Stewart’s Wilt outlook in WI corn for 2005 – and ID tips on springtail versus corn flea beetle
Eileen Cullen, Extension Entomologist
UW Entomology Department
Corn flea beetle can vector Stewart’s bacterial wilt of corn, and is of concern in susceptible sweet corn varieties and susceptible dent corn inbred seed production fields if 1) corn flea beetles have a high overwintering survival rate 2) Stewart’s wilt bacteria is available for the corn flea beetle to pick up and spread.
A Stewart’s wilt predictive model developed at IowaStateUniversity by Forrest Nutter uses air temperatures recorded during the winter months and assigns a risk score based on the number of winter months in which the average monthly air temperature exceeds 24ºF. If two or three months exceed this minimum temperature threshold, a moderate to severe risk of Stewart’s Wilt disease prevalence would be expected. A low risk would be expected if only one month exceeded 24ºF, and essentially no risk if zero months reached or exceeded 24ºF. In other words, the colder the winter – the lower the survival of overwintering corn flea beetles.
WI DATCP staff recently reviewed the mean monthly air temperatures for December, January and February at eight sites around Wisconsin. They applied these data using the Iowa State Model to get an indication of risk for significant corn flea beetle populations and Stewart’s wilt in seedling corn for 2005. Mean monthly temperatures across the eight WI sites ranged between 17.8 – 27.3ºF (December); 12.6 – 22.2ºF (January); and 21.6 to 30.1 (February). Mean temperature results suggest that winter conditions were mild enough in southern, south central, and parts of east and west central WI to permit winter survival of corn flea beetle.
Looking at monthly air temperatures alone, the Iowa model indicates risk for Stewart’s wilt in parts of southern Wisconsin may be moderate to high. Remember however that surviving flea beetles would need to encounter the Stewart’s wilt bacteria to pick it up and spread the disease. The Stewart’s wilt bacteria, Erwinia stewartii, can overwinter in the corn flea beetle, on plant debris, or in the soil. Keep in mind the virtual non-existence of Stewart’s bacterial wilt of corn in Wisconsin fields during the last couple of summers, and WI DATCP detection of only one corn field, a seed production field of susceptible inbreds, detected with Stewart’s wilt in 2004. Given the recent lack of the Stewart’s wilt bacteria, corn flea beetles that did survive the winter months are probably not carrying the Stewart’s wilt bacterium. Thus, the risk for Stewart’s wilt of corn in 2005 is low.
Scouting sweet corn and inbred corn from seedling emergence to V5 stage is still a good practice. Although leaf feeding injury by corn flea beetle does not usually cause economic yield loss, if flea beetle numbers are extremely high and corn is not growing vigorously due to beetle feeding, suggested treatment thresholds are available. An insecticide may be warranted if there are 5 or more corn flea beetles per plant in hybrid corn fields. In inbred seed production fields, treatment can be justified if 10% of the plants inspected show severe feeding injury (long, white/silvery streaks on the corn leaf surface) and 2 to 3 corn flea beetles per plant.
Finally, remember there can be a couple of different tiny, dark colored jumping “insects” in seedling corn fields next month. Springtails can be easily mis-identified as corn flea beetles on first glance with their minute size and jumping behavior. Springtails (Collembolla) belong to a primitive arthropod group Ellipura, and are not actually classified as insects. They do have six legs and a jumping ability due to the forked appendage (furcula) at the tip of the abdomen. Springtails generally feed on decaying organic matter, fungi, bacteria, etc. and are not considered a field, sweet or seed corn pest. By contrast corn flea beetles are shiny black and have enlarged hind legs to enable their jumping activity when disturbed.
Below are two pictures showing the difference between a springtail and a corn flea beetle. The picture on the left is a microscope shot that Phil Pellitteri, UW Insect Diagnostic specialist, took in May of 2003 when several calls came in on corn flea beetle sightings in corn fields. In those cases the samples were actually springtails. The picture on the right is that of a corn flea beetle. Once you look closely the differences are clear - notice the lack of a furcula on the tail end of the flea beetle and the distinct beetle wing covers (elytra). It can be difficult to study the specimens when they are jumping around, so if you can collect several into a small vial of alcohol you’ll be able to look at them closely.
Springtail (Collembola) UW Madison Entomology Dept.
Corn flea beetle
Purdue Univ. Entomology Dept
References
Tenuta, A. and T. Baute. 2003. Predicting Stewart’s wilt in corn. Ontario Ministry of Agriculture and Food.
http://www.gov.on.ca/OMAFRA/english/crops/field /news/croppest/2003/01cpo03a2.htm
WisconsinPest Bulletin, Volume 50, no. 1. April 15, 2005. page 4.
http://www.datcp.state.wi.us/arm/environment/insec ts/pest-bulletin/pdf/2005/April15.pdf
ISU researchers able to predict severity of disease in corn fields. 1998.
http://www.ag.iastate.edu/aginfo/news/1998releases /stewartswilt.html