BioControl Evolution

Salt cedar (Tamarisk) was introduced to North America as an ornamental. Unfortunately, Salt Cedar is an invasive species that causes a variety of ecological and economic problems. Salt Cedar grows along river and stream banks and displaces native species important for wildlife habitat. Salt Cedar reduces available water by high evaporation rates. In dry areas, it can be a forest fire hazard.

Once established, invasive plants are difficult to eradicate but they may be controlled by importation of natural enemies such as insects. Diorhabda carinulata, an Asian beetle that feeds only on species of Tamarisk was imported as a possible control agent. Early releases were disappointing. Beetle populations rarely increased to levels high enough to give the desired level of control. Why were the beetles not effective?

Salt Cedar Beetle
Photo: werc.usgs.gov

Bean, Dalin and Dudley have been studying the biology of Diorhabda carinulata. They found that the beetle uses photoperiod cues to time its diapause (the dormant overwintering period). Initially released beetles would enter diapause as early as August in some areas, even though environmental conditions would allow additional generations to successfully develop. In 2012, some areas experienced more desirable beetle populations. This is attributed in part to the warm conditions during 2012. However, Bean and colleagues* have found that the beetles have been adapting to their new home. In one field site, the beetles are active for 16 additional days. This may not seem like a long time, but beetle populations reach their peak in the last generation. An additional generation can mean much larger populations.

This rapid evolution is the result of natural selection. In their home range, beetles that failed to enter diapause on time had offspring that did not have enough time to fully develop before adverse winter conditions arrived. Thus, beetles that delay diapause produce offspring with lower rates of survival. In their new home, beetles that diapause at a later date produce many more offspring than beetles that diapause early. Over several generations, the time when most beetles diapause shifts to a later date.

Will this beetle be the answer to Salt Cedar control? Only time will tell. Understanding the biology of insects used for biocontrol informs future efforts to control many of our invasive species.

*Bean, Dalin and Dudley. July 2012. Evolution of critical day length for diapause induction enables range expansion of Diorhabda carinulata, a biological control agent against tamarisk. Volume 5, Issue 5, pages 511–523.
DOI: 10.1111/j.1752-4571.2012.00262.x