We have developed a simple, inexpensive scouting funnel that allows corn rootworm populations (Diabrotica spp.) to be scouted in cornfields throughout the field season, and even during the winter months prior to corn planting.  This scouting funnel is similar to a Berlese funnel, but is simpler, less expensive and can recover corn rootworm larvae from corn plants quickly with high efficiency.  The scouting funnel is made from plastic drink cups and plastic insect screen.  A corn plant to be scouted is dug from the field, the soil is shaken gently from the roots, and the plant is allowed to dry in the extraction device with no external heat source.  The absence of soil around the roots is essential for effective drying.  Rootworm larvae leave the slowly drying plant and drop into a small container of water with a white background, which allows the larvae to be counted easily with the naked eye.  Most of the neonate larvae in a young corn plant are recovered after 24 hours, and nearly all are recovered after 48 hours.  For larger plants, up to 6 days may be required for complete recovery of the larvae. Our scouting funnel has also been useful for detection of other species such as wireworms, seed corn maggots, and small grain beetles.
We have used the same scouting funnels to scout corn rootworm larvae from corn roots throughout the field season. For larger corn plants, we have developed a larger version of the scouting funnel made from a plastic milk jug, removing the bottom of the jug to allow introduction of plants, and attaching plastic insect screen to the mouth of the jug with the cored snap cap.  This has given us the advantage of being able to monitor the performance of various field treatments at weekly intervals from planting time until root ratings are done in mid to late July.  In this way, we can make better decisions about the efficacy of planting-time treatments, and about the need for cultivation-time treatments or later rescue treatments.
      In addition to scouting corn plants during the growing season, we have also used these scouting funnels to scout soil for rootworms pre-emptively during the winter.  We have done this by bringing in soil from the field to the laboratory, adding germinating corn seeds to the warmed soil, and allowing the eggs to hatch in the presence of young corn plants for 3 days.  The corn plants are then transferred to the scouting funnels, and the recovered larvae are counted daily until no more larvae are recovered.  We have made repeated additions of germinating corn to the same soil sample to recover additional larvae as they emerge from diapause.

      Approaches for recovery of corn rootworms from corn plants have previously been described by Edwards and Fletcher (1971), Apple et al. (1969), Apple et al. (1977), Chiang et al. (1969), and Gould (1971).  These approaches are often labor intensive or too bulky for field use, so they are rarely used in practice (Fisher and Bergman 1986).
For example, extraction by dissection was often necessary in our laboratory prior to the development of our device.   Corn plants suspected of containing larvae were soaked in a solution of Clorox until the corn roots were transparent.  Once the roots were transparent, the larvae were more obvious under a microscope, however, complete dissection was still required.  This method was limited to laboratory use because of the limitation of the time required and the fragile nature of the equipment.  Berlese funnels have been used, but their bulk limited the number of plants that could be sampled in a field (Fisher and Bergman 1986).   Scouting neonate larvae in the weeks after corn planting by pulling up young plants and sieving the soil around the roots is labor intensive, and it is extremely difficult to see the tiny larvae (1.5 mm).  This is rarely done in practice because in addition to their small size, neonate larvae quickly move inside the corn plant once they locate it, making it impossible to observe them.   Our technique avoids all these problems.  Our scouting funnel is simple, inexpensive, compact and can recover corn rootworm larvae from corn plants quickly with high efficiency.  We can sample hundreds of plants in only a few hours, and with results in less than 24 hours.

      Our scouting funnel is made from two 24 oz. Solo plastic drink cups, (Solo cup Co. Urbana, Ill. 61801) and plastic insect screen (Fig. 1). One cup is cut along the bottom rim, trimming carefully to a smooth edge (Fig. 1B).  The other cup is carefully cut such that the small lower portion is separated from the overall cup (Fig. 1C).  Both pieces should have smooth and even edges.  The larger of the two pieces is the inner cup of the scouting funnel.  A six inch diameter piece of insect screen is cut and then held in place over the small open end of the inner cup (Fig. 1E).  The outer cup is then pushed over the insect screen, pulling it tight to for a surface for the corn roots to dry on (Fig. 1F).  The smaller piece of the second cup is then placed over the bottom of the outer cup with a slip-fit.
 
 
Figure 1
Making the Scouting Funnel
 

 
 1A. Materials: Two 24 oz. plastic drink cups and insect    1B. Cut the bottom ring from the first cup.
screen.

 
1C. Cut the bottom cup portion from the second cup          1D. Make sure the edges of the three cup parts are
                                                                                                        smooth
 
  
 1E. Cut a six inch diameter piece of insect screen, and     1F. For final assembly, slide the cup over the second,
place it over the small end of the second cup.                       pulling the screen tight.
 
     A corn plant from which larvae are to be recovered is dug from the field or taken from a laboratory soil bioassay.  The soil is shaken gently from the roots, and the plant is allowed to dry in the scouting funnel with no external heat source (Fig. 2).  The absence of soil around the roots is essential for effective drying.  Rootworm larvae leave the slowly drying plant and drop into the tightly-fitting cup bottom, which contains 2 cm of water, and has a white background that allows the larvae to be counted easily with the naked eye (Fig. 2F).  Most of the neonate larvae in a young corn plant are recovered after 24 hours, and nearly all are recovered after 48 hours.  For larger plants, up to 6 days may be required for complete recovery of the larvae.  These scouting funnels are easily transported to and from the field in standard cardboard file boxes (Fig. 2E), 12 scouting funnels per file box.  The boxes can be stacked floor to ceiling in the laboratory to minimize the space required.
 
Figure 2
Using the Scouting Funnel
 

 
 2A. Add water to one half the volume of the bottom         2B. Gently remove roots from soil.
cup
 
 
 2C. Place the roots in the scouting funnel.                        2D. Assemble the bottom to the scouting funnel.
 
  
 2E. Store in twelve packs overnight.                                     2F. Remove the bottom of the cup and inspect for larvae,
                                                                                                      For larger plants, repeat steps D through F until the roots
                                                                                                      are completely dry.
 

We have used these scouting funnels to scout corn rootworm larvae from corn roots throughout the field season.  This has given us the advantage of being able to monitor various field treatments from planting time to root ratings in middle to late July.

METHOD:  For the last two growing seasons we have sampled plants from each field treatment and from controls.  The larval scouting funnels are transported to the field in cardboard file boxes.  A plant is removed from the soil, the roots are lightly cleaned of debris, and the plant is immediately placed in the scouting funnel.  When the file box is filled with 12 scouting funnels, it is transported back to the field truck.  The boxes are returned to the laboratory, and 24 hours after the field has been sampled the scouting funnels are checked for larvae (Fig. 2F).  Any larvae that have dropped into the recovery water in the lower cup are counted, then removed with the recovery water.  Clean recovery water is added to the bottom cup and the scouting funnel is reassembled, and checked 24 hours later for any additional larvae.   In this way, we can make better decisions about the efficacy of planting-time treatments, and about the need for cultivation-time treatments or later rescue treatments.
      We have used a larger scouting funnel to scout corn rootworm larvae from corn roots late in the field season. For these larger corn plants, we have developed a larger version of the scouting funnel made from plastic milk jugs (Fig. 3).  The bottoms of the milk jugs are cut off with scissors, along a molded edge.  Insect screen is placed over the mouth of the jug and affixed with the milk cap, which has had the top cut off, leaving a ring of plastic (Fig 3A).  Then four modified milk jugs are positioned together with their handles facing each other, and the sides are stapled together. This produces a group of four funnels that are stored in a cardboard file box that has clear Solo plastic drink cups (9 oz. size) hot melt glued to the inside bottom, to receive the openings of the four milk jugs (Fig. 3A).  An insert cup is placed in each of the glued cups, and these cups are filled with water to catch larvae.  Large plants are tagged in the field for removal (Fig 3B).  The plants are carefully washed to remove soil and to aid in drying the root system.  The plants are then “topped” and placed in the scouting funnels for return to the laboratory (Fig. 3C).  After 24 hours the cups in the bottom of the box are checked for larvae (Fig. 3D).  Often with large plants it can take more than a week to extract all the larvae from the roots.  Scouting the field this late in the season yields primarily 2nd and 3rd instar larvae.   Some 1st instar larvae are still found, however.

Figure 3
Using the larger Corn Rootworm Scouting Funnel

 

 
A.  The quad of scouting funnels is assembled with 1/2 inch of water in each of  the insert cups mounted to the bottom of the box.
 

B.  The plants are tagged and removed from the ground.

 
 
 
C.  The roots are washed carefully before the plants are placed in the scouting funnels.

D.  After 24 hours the scouting funnels are removed and the larvae are counted.  With plants this size it often takes 5 to 10 days to extract all the larvae.
 
 
 

     Our simple, inexpensive scouting funnel, when combined with a simple technique, has been used to recover neonate, 2nd instar and 3rd instar larvae from a cornfield as early as January.  Additionally, our scouting funnel is well-suited for surveys where rotation-resistant rootworms or extended diapause rootworms might be present.

METHOD:  We have used these scouting funnels to survey soil pre-emptively during the winter for rootworms by bringing soil into the laboratory from the field and allowing the eggs to hatch in the presence of young corn plants.  This is done by allowing the soil to warm to room temperature and allowing the eggs in the soil to incubate for 16 days, the normal time for non-diapausing eggs to hatch after being laid.  After this incubation time, germinating corn (3 days old) is added to the tub.  The hatching larvae are given 72 hours to find and enter the corn roots.  After 72 hours the corn plants are then transferred to the scouting funnel (Fig. 2C).  The corn plants from the tub are  replaced with additional germinating corn in order to continue recovering larvae.  The soil is removed gently from the roots, and the plant is allowed to dry in the scouting funnel with no external heat source.  The soil around the roots is removed to expedite effective drying.  As the roots dry, rootworm larvae leave the plant and drop into a small recovery cup of water.  The recovery cup has a white background, which allows the larvae to be counted easily with the naked eye.  Most of the neonate larvae are recovered after 24 hours, and nearly all are recovered after 48 hours.  In most cases, even with larger plants, 72 hours are required for complete recovery of the larvae (Fig.  2).  The process is repeated for 30 to 40 days in order to remove all the larvae from the tub.
  In this simple, inexpensive way, we can estimate the rootworm pressure on a cornfield. Only one trip is required to the cornfield, and very little actual laboratory time is used.

RESULTS:  Larvae were recovered effectively throughout the winter months prior to corn planting (Figures 4, 5, 6 and 7).  Figure 4 and 5 show the results from February and April 1998.  There were several key features of this initial study in the winter of 1998.   First, and perhaps most significant, larvae could be brought out of diapause prematurely in the laboratory.  Second, larvae could be recovered with as little as fifteen days of incubation using our simple technique.  Third, a sustained recovery period of 55 days was possible, indicating that not all eggs come out of diapause at the same rate.
 Figures 6 and 7 show the results for January and February of 1999. The key features found in the 1998 study are repeated here in 1999.  Refinements in handling technique resulted in an overall better growth curve and total larvae recovery for 1999.

     This scouting funnel is also a valuable laboratory tool, and has accelerated our laboratory testing of known and new rootworm treatments.  The results of bioassays comparing levels of COUNTER CR to a control has shown that this method is very sensitive, allowing a rapid determination of dose response to a particular insecticide.  Additionally, we have used this scouting funnel for separation and collection of neonate, 1st, 2nd and 3rd instar larvae for use in bioassays.  This has dramatically increased the number of replications that can be run on a daily basis.  Our scouting funnel is also useful in the laboratory for work with other corn pests such as seed corn maggots.

METHOD:  We have used these scouting funnels to determine rootworm susceptibility to known and new rootworm treatments by developing a bioassay that, when used with this device, is very effective.  The bioassay consists of a tub of soil which contains a young corn plant at one end and treated soil between the young corn plant and where the larvae are to be introduced.  For example, the insecticide COUNTER CR was tested in different concentrations mixed thoroughly with soil.  Larvae were recovered effectively from the control and COUNTER CR bioassays.  The results showed that as COUNTER CR concentrations increased the number of recovered larvae decreased (Fig. 8).
     The neonate larvae are given 24 hours to find and enter the corn roots.  After 24 hours the corn plants are then transferred to the scouting funnel.  The soil is shaken gently from the roots, and the plant is allowed to dry in the scouting funnel with no external heat source.  The absence of soil around the roots is essential for effective drying.  Rootworm larvae leave the slowly drying plant and drop into a small container of water with a white background, which allows the larvae to be counted easily with the naked eye.  Most of the neonate larvae in a young corn plant are recovered after 24 hours, and nearly all are recovered after 48 hours.  The number of larvae recovered from the treated soil are compared to the number recovered from a corn plant that was in a tub of control soil.   Any larvae which are in the treated area of the tub can also be recovered if they are not dead.  This is done by separating the treated soil from the tub and placing germinating corn in it, and after 24 hours the larvae in the soil are recovered using the scouting funnel.  This has been a valuable methodology for the study of insect behavior regulators (IBR), as well as insecticides.
 
 

     Growers are often advised to scout neonate larvae in the weeks after corn planting by pulling up young plants and searching for larvae in the soil around the roots, but this is rarely done in practice because it is extremely difficult to see the tiny larvae (1.5 mm).  In addition, neonate larvae quickly move inside the corn root once they locate it, and are no longer visible.  Our technique avoids both these problems.   Another common method for scouting corn rootworms is to survey the adults with sticky traps in late summer.  The traps are removed from the field after several weeks, and the beetles are counted. The findings are reported to the grower so a decision can be made regarding field treatments for the next year.  This method requires at least two trips to the field, and hours placing and replacing the sticky traps.  Additionally, sticky traps are often lost or destroyed by bad weather or curious animals. Our technique avoids these problems, and gives a more timely picture of rootworm pressure, before planting is done.  Our scouting funnel, when combined with a simple technique, has been used successfully to recover neonate larvae from a cornfield in the months prior to planting resulting in an accurate map of rootworm pressure.  This method may also be employed to estimate the hatch date in the field.  One of the more common methods for avoiding corn rootworm problems has been to rotate corn with other crops. However, corn rootworm populations have now been reported with behavioral resistance to rotation and also with extended diapause for two years or more.  Our scouting funnel is well-suited for scouting these rotation-resistant rootworm populations and extended-diapause rootworm populations.
 

Apple JW, Chiang HC, English LM, French LK, Keaster AJ, Krause GF, Mayo ZB, Munson GJ, Owens JC, Rasmussen EE, Sechriest RE,  Tollefson JJ, Wedberg JL (1977) Impact of northern corn rootworm larvae on field corn.  North Cent Res Pub No 239.

Apple JW,  Walgenbach ET,  Kruse WJ (1969) Northern corn rootworm control by granular insecticide application at planting and cultivation time.  J Econ Entomol 62:1033-1035

Chiang HC, Sisson V, Rasmussen DE (1969) Conversion of results of concentrated samples to density estimates of egg and larval populations of northern corn rootworm.  J Econ Entomol 62:578-583

Edwards CH, Fletcher KE (1971) A comparison of extraction methods for terrestrial arthropods.  In:  Methods of Study in Quantitative Soil Ecology:  Population, Production, and Energy Flow.  Blackwell, Oxford, UK.

Fisher JR, Bergman MK (1986) Field sampling of larvae and pupae. In: Methods for the Study of Pest Diabrotica. Krysan JL, Miller TA, editors. Springer-Verlag, NY.

Gould GE (1971)The corn rootworm problem in Indiana.  Proc Indiana Acad Sci 80:267-273.