Functional Response of Selected Cotton Arthropod Predators to Bollworm Eggs in the Laboratory
چکیده
The functional response study of the ten most common arthropod predators of cotton bollworm (Helicoverpa zea) eggs was conducted in the laboratory. Crab spider (Misumenops spp.) and assassin bug (Zelus renardii) did not consume any bollworm eggs under laboratory conditions and the scymnus lady beetle (Scymnus loewii) consumed very few (avg. 0.5 eggs per beetle) bollworm eggs in a 24-hour period. Functional response was measured by offering varying densities of bollworm eggs for each predator species and the functional response was recorded for 24 hours. Soft-winged flower beetle (Collops spp.), minute pirate bug (Orius spp.), hooded beetle (Notoxus spp.) and damsel bug (Nabis spp.) showed Type-I response, while lady beetle (Hippodamia convergens) adult, lady beetle larva, and green lacewing larva (Chrysoperla spp.) showed Type-II response; big-eyed bug adult (Geocoris spp.) showed Type-III functional response. Among the predators with TypeI functional response, the Collops beetle had the highest slope. Lady beetle larva had the highest handling time (0.185 h) followed by lady beetle adult (0.14 h) and green lacewing larva (0.08 h). Lady beetle adult and green lacewing larva showed a significantly higher attack rate or searching rate compared to lady beetle larva. Introduction The cotton bollworm is a key pest of cotton, which reportably infested 79.4% of the 2002 U. S. cotton crop causing a 2.31% (613,102 bales) yield loss (Williams 2002). Although many arthropod predators of bollworm eggs and larva have been reported and studies have been conducted to evaluate the effect of different predator species in suppressing bollworm eggs and larval populations (Ewing and Thomas 1943, Lingren et al. 1968, Ridgway and Lingren 1972, Bryson and Schuster 1975, McDaniel and Sterling 1979), the information on functional response and the comparative ability of those predator species to consume the target pest is lacking. This information is very important in natural enemy conservation and biological control programs. The number of prey that an individual predator kills is a function of prey density, and is known as functional response (Holling 1966). Effectiveness of a predator is directly related to the type of their functional response. The objective of this study was to evaluate the functional response of the most common arthropod predators to bollworm eggs. Following are the three major types of functional response of arthropod predators. Type-I Functional Response In this type of functional response the number of prey killed increase linearly in a constant rate up to maximum and then remains constant as prey density increases (density independent). Ne = α + β N ------------------------------------------------------------------------------------------------------(1) Where, Ne= number of prey eaten, N= prey density (number of prey offered), α =Intercept and β = slope of the prediction line. Type-II Functional Response In this type of functional response the number of prey killed approaches asymptote hyperbolically as prey density increases (declining proportion of prey killed or the inverse density dependence). Holling’s “Disc equation” (equation-2) is the most commonly used curvilinear model for Type–II functional response. This function assumes the encounter rate with prey is linearly related to the prey density. Prey density remains constant and the handling time limits the number of prey eaten within a fixed time period. e h a N T N = 1+a N T ------------------------------------------------------------------------------------------------------(2) Where, Ne= number of prey eaten, a= the attack constant or instantaneous search rate, N=prey density, T= total available time, Th =handling time (The time taken by the predator to search, capture, consume and digest one prey), and aN= encounter rate with N prey density. Since the experiment was conducted without prey replacement, the prey density declined as the experiment continued from hour 1 to hour 24, a random predator equation (Rogers 1972, Juliano 1993) was used to represent the Type-II functional response as shown below. N e = N0 {1-exp [a (Th Ne-T)]} ------------------------------------------------------------------------(3) Where, N e = number of prey eaten, a = the attack constant or instantaneous search rate or area of discovery, N0=initial prey density, T= total time, and T h = handling time for one prey. Type-III Functional Response In this type of functional response the number of prey killed approaches asymptote as a sigmoid function (increase in proportion of prey killed up to inflection point and then decrease in proportion). This is the density dependent response. The Type-III functional response can be represented by the Hassell model where the attack rate (a) is hyperbolic function of the prey density as given below. d+bN a = 1+cN By substituting the value of ‘a’ to the disc equation we get the Type–III model as below. 2 2 h h dNT+bN T Ne = 1+cN+dNT +bN T ------------------------------------------------------------------------(4) Where, N e = number of prey eaten, a = the attack constant or instantaneous search rate, N0=initial prey density, T= total time, T h = handling time; b, c, and d are parameter constants. The random predator equation (Rogers 1972, Juliano 1993) was used to represent the type-III functional response to account for the fixed prey density in our study as given below. Ne = N0 {1-exp [(d+bN0)(Th Ne-T)/(1+cN0)]} ----------------------------------------------------------(5) Where, N e = number of prey eaten, a = the attack constant or instantaneous search rate, N0= initial prey density (number of prey offered), T= time, T h = handling time; b, c, and d are parameter constants. Materials and Methods The study was conducted in the Cotton Entomology Laboratory at the Agricultural Experimental Station, Lubbock, Texas in July–August 2003. The functional response of the10 most prevailing arthropod predators was evaluated (Table 1). All predators were collected from a cotton field with a sweep net or beat bucket. Uniform size, uninjured predators of mixed sex were sorted out and held without food and water for 24 hours. The 2 and 3 instars of both the lady beetle and green lacewing were used for the study while the remainders of the predators were in the adult stage. Bollworm eggs were used as prey, which were obtained from an insectary at USDA-ARS, SIMRU, Stoneville, MS and stored in a refrigerator at 10 C until they were used in the experiment. The functional response was measured by randomly assigning different densities of bollworm eggs (1 to 150 eggs per density treatment) to each predator species in a petridish. Each treatment was replicated 5 to 10 times. The maximum and minimum prey densities for each predator were determined based on a preliminary feeding study. The treatment arena consisted of bollworm eggs placed singly in rows on a fresh cotton leaf in a plastic petridish (100 mm x 15 mm). The experiment was conducted at 29 C and a photoperiod of 12:12 (L: D). Moist cotton swab and filter paper (9 cm) were used to keep the bollworm eggs and cotton leaf moist during the experiment. Predators were allowed to feed on bollworm eggs for 24 hours. The numbers of bollworm eggs eaten by each predator were recorded after 24 hours of predator exposure. Data Analysis Model Selection The maximum likelihood ratio test of hypothesis was performed with logistic regression analysis of the proportion of prey eaten vs. number of prey present using SAS8.2 -CATMOD program for the selection of best fit model (Juliano 1993). The following cubic expression of the polynomial function was used to describe the relation between Ne/N0 vs. N0 2 3 0 1 0 2 3 2 3 0 0 1 0 2 3 Exp (P +P N +P N 0+P N 0) Ne N 1+(Exp (P +P N +P N 0+P N 0)) = ----------------------------------------------------------(6) Where, Ne= number of prey eaten, N0= initial prey density, and P0, P1, P2, P3.... Pz = parameters to be estimated. This analysis tests the null hypothesis that the estimated parameters are equal to 0 (H0: P0=P1=P2=P3=0). If the linear parameter (P1) was not significantly different from 0 the Type –I (Eqn-1)) model was selected as the best-fit model. If the linear parameter P1 0 the Type-II (Eqn-3) model was selected as the best-fit model. When P1 >0 and P2 /t/ Parameter SE p>/t/ S.W. flower beetle 6.0168482 3.706775 0.1655 0.5101576* 0.048214 0.0001 Minute pirate bug 0.6968585 0.274217 0.0440 0.0314390* 0.019887 0.1650 Hooded beetle 0.2565217 0.727959 0.7582 0.2456522* 0.031583 0.0161 Damsel bug 1.2162162 2.225171 0.6227 0.4198198* 0.122492 0.0416 * Significant at 95% significance level two-tailed test. (P<0.025) Table 4. Estimated handling time and attack rate of different predators that displayed Type-II functional response. Attack rate (a) Handling time (Th) Predator Species Parameter SE Parameter SE Lady beetle adult 0.2643687676* 0.06839045 0.1437737248* 0.01448963 Lady beetle larva 0.0296229172* 0.00773691 0.1857624316 0.09092348 Green lacewing 0.0595875791* 0.01032374 0.0790673603* 0.03679727 * Significant at 95% confidence interval.