Tamarisk biocontrol: A case study in rapid evolution and range expansion of an introduced species

Background/Question/Methods

The northern tamarisk beetle, Diorhabda carinulata, was originally collected in Central Asia and released in North America in 2001 to control the exotic shrub, Tamarix spp.   This foliage-feeding beetle has expanded its range in North America, moving southward into areas that had been previously un-colonized due to phenology mismatches with the host plant arising from miscued photoperiodically-induced diapause.  A shift in phenology has been noted in the field with beetles remaining reproductive later into the season, bringing the southernmost beetle populations more into synchrony with the growing season for Tamarix. How has the control and timing of diapause induction evolved to enable this synchrony, and in a short amount of time?   To answer this question we used beetle populations collected along a north-south gradient to test the hypothesis that photoperiodic diapause induction cues have become more sensitive to temperature, increasing phenotypic plasticity in the trait of the photoperiodic cue for diapause.  We have subjected developing beetles to an array of photoperiods overlaid with thermoperiods in order to measure the critical day length for diapause induction (CDL; the day length at which 50% of the population enters diapause) as a function of temperature.  

Results/Conclusions

Beetles collected from the northern most site near the town of Delta, Utah (39.5° N) had a CDL that matched closely with the CDL originally measured in this beetle shortly after it arrived from central Asia (14.6 hours light).  This CDL was constant over a wide range of temperatures (20-30°C average). At the southernmost site, at 36.5°N, beetles had a CDL of 14.28 hr at 30°C indicating that evolution of CDL has enabled beetles to remain reproductive later into the season at the high temperatures encountered in the lower Colorado River.  In spite of a shifting CDL, beetles on the Virgin River and lower Colorado remained out of phenological synchrony with Tamarix.  They entered diapause in late July to mid-August, three months before Tamarix has senesced.  This asynchrony is one reason why the southward moving front of D. carinulata has slowed at approximately the 36^th parallel.