe box with Lid, made specifically for K-mart stores).  The samples will be gently but completely mixed and water will be added as needed to bring the soil to 15% moisture content.  The samples will be then returned to the lab where they will be kept in relatively constant temperature conditions (27 C) for 15 to 17 days before corn will be added to extract larvae.  Germinating corn seeds, 3 days old, are added to the warmed soil, and eggs are allowed to hatch in the presence of the young corn plants for 3 days. Neonate larvae locate germinating corn with high efficiency, and in tubs this size, nearly 100% of the larvae tunnel inside the corn roots within 24 hours of hatching. The small corn plants containing the larvae are then transferred to scouting funnels, and the recovered larvae are counted daily until no more larvae are recovered.  Repeated additions of germinating corn to the same soil sample can be made to recover additional larvae as they emerge from diapause.  During the winter, eggs typically hatch about 15 days after being brought indoors, and germinating corn seeds are replaced repeatedly for 30 to 40 days in order to recover all the larvae from the tub.The small corn plants containing the larvae are then transferred to scouting funnels, and the recovered larvae are counted daily until no more larvae are recovered.  Repeated additions of germinating corn to the same soil sample can be made to recover additional larvae as they emerge from diapause.  During the winter, eggs typically hatch about 15 days after being brought indoors, and germinating corn seeds are replaced repeatedly for 30 to 40 days in order to recover all the larvae from the tub.

 
 
The scouting funnels are organized and designated for each tub.  The tubs are arranged with the corresponding
scouting funnel for ease of identification.
 
After three to five days of root development the corn seedlings are removed from the tub.  The corn is not placed
under the soil to avoid disrupting the Diabrotica eggs.
 
 The corn seedlings are gently added to the scouting funnel.
 
 Fresh three day old germinating corn is then added to the tubs.
 
The tubs are misted to maintain the 15% to 20% moisture content.
 
 

Field History

Location:    Yuma County, NE ¼, Section 17, 4N-46W
Soil type:    Haxtun Sandy Loam
Cultivar:     Pioneer “34K77”
Plant population    32,000/Acre
Planting date    28 April 1999
Starter Fertilizer:    180-N, 15-P, 8-S
Irrigation:    Valley electric pivot sprinkler, with drops and rotators
Herbicide:    14oz/A-Basis Gold and 4oz/A-Clarity
Crop History:    Continuous Corn for 3 years
 


Results

Larvae were recovered at a variety of locations in the Haxtun plot  for a single month in the winter of 1999 (Figure 1).  Winter 2000 scouting was conducted in the same Haxtun plot (Table 1).  This table will be updated weekly for the months of November 1999 through May 2000.  Graphs and evaluation of data will also follow the scouting season.
 

Figure 1. Winter Soil Scouting for Western Corn Rootworm Larvae
 
Table 1. Winter 2000 Scouting Raw Data
Figure 2. Winter 2000 Scouting Raw Data
 

Conclusions

1. In 1998, larvae were brought out of diapause prematurely in the laboratory as early as January, the earliest month that pre-emptive soil scouting was attempted in 1998.

2. In 1999, soil was brought in from the field even earlier, and larvae were brought out of diapause prematurely in the laboratory as early as October.

3. Larvae can be recovered with as little as 15 days of incubation.

4. A sustained recovery period of 55 days was possible, indicating that not all eggs come out of diapause at the same rate.