Effect of Photoperiod and Temperature on the Life Table Parameters of Onion Thrips (Thrips tabaci) Lineages

The stock cultures of the T. tabaci lineages were maintained in the laboratory at 23 °C under long daylight (16L/8D) regime. Cabbage leaf discs excised from head-forming leaves were used in the bioassays of the L1 and L2 T. tabaci lineages, and tobacco leaf discs excised from middle-aged leaves were used in the bioassays of the T lineage. The number of mothers in the experiment was different in each treatment. In the treatment of 16L/8D and 23 °C it was 48, 49, and 48 for L1, L2, and T, respectively; in the treatment 8L:/6D and 23 °C it was 50, 50, and 29, for L1, L2, and T, respectively; and in the treatment 8L/16D and 15 °C it was 11, 22, and 32 for L1, L2, and T, respectively (Table 1).

Newly emerged virgin female adults of the L1, L2, and T lineages were kept isolated individually and transferred to a new microcentrifuge tube containing a leaf disc of their preferred host plant every 24 h. The preoviposition period was calculated as the time from adult emergence to the beginning of oviposition. Leaf discs were changed daily until the observation of the first egg using the bottom light of a stereomicroscope (Alpha, NSZ-606, Novel optics, Ningo Yongxin, China). When females began laying eggs, leaf discs were changed regularly at 48 h and diapausing females were provided new leaf discs in a similar way until they died.

To measure the incidence of reproductive dia-pause, the oviposition of females was monitored during their entire lifetime. The criteria employed to detect females in reproductive diapause was the failure to oviposit during their lifetime. Females that did not lay a single egg during their lifetime were considered to be in reproductive diapause, and females that laid eggs during their lifetime were considered reproducing females. However, some females died within a relatively short period of time without laying a single egg. Those females that died before reaching the age of the upper bound of the 95% confidence interval of average preoviposition time were excluded from this test. Therefore, the females that lived longer than the upper bound of the 95% confidence interval of an average preoviposition time and produced some eggs were considered reproducing, and those that did not lay a single egg as being in reproductive diapause.

The length of the oviposition period (in days) was calculated as the period between the first and the last egg laid. Longevity (in days) was measured as the period between the emergence and the death of the adult. Fecundity was calculated as the total number of eggs laid for each female.

All data analyses were performed using IBM SPSS 25 (SPSS, Chicago, USA). Means of female lifespan, fecundity, preoviposition, and oviposition period were analyzed separately using GLM of univariate analysis of variance to test the hypothesis that there would be meaningful differences between the lineages and treatments. All means are reported with their 95% confidence interval. Prior to analysis, data were checked for normality using nonparametric Kolmogorov–Smirnov and Shapiro–Wilk tests (p > 0.05) as well as studying skewness and kurtosis. The normality of lifespan and preoviposition data were violated, and to normalize the distributions, these variables were log-transformed. Prior to conducting a series of follow-up t-tests, the homogeneity of variance assumption was tested, and for multiple pairwise comparisons, the Games–Howell post-hoc test was performed.

The average preoviposition periods of females in the L1, L2, and T lineages at different photoperiod regimes and temperature levels are given in Table 2. The females in the three lineages examined at 23 °C under 16L/8D periods laid eggs within 3 days. The photoperiod 8L/16D at the same temperature increased the preoviposition period for T lineage by 1–2 days. The temperature of 15 °C significantly extended the period of preoviposition by 13, 4, and 3 times for L1, L2, and T, respectively, compared to treatment 2. The differences between lines in this treatment were significant at p < 0.001.

The duration of the preoviposition periods of the L2 lineage at 23 °C under short daylight was about 5 days, but it was less than 3 days in the L1 and T lineages. Thus, the differences of females in the preoviposition period, due to photoperiod responses, could be one of the causes of the population fluctuations among the different T. tabaci line-ages. Because a lineage that has a shorter preoviposition period can easily build up its population, while a lineage that has a longer preoviposition can build up its population slowly (Woldemelak et al. 2021). Murai (1990) reported prolonged preoviposition period in the thelytokous type of T. tabaci under short photoperiod. Additionally, the preoviposition periods of L1, L2, and T T. tabaci lineages were increased at 15 °C. Murai (2001) and Sakimura (1937) found a longer preoviposition period in the thelytokous females of T. tabaci at 15 °C.

Photoperiod and temperature both influenced the incidence of reproductive diapause of the T. tabaci lineages as shown in Table 3. The incidence of reproductive diapause in the T lineage was increased as the length of daylight decreased, and in the L1 and L2 lineages increased as temperature decreased. Dissections of females exposed to 23 °C under 8L/16D revealed that 40% of the females tested in the T lineage enter reproductive diapause. However, reproductive diapause was not detected in the females of L1 and L2 lineages at this condition. However, it was detected in 50, 42, and 28% of the L1, L2, and T lineage females tested, respectively, at 15 °C. There was no significant difference in reproductive diapause in the T lineage between 23 and 15 °C. All females of the three lineages laid eggs at 23 °C under 16L/8D.

Reproductive diapause was not detected in the arrhenotokous type of T. tabaci, but it was detected in the thelytokous T. tabaci type under short photo-period (Murai 1990). Woldemelak et al. (2021) reported reproductive diapause in the L1 and T line-ages at 15 °C under 16L/8D. Therefore, reproductive diapause seems to occur in the T lineage likely due to the effect of short daylight, and in L1 and L2 lineages likely due to the effects of low temperature. Photoperiod does not induce diapause in the development of T. tabaci rather the adult has a temperature-induced reproductive quiescence (Jenser & Szénási 2004). Murai (1987) has also reported that under 10L/14D at 23 °C F. intonsa females produce eggs, but 100% of these females entered reproductive diapause at 20, 16, and 12 °C under 10 h daylight. Kamm (1972) reported reproduction diapause induced in the Anaphothrips obscurus (Müller) by exposing larvae to short days (10L/14D), and Lewis (1973) also observed it in Limothrips cerealium (Haliday). The incidence of reproductive diapause varied across different geographical locations in the Haplothrips brevitubus (Karny) (Fujimoto et al. 2014).

The average oviposition periods of females in the L1, L2, and T lineages at different photoperiod regimes and temperature levels are given in Table 4. The length of oviposition periods of female L1 and T lineages was significantly influenced by the photoperiod treatment (p = 0.001), but the oviposition periods of the L2 lineage showed no difference due to the photoperiod effect. The shortest oviposition period was observed in the L1 and L2 lineages of treatment 3. In all three lineages, there was no significant difference in oviposition period between 15 and 23 °C under the 8L/16D photoperiod.

The egg production of females in all three lineages was irregular and sporadic at 15 °C and 23 °C under 8L/16D. Females of L1, L2, and T lineages reared at 15 °C under 8L/16D laid very few eggs. Ekesi et al. (1999) also observed irregular and sporadic egg production under long daylight (16L/8D) at 29 °C in the M. sjostedti. The effect of photoperiod at low temperature on oviposition periods in lineages L1, L2, and T contradicts the report of Ishida et al. (2003) where F. occidentalis continuously oviposited at 15 °C under short daylight (10L/4D). However, females in the L1, L2, and T lineages did not lay eggs continuously under short daylight at 15 °C and 23 °C.

Fifty percent of adult L1 females survived at 8L/16D at 23 °C for periods from 30 to 40 days, whereas about 71% survived under short photoperiod (Fig. 1). The longevity of L1 lineage at 15 °C and short photoperiod was much lower because only about 10% of adult female survived periods of 30 and more days and about 40% survived for 10–20 days. The longevity of adult L2 females was similar under different experimental conditions because they survived in about 40% for 20–40 days. The longevity of T line-age was different from that of L1 and L2. 50% of T females survived 20–30 days at 23 °C and 16L/8D, and about 25% survived 30–50 days in the shorter photoperiod. Nevertheless, the differences were at 15 °C and 8L/16D. Under this condition, about 20% survived for 100–110 days. Generally, such conditions were conductive to extend live time of all lineages but especially the lineage T (Table 5).

Figure 1

All reproducing females in the L1 and L2 line-ages reared under 8L/16D at 15 °C died within 49 days, whereas, reproducing females in the T lineage died within 76 days. The longest longevity under short daylight at 25 °C has been reported in F. occidentalis (Brødsgaard 1994), and adult longevity increased at 14 °C and 12L/12D in M. sjostedti (Ekesi et al. 1999) and at 10 °C under 16L/8D in T. obscuratus (Teulon & Penman 1991). There was a difference in longevity between reproducing and diapausing females in lineages L1, L2, and T, where the reproducing females showed a shorter longevity than diapausing females, and it could be due to a direct relationship between fecundity and longevity. Fecundity is a key factor in terms of reducing longevity (Woldemelak et al. 2021). Therefore, diapausing females could have longer longevity due to the trade-off of lower investment in producing eggs, and reproducing females could have shorter longevity due to the overall cost of investment in producing eggs.

The average fecundity of females in the L1, L2, and T lineages at different photoperiod regimes and temperature levels are given in Table 6. It was significantly (p = 0.001) influenced by temperature and photoperiod. In all three lineages, the lowest fecundity (7–17) was recorded at 15 °C and under 8L/16D compared to both treatments at 23 °C. Twice the highest fecundity was at 23 °C and 16L/8D (86–89) compared to 23 °C and 8L/16D (39–46). No significant differences were noted between lineages under treatment 1 and 3.

Short daylight is likely to have a direct negative effect on the fecundity of all three T. tabaci lineages. Based on the observed fecundity, the optimum photoperiod for maximum egg laying was 16L/8D. Furthermore, sharp fecundity reduction was recorded in the three lineages due to low temperature (15 °C). Murai (2001) and Sakimura (1937) found temperature as a factor to increase and decrease the fecundity of thelytokous (L2) females. F. occidentalis fecundity decreased under short daylight (Whittaker & Kirk 2004) and in T. nigropilosus (Nakao 1994) where fecundity decreased under short daylight and low temperature. There were few symptoms of damage on the leaf discs under short daylight (8L/16D) at 15 °C than under long daylight (16L/8D) at 23 °C. This implied that the feeding level of these lineages is directly interlinked with the temperature and length of the photophase. Murai (1987) has reported that the general activity, including feeding activity of thrips, is known to be higher in long daylight than in short daylight.