Sources of H. axyridis, C. juglandicola, and P. juglandis
The adult lady beetles of H. axyridis and the two aphid species, C. juglandicola and P. juglandis, used in this study were collected from fresh leaves of walnut trees in Yili(N43°14′53″, E82°49′40″), Xinjiang, China. Aphid colonies were maintained on young walnut plants in a controlled-climate room held at 25 ± 1 °C, 60 ± 10% RH, and under a photoperiod of 14:10 h (L:D).
Functional response: predation by H. axyridis on aphids at various densities and four temperatures
The functional response (number of aphids consumed per day) of H. axyridis was determined by offering varying densities of 2nd/3rd instar nymphs of each aphid species separately on a walnut leaf disc placed in a Petri dish (90 mm by 18 mm) lined on the base with moist filter paper. Prey densities were 80, 160, 240, 320, 400, and 600 aphid nymphs of C. juglandicola and 60, 120, 180, 240, 300, 360 aphid nymphs of P. juglandis. After adding the prey, one adult of H. axyridis, starved for 24 h, was added to and confined in the Petri dish. The Petri dishes were kept in environmental chambers under four different constant temperatures: 15, 20, 25, and 30 ± 1 °C, with 60 ± 10% RH and a light: dark photoperiod of 14:10 h. The number of aphids consumed by the H. axyridis was recorded after 24 h. Each aphid density and aphid species was replicated six times for each temperature, simultaneously.
Intraspecies interference within H. axyridis whilst feeding on aphids
To test how intraspecific interference affected predation by H. axyridis on walnut aphids, one, two, three, four, or five adult(s) of H. axyridis lady beetles were starved for 24 h and then added to Petri dishes (as above) containing 600 2nd/3rd instar nymphs of each aphid species separately. The number of aphids consumed was recorded after 24 h. Experiments were conducted under 25 ± 1 °C, 60 ± 10% RH and a photoperiod of 14:10 h (L:D). Six replicate experiments using each H. axyridis density were undertaken.
Data analysis
The Holling’s type II equation (Holling 1959, 2003) was used to model the functional response of H. axyridis preying on aphids:
$$ {N}_a={ aT N}_0/\left(1+{aT}_h{N}_0\right) $$
(1)
where Na is the number of aphids consumed per day and N0 is the initial number of prey (aphids); Th is the handling time of H. axyridis preying on aphids, a is the successful attack rate, and T is the time of the experiment, which in this case was one day.
The values of a and Th were found by linearly regressing 1/Na against 1/N0. The resultant y-intercept is the initial estimate of Th and the reciprocal of the regression coefficient (slope) is an estimate of a (Livdahl and Stiven 1983). We used equation (1) Na = aTN0/(1 + aThN0), when N0 was infinitely great. The equation, Na/N0 = T/Th, estimated maximum predation number.
The searching efficiency was calculated as
$$ S=a/\left(1+{aT}_hN\right) $$
(2)
where S is searching efficiency, a is the successful attack rate, Th is the handling time and N is the number of prey.
The average predation rate was calculated using Watt’s (1959) equation:
where A is the average predation rate, P is predator density of 1, 2, 3, 4, or 5 H. axyridis adults, m is the coefficient of interference, and Q is the seeking constant. The values of Q and m were found by power-exponential regressing A and P.
Model fitting was performed using Origin version 7.5 for Windows.