Comparative Bionomics of Frankliniella occidentalis and Frankliniella tritici

Two of the most abundant thrips in the south eastern USA are Frankliniella occidentalis (Per gande) and Frankliniella tritici (Fitch) (Thysan optera: Thripidae). Frankliniella occidentalis is a key pest because of damage caused by its feeding and oviposition in many crops and because it is a vector oiTomato spotted wilt virus (Childers 1997; Ullman et al. 1997). Although F. tritici is not a vector oiTomato spotted wilt virus (Assis Filho et al. 2005), its abundance and morphological simi larity to F. occidentalis makes it a management concern for growers and crop consultants (Reitz 2002). Despite superficial similarities in their habits and host plants, the seasonal population dynamics of these species differ dramatically (Eckel et al. 1996; Funderburk et al. 2000; Reitz et al. 2002), yet efforts to better understand their population dynamics are hindered by a lack of comparative biological data and because observ ing individual thrips in the field is almost impos sible. Therefore, laboratory studies can provide critical information to better understand their population dynamics. The objectives of this study were to compare development and reproductive success oiF. occidentalis and F tritici. To determine immature development time, in dividual adult females were allowed to oviposit for 8 h in 2.5-cm sections of green bean pods con tained in 30-ml clear plastic cups. Then females were removed, and the bean sections were held in an environmental chamber (28.0 ? 1.0?C, 65.0 ? 10.0% r.h., 14:10 L: D photoperiod). Bean sections were observed every 12 h under a stereomicro scope to determine when larvae began to emerge. Then a single newly emerged larva from each fe male was transferred to fresh bean pod section. These individuals were checked every 12 h to de termine when molting occurred. There are 2 ac tive larval stages followed by 2 relatively quies cent non-feeding stages, the propupa and pupa (Moritz 1997). Because data were recorded on an interval scale and composed of different events (molts), survival analyses based on the Wei, Lin and Weissfeld model were used to compare devel opment of the life stages (Li & Lagakos 1997; Wei et al. 1989). In this model, molts were stratified according to their order of occurrence with adult eclosi?n as the terminating event. Because of the stratification, separate hazard ratios can be esti mated for each life stage as well as a comparison of the overall development of the species. Subjects that did not complete development were censored. To determine female adult longevity and fecun dity, newly emerged adult females were placed in dividually in small Petri dishes containing a 2.5 cm piece of green bean, with the ends sealed with Parafilm. Beans were replaced every 24 h, and this process continued for each thrips until death. Beans were subsequently examined over 7 d for l rval emergence. The total number of offspring produced per adult female was recorded. Fecundity was analyzed by repeated measures analysis of co variance, with species as a treatment and longev ity as a covariate. This analysis allowed a compar ison of total fecundity and daily fecundity of thrips. Adult longevity was compared based on survival analysis in which mortality was examined by com paring the Cox proportional hazard for each spe cies (Cox 1972) with ties handled by Breslow's method (Breslow 1974). Insects that were acciden tally lost during the experiment were considered censored observations and not included in fecun dity analyses. Experiments were replicated 3 times with colonies that had been established for 4-8 months from specimens collected in Gadsden County, FL. Voucher specimens are deposited in the collections of the author. All statistical analy s s were performed in SAS ver. 9.1 (SAS 2004). Time to egg hatch was approximately 3 d for both F occidentalis (range 2.5-3.5 d) and F. tritici (range 2.5-3 d). Overall immature development from oviposition to adult eclosi?n was signifi cantly faster for F tritici (median = 11.0 d, range = 9.5-14.0 d, n = 26) than for F. occidentalis (me dian = 12.0 d, range = 9.5-14.5 d, n = 20; %2 = 14.10, df=5,P = 0.015). There were no significant differences between the species in development times of the 2 larval stages (median = 2.0 d, range = 1.0-4.0 d, and 3.0 d, 2-4 d, for first and second stadia, respectively), and the propupal stage (me dian = 1.0 d, range = 0.5-3 d; P > 0.25 for each stage comparison). The difference in total devel opment resulted from the shorter pupal stage of F tritici (median = 2.0 d, range = 1-3 d) compared with/*! occidentalis (median = 3.0 d, range = 2-5 d; X2 = 5.14,df=l,P = 0.02). Adult longevity of females differed between the species, with F occidentalis having a signifi cantly greater longevity than F. tritici (log rank test x2 = 8.2, df=l,P = 0.004). The quartile esti mates of times at which 25%, 50%, and 75% of the individuals had died were all significantly greater for F occidentalis (Fig. 1A) indicating that the greater survivorship of F occidentalis was consis tent throughout the adulthood. Initial survivor ship was high for both species, with less than 10% mortality in the first week after eclosi?n. The maximum longevity was similar for both species,