Concentrations of diapausing Diorhabda elongata (Coleoptera: Chrysomelidae), the tamarisk leaf beetle, in relation to abundance patterns the previous summer and litter characteristics of diapause sites

Background/Question/Methods

Tamarix species (tamarisk) are invasive shrubby trees introduced to North America in the 1800’s. Research into biological control of tamarisk began in the 1960’s, culminating in the release of the tamarisk beetle, Diorhabda elongata, in 2001. Grand County, Utah, officials began releasing tamarisk beetles in 2004 as part of their tamarisk control efforts; beetle populations have increased exponentially, defoliating an estimated 646,279 ha of tamarisk in 2008. The tamarisk beetle overwinters as an adult in tamarisk leaf litter. Reproductive diapause is initiated at daylengths shorter than about 14 hours 20 minutes, but active adults are found into October. Knowing where beetles overwinter is important because other control efforts during winter (e.g., burning, cutting or bulldozing) may inadvertently kill beetles. We have been monitoring beetle numbers throughout Grand County for the past two years and in fall 2008 initiated a study of diapausing beetles. We use randomly selected sentinel trees for beetle monitoring; in November we collected litter from the bases of sentinel trees at sites with high beetle abundance in the first three survey periods (May 2008), the middle of the season (July, 2008), and the last three surveys (September, 2008). We collected samples from the cardinal points at the base of each sentinel tree, separating the litter into upper and lower layers if there was a clear demarcation based on litter color and compactness. We sieved samples through screens (6 mm, 3 mm, and 1.5 mm) to recover beetles. We pooled counts for each site, regressing numbers of beetles in litter on number of beetles counted each monitoring visit during the season—a separate linear regression for each survey period (5 May to 6 October 2008).
Results/Conclusions

Number of larvae counted during surveys 4 to 11 August and number of adults counted 12 to 18 August were each significantly related to the number of beetles found in litter (R² = 0.54; P < 0.001; R² = 0.486; P < 0.001, respectively); there was little or no relation to survey counts at any other times. We found more beetles in the lower layer than upper layer; litter depth was weakly correlated with beetle abundance. Aspect did not influence number of beetles in the sample. Land managers can use the results of this study as a guide in planning other tamarisk control efforts that involve litter disturbance to minimize impact on beetle control the following year.