Iowa State University

Insect Interactions with Herbicides   
by Bob Hartzler

May 13, 2004 -  In recent years there has been an increase in herbicide failures caused by tunneling of insects in the stems of weeds.  It is believed that the insect feeding stresses the weed and reduces translocation of the herbicide, therefore reducing performance and increasing weed survival.  Giant ragweed is the species most affected by this phenomenon, but similar problems have been observed in waterhemp and other weeds.  The photos (courtesy of Dawn Nordby and Kelly Cook, Univ. Illinois) depict giant ragweed plants damaged by common stalk borer.  Numerous other insect species have been found to cause similar damage to weed stems.

The increase in problems with insect feeding is probably not due to changes in feeding habits of the insects (i.e. switching host plants), but rather due to changes in weed management practices.  All of the problems that have been investigated involve the use of glyphosate in Roundup Ready crops.  Glyphosate should not be any more sensitive to the effects of insect feeding than other herbicides, but rather glyphosate is the only herbicide that allows herbicide application to be delayed until weeds grow large enough for insect feeding to become a significant problem.  Giant ragweed is the most commonly afflicted plant because it is the most rapid growing weed early in the season.  Researchers at the University of Illinois found few insects in stems of 3" tall giant ragweed, whereas 65 and 87% of 6 and 12" giant ragweed were infested with burrowing insects (Table 1).

Table 1.  Effect of herbicide application timing (based on weed size) on percent of giant ragweed plants
with insect tunneling (Nordby and Cook.  2003.  University of Illinois).

The simplest means of dealing with this problem is to use timely applications of herbicides to control small weeds.  Not only will timely application avoid problems with reduced performance due to insect damage, but it also will protect crop yields from early-season competition.  If application is delayed until weeds reach sizes that make attractive hosts to insects, increasing herbicide rates may improve performance but is unlikely to completely overcome the effect of insect feeding on herbicide activity.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University