INSECTS

A laboratory colony of M. usitatus came from Wanning City in Hainan province of China (18.7851°N, 110.3910°E), and was fed on kidney bean pods (Phaseolus vulgaris L.; Fabaceae) in 2 L glass jars. Orius sauteri were provided by the Biological Control Lab of China Agricultural University, Beijing, China, and were fed with the eggs of Angoumois grain moth (Sitotroga cerealella (Olivier); Lepidoptera: Gelechiidae) in the glass jars. In addition, the kidney bean pods were used as the oviposition substrate for both species. All experiments were conducted with reared insects at 26 ± 1 °C, 65% RH, and 16:8 h (L:D) in incubators (QX-256, Jiangnan Instrument Factory, Ningbo, China).

FUNCTIONAL RESPONSE TESTS

Three-d-old adult females of O. sauteri were fed with 3-d-old adult females of M. usitatus for 48 h and starved for 24 h. Then the bugs were transferred individually to plastic petri dishes (9 cm diam) and provided with 3-d-old adult females of M. usitatus at different densities (10, 20, 40, 60, 80, 100, 120 prey per predator, respectively). After 24 h, the number of prey consumed by each bug was recorded. Bugs were used once only. Meanwhile, a control treatment was carried out without predators. Each density included 5 replicates.

INTRASPECIFIC INTERFERENCE EXPERIMENT

To measure the reduction in prey consumption when various numbers of adult Orius bugs were held in the same arena with prey, we ran an intraspecific interference experiment in which 45 adult female thrips were held in an arena with different numbers of adult Orius (1, 2, 3, or 4). The number of prey consumed by the group of predators was checked after 24 h, and converted to a per predator basis. A control treatment was conducted without predators. Each treatment included 5 replicates.

LIFE TABLE STUDY

To determine the life table parameter for O. sauteri when fed with M. usitatus, the survival of a group of bugs under those conditions was followed. Kidney bean pods with O. sauteri eggs that were < 24 hrs old were isolated from the rearing colony, and held in the incubator as described above. After egg hatch, 30 first instar nymphs were chosen randomly and reared individually with 45 M. usitatus 3-d-old adult females daily as prey in 10 mL bottles. The kidney bean pods provided for the M. usitatus prey as their food was replaced daily. Mortality and nymphal instar were recorded daily for each insect under a stereomicroscope (SZ656, Optec, Chongqing, China) until nymphs molted to the adult stage.

Separately, to measure adult fecundity of O. sauteri on this prey, a male and a female of O. sauteri that emerged the same d were placed in the same bottle, and their survival and the number of eggs laid per female were recorded daily. Females inserted eggs in the bean pods and eggs were visible under the stereomicroscope. Pods with O. sauteri eggs were held in the incubator, where the hatching rate and progeny sex ratio were recorded. One female from each pair was a replicate, and there were 30 replicates.

PREDATION RATE

Newly hatched first instar nymphs were provided 45 three-d-old adult female thrips daily during the life table experiment. There were 30 replicate Orius for this study. Their daily predation was determined by counting the dead thrips and then replacing them with another groups of 45 three-d-old adult female thrips at the end of each day.

DATA ANALYSIS

Data from the functional response experiment were analyzed with SPSS 21 and fitted to the Holling's disc equation (Holling 1959):

where N _A is the number of prey killed by predators during time T, which in this experiment was 1 d. T _h is the predator handling time for 1 prey (= T divided by maximum predation rate), while a is a constant equal to the search rate multiplied by the probability of finding a prey. N is the density of prey. The search rate was calculated as per Ding (1994):

where E is the search rate.

The results of the intraspecific interference experiment were analyzed according to Hassell & Varley (1969):

where E is the predation rate, Q is the quest constant, P is the density of predators, N _a is the number of the prey killed by predators, m is the mutual interference constant, and N is the density of the prey. Moreover, the intensity of scramble competition (I) was calculated as per Zou et al. (1996):

where E _I is the predation rate for 1 predator, and E _P is the predation rate for the predators with a density of P.

Life table parameters, as per Birch (1948), were calculated from the data as follows:

where l _x was the age-specific survival rate; m _x was the age-specific fecundity. Population parameters are r, the intrinsic rate of increase; λ, the finite rate of increase; R ₀, the net reproductive rate, and T, the mean generation time.

The predation rates of O. sauteri at all stages on M. usitatus were analyzed according to the following formulas (Chi & Liu 1985; Chi 1988):

where s _xj refers to the age-stage specific survival rate (where x = age and j = stage); c _xj is the age-stage specific consumption rate; k _x is the age-specific predation rate; C ₀ is the net predation rate, and Q _p is the transformation rate from the prey population to predator progeny. The parameters were calculated by using the TWOSEX-MSChart and CONSUME-MSChart programs designed by Chi (2009a, b).

FUNCTIONAL RESPONSE OF ORIUS SAUTERI ON MEGALUROTHRIPS USITATUS

We found that O. sauteri was able to efficiently prey on M. usitatus, and its predation showed a decelerating (Type II) functional response to increasing M. usitatus number. When only 10 thrips were provided, O. sauteri consumed a mean of 9.4 thrips per predator per d, which indicates that the predators can efficiently find the thrips at low densities (Fig. 1). The functional response of O. sauteri adults to M. usitatus adults was calculated using Holling's disc equation, and the functional response parameters were estimated with equation (1). The maximum predation rate O. sauteri per individual adult female was 45.3 thrips per d (Table 1). At higher prey density, the predator search rate (per predator per d) declined, showing that O. sauteri needed to spend less time searching for prey at higher prey densities (Fig. 2).

Fig. 1.

Predation capacity of Orius sauteri adults for Megalurothrips usitatus adults at 26 °. Each value is the mean (± SE) of 5 replicates.

Table 1.

Functional response of Orius sauteri adults to Megalurothrips usitatus adults as prey when reared at 26 °C. T _h = predator handling time (for 1 prey); a = a constant equal to the search rate multiplied by the probability of finding a prey.

INTRASPECIFIC INTERFERENCE OF ORIUS SAUTERI

The predation capacity (prey consumption per predator) of O. sauteri decreased with increasing O. sauteri density within the test arena, due to scramble competition. When the predator density was 4, the intensity of scramble competition (I) was 0.76, indicating that the intraspecific interference of O. sauteri was significant (Table 2, Fig. 3).

AGE-STAGE, TWO-SEX LIFE TABLE

We found that O. sauteri eggs from adults that were fed on M. usitatus had a high hatch rate, and resulting nymphs had high survival rates at 26 °C (Fig. 4). In addition, O. sauteri adults fed on M. usitatus had a long oviposition period. The hatch rate of O. sauteri eggs was 59.7%. The mortality of first instar nymphs of O. sauteri was higher, whereas no mortality occurred in the fourth or fifth instars. The mean development period, pre-oviposition period, sex ratio, fecundity, female adult longevity, and male adult longevity of O. sauteri were 15.9 d, 2.1 d, 2.8 females per male, 95.4 eggs per female, 21.1 d, and 9.7 d, respectively (Table 3). The longevity of adult males of O. sauteri was much shorter than of females. At peak oviposition, O. sauteri laid 5 to 8 eggs per d, declining thereafter to 1 to 4 eggs each d until death (Fig. 5). Under the experimental conditions, net reproductive rate (R ₀), mean generation time (T), the intrinsic rate of increase (r), and finite rate of increase (λ) of O. sauteri were 51.1 offspring per individual, 24.6 d, 0.16 d-1, and 1.2 d-1, respectively (Table 4).

Fig. 2.

Relationship between search rate of Orius sauteri adults and density of Megalurothrips usitatus adults at 26 °C.

Table 2.

Predation (prey consumption per predator, mean ± SE), E _I (predation rate for 1 predator) and I (intensity of scramble competition) among Orius sauteri adults at 26 °C.

PREDATION RATE

C ₀ and Q _P, calculated using equations 11 and 12, showed that O. sauteri consumed an average of 186.0 thrips over the course of its life, and that to lay 1 egg required the consumption of 3.6 thrips (Table 4). Except for the egg stage, O. sauteri of all life stages could kill thrips (Fig. 6). Nymphs increased their thrips consumption from about 2 to 10 thrips a d. Male consumption was 3 to 9 thrips each d, which was less than that of females, while survival time for males was much shorter than for females. The total predation by O. sauteri nymphs, and by adult females and males, was 3.5 to 30.8, 233.1, and 39.5 per stadium, respectively (Table 5). Thrips consumption increased as O. sauteri progressed, and after the fourth instar the predation rate increased rapidly. Males consumed an average of 104.0 thrips over their entire lifetime, whereas females consumed an average of 304.7 thrips during their lifetime. The age-stage specific predation rate (k _x), the survival rate (l _x), and the age-specific net predation rate (l _x k _x) were calculated with equation 10. Predation peaked in 5 to 9-d-old adults, which coincided with peak oviposition (Fig. 7).