BIOLOGICAL AND MICROBIAL CONTROL Functional Response, Prey Stage Preference, and Mutual Interference of the Tamarixia triozae (: ) on Tomato and Bell Pepper

1 XIANG-BING YANG, MANUEL CAMPOS-FIGUEROA, ADRIAN SILVA, AND DONALD C. HENNE

Texas A&M AgriLife Research and Extension Center, Texas A&M University System, Weslaco, TX 78596 USA.

J. Econ. Entomol. 108(2): 414–424 (2015); DOI: 10.1093/jee/tou048 ABSTRACT The potato psyllid, Bactericera cockerelli (sˇulc), has been detrimental to potato, tomato, and other solanaceous crop production in many countries. Management of B. cockerelli is dominated by frequent insecticide applications, but other approaches need consideration, including biological control. The sole arrhenotokous ectoparasitoid of nymphal potato psyllids is Tamarixia triozae (Burks) (Hyme-

noptera: Eulophidae). Here, laboratory evaluations of host stage preference, parasitoid mutual interfer- Downloaded from ence, and functional response of T. triozae were conducted with varying host B. cockerelli nymphal stages and densities on both tomato and bell pepper plant leaves. Significant differences in prey stage prefer- ences were found on both host plants. In a no-choice host stage test, significantly greater parasitism of fourth- and fifth-instar B. cockerelli nymphs occurred, and no parasitism of first or second instars was found. Similar preferences were found in a host stage choice test. Effect of mutual interference on per

capita female parasitism was significant when confining two or three simultaneously ovipositing female http://jee.oxfordjournals.org/ T. triozae adults on a given host density versus solitary females. The per capita search efficiency (s)of female T. triozae was significantly and negatively correlated with T. triozae density. The functional response of T. triozae to nymphal B. cockerelli was a Type III form on both host plants. In addition, host plant type did not exert a significant bottom-up effect on either parasitism or functional response of female T. triozae. The feasibility of using bell pepper as a potential banker plant for T. triozae augmentation is also discussed.

KEY WORDS Tamarixia triozae, functional response, searching efficiency, mutual interference, parasitism by guest on October 3, 2016

Introduction transmission (Butler et al. 2011). Therefore, biological control of B. cockerelli by natural enemies and its com- The potato psyllid, Bactericera cockerelli (sˇulc), is a patibility with insecticide programs needs more atten- damaging pest of potato, tomato, and other culti- tion. Previous studies of the potential for effective vated solanaceous crops in the United States, Mexico, biological control of B. cockerelli arelimited.Biological Central America, and New Zealand (Munyaneza et al. control has been used successfully in pest management 2007; Hansen et al. 2008; Liefting et al. 2008; Munya- programs for decades (van Lenteren and Woets 1988; neza and Henne 2012). Potato psyllids damage plants Bailey et al. 2009), and incorporating beneficial preda- by direct feeding and, more importantly, by transmit- tors and parasitoids in integrated pest management ting the bacterial pathogen ‘Candidatus Liberibacter programs have helped reduce reliance on chemical solanacearum (syn. psyllaurous)’ to solanaceous plants control. and causing zebra chip disease of potato (Hansen et al. Tamarixia triozae (Burks) (Hymenoptera: Eulophidae) 2008; Liefting et al. 2008). In the United States, mil- is the only known arrhenotokous ectoparasitoid of nym- lions of dollars in potato production losses have been phal B. cockerelli. Described previously as Tetrastichus reported, with severely affected potato farms being sp. and Tetrastichus triozae, T. triozae is the current ac- devastated by zebra chip disease, particularly in Texas cepted name (Romney 1939; Burks 1943). In the United (Goolsby et al. 2007). Currently, frequent insecticide States and Mexico, T. triozae occur naturally in fields applications to control B. cockerelli remain the optimal and greenhouses (Lomeli-Flores and Bueno 2002; Bravo management option because of the need for immediate and Lopez 2007). Previous field studies of T. triozae efficacy to control bacterialiferous adults (Goolsby used brief scouting in potato and tomato fields, and high et al. 2007; Gharalari et al. 2009; Yang et al. 2010). parasitoid pupal mortality (38–100%) and low parasitism However, B. cockerelli are becoming resistant to some rates (<20%) were present owing to factors such as fun- insecticides, leading to increased Liberibacter gal diseases and predation (Johnson 1971;Yangand Henne unpublished data). Otherwise, no comprehensive studies of T. triozae host–parasitoid interactions have been reported, particularly functional responses, host 1 Corresponding author, e-mail: [email protected]. stage preferences, or parasitoid mutual interference.

VC The Authors 2015. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For Permissions, please email: [email protected] April 2015 YANG ET AL.: FUNCTIONAL RESPONSE,PREY STAGE PREFERENCE, AND MUTUAL INTERFERENCE 415

Functional responses elucidate the relationship be- the Texas A&M AgriLife Research and Extension Cen- tween proportions of hosts consumed/parasitized by a ter at Weslaco, TX, in 2006. B. cockerelli from this col- predator/parasitoid at increasing host densities up to a lection were maintained within screen cages on tomato, maximum, or satiation, level and are key to understand- Solanum lycopersicum L. (variety “Florida Lanai”), for ing how a predator or parasitoid behaves under differ- >7 yr under laboratory conditions of 26.7 6 2C, ent host population densities (Holling 1959). Such 75 6 5% relative humidity (RH), and a photoperiod of studies are important for predicting successful field re- 14:10 (L:D) h. The B. cockerelli nymphs from heavily leases of biological control agents (Xiao and Fadamiro infested tomato plants were prepared for colonizing by 2010). Predator–prey functional responses relationships T. triozae. are generally modeled by three empirical relationships: T. triozae was originally collected in March 2013 linear (Type I), convex (Type II), and sigmoid (Type from insecticide-free potato research fields located at III; Holling 1959, 1961, 1966; Trexler et al. 1988; the Texas A&M AgriLife Research and Extension Cen- Timms et al. 2008). Type I models characterize an line- ter at Weslaco, TX. Because T. triozae only parasitizes arly increasing predation rate where predators attack host nymphs, leaf samples containing B. cockerelli prey randomly, and the proportion of prey consumed mummies were placed inside a polyvinyl chloride increases when prey density increases, up to a maxi- bucket (30 cm in diameter by 30 cm in height). mum level where the attack rate becomes constant. A removable transparent container was attached to the Type II and III models describe predator–prey rela- top of the bucket lid, covering a 13-cm-diameter hole tionships that are nonlinear between the proportion of that was excised on the bucket lid to allow emerging Downloaded from prey consumed and prey density; in a Type II func- T. triozae adults access to the transparent container. tional response, the proportion of prey consumed is a The lid of the transparent container was glued over the hyperbolic curve, and in Type III, it is a sigmoidal hole, and a smaller diameter hole was bored out of the curve. In both Type II and Type III models, the pro- lid. The buckets containing the mummy samples were portion of prey consumed reaches an asymptote be- placed on a light rack and emerging T. triozae inside cause of the time required for handling prey (Chong the bucket accessed the transparent container by pho- http://jee.oxfordjournals.org/ and Oetting 2006; Parajulee et al. 2006). Hence, under- totaxis. Newly emerged T. triozae adults were aspirated standing the functional response of T. triozae is critical from the transparent container and released into a to capture details of its predation and parasitism behav- screen cage (60 by 60 by 60 cm [L by W by H]) con- ior, and will also facilitate conservation of beneficial taining tomato plants that were heavily infested with natural enemies (Parajulee et al. 2006). B. cockerelli nymphs. A cup holding a sterilized sponge Augmentation of natural enemies is an important as- that was soaked with a 10% honey (v:v) solution was pect of biological control. Banker plants application hung inside the cage to feed adult T. triozae.Whenthe provides a sustainable reproducing population of natu- nymph populations were low, additional B. cockerelli

ral enemies within the selected crops or plants to pro- nymphs were supplied as needed. The T. triozae colony by guest on October 3, 2016 vide long-term pest suppression (Frank 2009). The was maintained under similar laboratory conditions as critical step in a successful banker plant system is to that mentioned above. find host plants, often noncultivated, that are tolerant Tomato, S. lycopersicum L. (variety Florida Lanai), of the pest host, will attract beneficial natural enemies, and bell pepper, Capsicum annuum L. (variety and will serve as alternative food or host for natural en- “Capistrano”) were seeded first in a foam tray with emies to reproduce sustainably to control the target cone-shaped pots (3 by 3 by 4 cm) and maintained in a pest on the primary crop (Bennison 1992; Bennison greenhouse at 28–32C and under natural light condi- and Corless 1993). However, little attention has been tions. One week after germination, the plant seedlings directed toward banker plant system applications, de- were individually transplanted into 1-liter plastic pots. spite its potential for improving biological control effi- The seedlings were fertilized weekly with 0.6 g liter1 cacy or application (Frank 2009). Water Soluble Plant Food (N:P:K ¼ 15:30:15; Chem- In this study, we conducted a series of laboratory bioas- isco, Division of United Industries Corp. St. Louis, says to evaluate the biology and behavior of T. triozae at- MO) and watered as needed. Four-week-old plants tacking B. cockerelli nymphs feeding on two host plants, were used in all experiments. tomato and bell pepper. The objectives of this study were to: 1) determine B. cockerelli nymphal stage preferences Host stage preference of T. triozae, 2) determine the effect of mutual interfer- ence where simultaneous ovipositing T. triozae females No-Choice Test. A no-choice experiment was are confined with B. cockerelli nymphs, 3) determine the conducted under laboratory conditions to determine type of functional response of individual ovipositing T. tri- female T. triozae parasitism efficiency on each of the ozae females at varying B. cockerelli nymphal densities, five B. cockerelli nymphal stages. Using a sterilized and 4) discuss the potential bottom-up effect of prey host blade, the top two or three fully expanded tomato and plant on fitness of T. triozae. bell pepper leaves were excised from 4-wk-old green- house plants, and leaf petioles were immediately immersed into plastic transparent vials (10 ml) filled Materials and Methods with reverse osmosis water. The vials and leaves were Insect Colonies and Host Plants. Potato psyllids placed individually into transparent plastic containers were collected from a potato research field located at (0.9 liter), of which the open top was covered with a 416 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 108, no. 2

52-mesh polyethylene screen (Yang and Liu 2009). leaves for parasitism. As T. triozae only parasitizes (and This setup was used for all experiments in the study. superparasitizes, Yang and Henne unpublished data) Using a camel hair paintbrush, 40 newly exuviated and does not feed on fourth and fifth instars of prey, no B. cockerelli nymphs of each instar were gently trans- prey replacement was needed in this experiment. Thus, ferred onto the leaf within the transparent container to the number of hosts in each density remained constant expose them to T. triozae parasitism. A preliminary throughout the experiment. Forty-eight hours later, experiment revealed B. cockerelli nymphs suffered no T. triozae females were removed and post-experiment mortality when gently transferred with a camel hair observations were the same as described above, where paintbrush. Tomato leaves with numerous B. cockerelli the number of mummified B. cockerelli nymphs from mummies were collected from a T. triozae colony and each trial was documented. Because the difference placed in the clear container for T. triozae adult collec- between Type II and Type III functional responses are tion. Newly emerged T. triozae adults <2h old were primarily the result of different parasitism rates at collected using an aspirator, sexed, paired, and lower host densities, more replications of lower host observed for mating occurred. One pair of mated T. tri- densities are suggested. Therefore, for trials with host ozae adults (1? and 1/) was introduced into the con- densities of between 5 and 25 nymphs, nine replica- tainer hosting a leaf with 40 B. cockerelli nymphs of tions were conducted on tomato and eight replications each instar for parasitism. After 48 h, the T. triozae were conducted on bell pepper, and on host densities adults were removed and the B. cockerelli nymphs of 35, five replications were conducted for each den- were left for 10 d to document the number of mum- sity on both tomato and bell pepper. Downloaded from mies. The percentage of nymphs that became mum- Statistical Analysis. As 0 first- and second-instar mies was calculated for each of the five B. cockerelli nymphs were parasitized, 0.5 was first added to the instars. Each treatment was replicated five times. original data prior to square root transformation, and Choice Test. Multiple-choice tests were conducted then the transformed total was used to analyze prey under laboratory conditions to determine host stage stage preference results (Zar 2010). Transformed data preference. Ten newly exuviated B. cockerelli nymphs from the prey stage preference experiment were first http://jee.oxfordjournals.org/ of each of the five instars were selected and introduced tested for homogeneity of variances, and data were onto the host plant leaf confined within the clear con- then analyzed with a two-way (host plant and prey tainer (i.e., 50 newly hatched or exuviated B. cockerelli stage) analysis of variance. Means were separated using nymphs, 10 from each of the first to fifth instars, were Tukey’s (honestly significant difference) test at P ¼ 0.05 randomly deposited onto the single host plant leaf). (SAS Institute 2010, Cary, NC). One pair of newly emerged and mated T. triozae Where mutual interference occurs among multiple adults (1? and 1/) was introduced into the container conspecific female parasitoids, the effect on search and allowed to parasitize for 48 h. Post-experiment efficiency (s) of per capita female parasitism can be

steps were the same as mentioned above in the quantified, whereby the rate of parasitoid searching by guest on October 3, 2016 no-choice test. These treatments were replicated five efficiency declines as parasitoid density increases times. (Hassell 2000). Per capita search efficiency of T. triozae Mutual Interference. To test for mutual interfer- in relation to conspecific parasitoid density was esti- ence among conspecific female T. triozae, three labora- mated from the equation below (Visser and Driessen tory trials evaluating different densities of conspecific 1991): female T. triozae adults were conducted on both B. cockerelli host plants, based on results of the nym- h i 1 Nt phal stage preference experiments. Therefore, 40 newly s ¼ In (1) exuviated fourth and fifth B. cockerelli nymphal instars Pt Nt Na (20 of each instar) were randomly selected and intro- duced into the transparent container containing a host Where Pt is the density of T. triozae females, Nt is the plant leaf. Next, one, two, or three pairs (? and /)of number of B. cockerelli nymphs, and Na is the number newly emerged and mated T. triozae adults were intro- of nymphs parasitized. Searching efficiency (s)was duced into transparent containers to allow parasitism regressed on log10-transformed female T. triozae den- for 48 h. Afterwards, similar post experiment steps sities using least squares regression (Zar 2010). were processed as mentioned above. The number of A two-step approach was used to distinguish among mummified nymphs was documented and the per cap- the three types of functional responses (Juliano 2001). ita search efficiency of female T. triozae was calculated First, a logistic maximum likelihood regression was as described in Equation (1) in Statistical Analysis sec- applied to determine the shape and to select the best tion. These treatments were replicated 10 times. model for the functional response curve relating the Functional Response. To determine the shape of percentage of B. cockerelli nymphs parasitized by T. tri- the functional response, laboratory trials were con- ozae as a function of B. cockerelli density (PROC ducted that confined individual newly emerged and LOGISTIC, SAS Institute 2010). To select the best mated solitary T. triozae females with variable B. cock- model, a stepwise regression model selection method erelli nymph densities using the clear containers was used in a polynomial logistic regression to deter- described above. Either 5, 10, 15, 25, 35, 50, 85, or 110 mine the number of parameters (P1, P2, P3,...and Pk) newly exuviated fourth- and fifth-instar B. cockerelli in the model, and the polynomial model with parame- nymphs were introduced on each of the two host plant ters up to the third degree is sufficient for a biological April 2015 YANG ET AL.: FUNCTIONAL RESPONSE,PREY STAGE PREFERENCE, AND MUTUAL INTERFERENCE 417 study (Juliano 2001). Therefore, the logistic model Table 1. No-choice test results of T. triozae parasitism of dif- below was used in this analysis: ferent prey stages on tomato and bell pepper plants under labora- tory conditions at 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h 2 3 Na expðP0 þ P1N0 þ P2N0 þ P3N0Þ Nymphal instar Tomato Bell pepper F P ¼ 2 3 (2) 1, 8 N0 1 þ expðP0 þ P1N0 þ P2N þ P3N Þ 0 0 Mean 6 SE (%) Mean 6 SE (%) First 0cA 0cA – – Where the parameter N0 is the host density; Na is the Second 0cA 0cA – – number of hosts parasitized; P0 is the intercept (con- Third 40.5 6 5.3bA 36.5 6 3.0bA 0.35 0.5686 stant); and P1, P2,andP3 are the linear, quadratic, and Fourth 81.0 6 2.7aA 74.0 6 4.2aA 2.05 0.1901 cubic coefficients, respectively, which describe the Fifth 84.0 6 3.3aA 77.5 6 1.8aA 2.91 0.1264 slope of the curve. The model selection index, Akaike F4, 20 364.82 602.74 < < information criterion (AIC), was used to select the best P 0.0001 0.0001 model (Akaike 1973; Burnham and Anderson 2002). Different uppercase letters indicate a significant difference The model with the lowest AIC value is considered the between host plants, and different lowercase letters indicate a signifi- cant difference among prey stages at P ¼ 0.05. model best fitted to the functional response. The type of functional response can be briefly determined by significant values of P0, P1, P2,andP3 that are different Results Downloaded from from 0. For example, a negative value of P0 indicates a Type II functional response, and a Type III functional Prey Stage Preference. No-Choice Test. Signifi- response is indicated by positive P0 and negative P1 val- cant differences in parasitism among B. cockerelli ues, etc. instars were found on both host plants (Table 1). There Second, after shape determination, data were fitted was no parasitism of first or second instars on either with Holling’s mechanistic model for further interpreta- host plant. However, parasitism rates were significantly tion of the parameters. Below is Holling’s model for a higher on the fourth and fifth instars than on the third http://jee.oxfordjournals.org/ Type II functional response: instar on both host plants (tomato: F ¼ 364.82; df ¼ 4, 20; P < 0.0001; bell pepper: F ¼ 602.74; df ¼ 4, 20; aNT P < 0.0001), but there was no significant difference Ne ¼ (3) between fourth- and fifth-instar parasitism on either 1 þ aNTh host plant. There were no significant differences in par- asitism between the two host plants for all instars Where a (attack constant) is the parameter to be esti- (F ¼ 0.35 to 2.91; df ¼ 1, 8; P ¼ 0.5686 to 0.1264). mated, Ne is the number of hosts parasitized, N is the Choice Test. Results were similar to the no-choice

host density, T is the total exposure time in the experi- test. In the choice test, significant differences in prey by guest on October 3, 2016 ment, and Th is the handling time per host. For a Type stage preference were found on both host plants (Table III mechanistic model, Hassell (1978) reported that a is 2). No first- and second-instar B. cockerelli nymphs on generally a hyperbolic function of prey density N: either host plant were parasitized. However, signifi- cantly more fourth- and fifth-instar nymphs were para- d þ bN sitized than the third instar on both tomato a ¼ (4) (F ¼ 221.15; df ¼ 4, 20; P < 0.0001) and bell pepper 1 þ cN plant leaves (F ¼ 310.42; df ¼ 4, 20; P < 0.0001). No significant preference between fourth and fifth instars Where b, c,andd are parameters to be estimated. was found on either host plant. Furthermore, there was Then, substituting equation 4 into equation 3 yields no significant difference between the two host plants equation 5 for Type III mechanistic model: for any of the five instars (F ¼ 0.01 to 0.24; df ¼ 4, 20; P ¼ 0.6408 to 0.9887). dNT þ bN2T Mutual Interference. Figure 1 shows the number N ¼ (5) e þ þ þ 2 B. cockerelli nymphs parasitized by different densities 1 cN dNTh bN Th of conspecific female T. triozae. One female and three conspecific female T. triozae adults parasitized signifi- To select the model that best fits our data, a nonlin- cantly more nymphs than two female adults, on both ear regression (PROC NLIN, SAS Institute 2010)with tomato (v2 ¼ 19.23; df ¼ 2; P < 0.0001) and bell pepper stepwise model selection was used for the analysis. In leaves (v2 ¼ 14.84; df ¼ 2; P < 0.0001). On both host the model selection procedure, parameters b, c,andd plants, no significant difference in the number of 2 were stepwise eliminated and then grouped with Th, nymphs parasitized was found at either two (v ¼ 0.21; 2 except for the model with only parameters c and Th (as df ¼ 1; P ¼ 0.6483) or three (v ¼ 2.19; df ¼ 2; there is no functional response if both b and d are 0). P ¼ 0.139) female T. triozae adults per container, except Furthermore, models were compared with AIC values where there was only one T. triozae female per con- to select the best model. Notably, when b ¼ 0and tainer (v2 ¼ 4.66; df ¼ 1; P ¼ 0.0308). c ¼ 0, equation 5 returns to equation 3, indicating a A significant negative correlation was found between Type II function response model (Holling 1966; Hassell the log10 searching efficiency (s) on both host plants 1978). (Fig. 2). Along with a declining trend in per capita 418 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 108, no. 2

search efficiency (s), a significantly lower s was significant negative quadratic estimation value (P2; observed at a higher density of female T. triozae on tomato: v2 ¼ 14.39; df ¼ 1; P ¼ 0.0001; bell pepper: both tomato (R2 ¼ 0.443; F ¼ 36.91; df ¼ 1, 28; v2 ¼ 19.29; df ¼ 1; P < 0.0001), significant positive lin- 2 2 P < 0.0001) and bell pepper (R ¼ 0.5687; F ¼ 36.91; ear (P1;tomato:v ¼ 8.62; df ¼ 1; P ¼ 0.0033; bell pep- 2 df ¼ 1, 28; P < 0.0001). Therefore, mutual interference per: v ¼ 12.76; df ¼ 1; P < 0.0004), and cubic (P3; significantly affected the searching efficiency of female tomato: v2 ¼ 13.19; df ¼ 1; P ¼ 0.0003; bell pepper: T. triozae at densities of more than one ovipositing v2 ¼ 17.67; df ¼ 1; P < 0.0001) estimation values were female. found (Table 4), and this model with estimated param- Functional Response. Logistic maximum likelihood eters is plotted in Figure 3C and D. Although the num- estimates and AIC of the functional response models of ber of parasitized prey increased in tandem with T. triozae on the two host plants are shown in Tables 3 increasing densities, the mean proportion of parasitized and 4. All models showed a significant regression on prey as a function of prey density increases initially to either host plant (P < 0.0001). The lowest AIC value an inflection point, and then it is followed by a (Table 3) indicated that the best model incorporated decrease in the proportion of B. cockerelli parasitized. four parameters (P0, P1, P2,andP3), and the parameter The maximum proportion of parasitized prey was esti- estimates are shown in Table 4. The functional mated to be a host density of 25, on both host plants. response form for T. triozae was thus determined to be Fitting Holling’s mechanistic model using nonlinear a Type III model on both host plants because of the regression indicated all models were significantly regressed (Table 5). Six models were fitted and com- Downloaded from Table 2. Multiple-choice test results of prey stage preference pared. The model (number 4) with parameters b, c, of T. triozae on tomato and bell pepper plants under laboratory and T on both host plants had the lowest AIC value, h conditions of 26.7 6 2 C, 75 6 5% RH, and a photoperiod of indicating the best fitted model. Therefore, the esti- 14:10 (L:D) h mated parameters in model 4 were used to plot the curves (Fig. 3A for tomato and 3B for bell pepper). Nymphal instar Tomato Bell pepper F1, 8 P The curves show the number of parasitized B. cocker- http://jee.oxfordjournals.org/ Mean 6 SE (%) Mean 6 SE (%) elli nymphs as a function of nymphal density. Both First 0cA 0cA – – curves were sigmoid in shape, indicating a Type III Second 0cA 0cA – – functional response. Third 30.0 6 3.2bA 28.0 6 2.0bA 0.24 0.6408 Fourth 76.0 6 5.1aA 76.0 6 6.0aA 0.01 0.9887 Fifth 78.0 6 5.8aA 78.0 6 3.7aA 0.01 0.9715 F4, 20 221.15 310.42 Discussion P < 0.0001 <0.0001 Parasitoid behavior and biology ranks among the Different uppercase letters indicate a significant difference most complex in the kingdom (Stehr 1982). Our

between host plants, and different lowercase letters indicate a signifi- by guest on October 3, 2016 cant differences among prey stages at P ¼ 0.05. study provided important information on T. triozae

Fig. 1. Effect of mutual interference on parasitism of B. cockerelli at different densities of conspecific female T. triozae on two host plants under laboratory conditions at 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h. Different uppercase letters indicate a significant difference found among the three levels of T. triozae density, and different lowercase letters indicate a significant difference between host plants at each T. triozae density (P ¼ 0.05). April 2015 YANG ET AL.: FUNCTIONAL RESPONSE,PREY STAGE PREFERENCE, AND MUTUAL INTERFERENCE 419 Downloaded from http://jee.oxfordjournals.org/ by guest on October 3, 2016

Fig. 2. Mutual interference of T. triozae females under laboratory conditions at 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h. Per capita searching efficiency of T. triozae is shown in relation to conspecific female density on (A) tomato and (B) bell pepper.

Table 3. Model selection by stepwise elimination of independent variables (PROC LOGISTIC) for T. triozae functional response to B. cockerelli on tomato and bell pepper host plants under laboratory conditions of 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h

Host plant for prey Models incorporating regression coefficientsa df Likelihood ratio (v2) AIC P-value

Tomato P0 and P1 1 182.7 2,292.5 <0.0001 P0, P1, and P2 2 189.0 2,288.2 <0.0001 P0, P1, P2, and P3 3 197.5 2,281.7 <0.0001 Bell pepper P0 and P1 1 149.6 2,326.1 <0.0001 P0, P1, and P2 2 155.6 2,322.1 <0.0001 P0, P1, P2, and P3 3 168.3 2,311.4 <0.0001 a P0 is the intercept (constant); P1, P2, and P3 are the linear, quadratic, and cubic coefficients, respectively. 420 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 108, no. 2

Table 4. Maximum likelihood estimates (PROC LOGISTIC) the best model for T. triozae functional response to B. cockerelli on tomato and bell pepper host plants under laboratory conditions of 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h

Host plant for prey Logistic regression parameter df Estimate 6 SE v2 P-value

Tomato Intercept (P0) 1 0.7269 6 0.2811 6.68 0.0097 P1 1 0.0570 6 0.0194 8.62 0.0033 P2 1 0.00134 6 0.000354 14.39 0.0001 6 6 P3 1 6.77 10 6 1.86 10 13.19 0.0003 Likelihood ratio 50 197.5 <0.0001 Bell pepper Intercept (P0)10.1025 6 0.2616 0.15 0.6953 P1 1 0.0645 6 0.0181 12.76 0.0004 P2 1 0.00148 6 0.000337 19.29 <0.0001 6 6 P3 1 7.60 10 6 1.81 10 17.67 <0.0001 Likelihood ratio 46 80.63 0.0012 Downloaded from http://jee.oxfordjournals.org/ by guest on October 3, 2016

Fig. 3. Functional response of T. triozae to B. cockerelli nymphs on tomato and pepper host plants under laboratory conditions at 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h. A and B are the number of parasitized hosts/ female T. triozae at variable host densities on tomato and bell pepper, respectively; C and D are mean proportion of prey parasitized/female T. triozae at variable prey density on tomato and bell pepper, respectively. biology (Fig. 4A–F) and parasitism behavior (Fig. the optimization of both parasitoid reproduction and 4G–J), which have not been comprehensively reported nutrition acquisition (Strand and Casas 2008). In our previously. Based on our results, significant differences study, third, fourth, and fifth B. cockerelli nymphal in host nymphal stage preference of T. triozae were instars were preferred by T. triozae, perhaps because found, and a significant effect of mutual interference later instars provided sufficient nutrient for parasitoid was determined under variable female parasitoid den- development so that lower mortality rate of parasitoid sities. Our data also showed that the form of functional wouldbesuffered(Price 1974; Mackauer and Sequeira response of T. triozae was a Type III model, which had 1993; Godfray 1994). Similar results were reported pre- been commonly considered to be a hallmark of effi- viously with the Asian citrus psyllid parasitoid, Tamar- cient biological control agents (Ferna´ndez-Arhex and ixia radiata (Waterston), on citrus (Qureshi et al. 2009; Corley 2003; Pervez and Omkar 2005). Paiva and Parra 2012). In the choice test, host feeding Preferences for certain prey stages are a common on smaller nymphs (first and second instars) was also feature of parasitoid behavioral biology as it represents observed, although only larger nymphs (third, fourth, April 2015 YANG ET AL.: FUNCTIONAL RESPONSE,PREY STAGE PREFERENCE, AND MUTUAL INTERFERENCE 421

Table 5. Mechanistic model fitting by stepwise elimination of coefficients for T. triozae functional response to B. cockerelli on tomato and bell pepper host plants under laboratory conditions of 26.7 6 2C, 75 6 5% RH, and a photoperiod of 14:10 (L:D) h

Host plant for prey Model no. Model coefficients Coefficient estimates df AIC P-value

Tomato 1 b, c, d, and Th b ¼ 12193.2; c ¼ 543311; d ¼ 1.0E-8; Th ¼ 0.5703 3 122.6 <0.0001 2 c, d, and Th c ¼ 0.0128; d ¼ 0.0224; Th ¼ 1.0E-8 2 120.6 <0.0001 3 b, d, and Th b ¼ 0.000657 d ¼ 0.0123; Th ¼ 0.9091 2 101.5 <0.0001 4 b, c, and Th b ¼ 0.00331; c ¼ 0.0844; Th ¼ 0.7481 2 99.3 <0.0001 5 b and Th b ¼ 0.00132; Th ¼ 0.9890 1 118.7 <0.0001 6 d and Th d ¼ 0.0224; Th ¼ 0.5703 1 118.6 <0.0001 Bell pepper 1 b, c, d, and Th b ¼ 14906.1; c ¼ 746321; d ¼ 1.0E-8; Th ¼ 0.9150 3 137.8 <0.0001 2 c, d, and Th c ¼ 0.0183; d ¼ 0.02; Th ¼ 1.0E-8 2 135.8 <0.0001 3 b, d, and Th b ¼ 0.001; d ¼ 0.00459 Th ¼ 1.3731 2 119.6 <0.0001 4 b, c, and Th b ¼ 0.00161; c ¼ 0.0157; Th ¼ 1.3066 2 118.1 <0.0001 5 b and Th b ¼ 0.00127; Th ¼ 1.4048 1 120.1 <0.0001 6 d and Th d ¼ 0.02; Th ¼ 0.9150 1 133.8 <0.0001 Downloaded from http://jee.oxfordjournals.org/ by guest on October 3, 2016

Fig. 4. Biology of T. triozae: (A) T. triozae egg (indicated by arrow). (B) Young T. triozae larvae (indicated by arrow). (C) Mature T. triozae larva. (D) T. triozae pupa. (E) Mummified fifth-instar B. cockerelli nymph. (F) T. triozae adult emerging from mummified B. cockerelli nymph. (G) Superparasitism (T. triozae larvae indicated by arrows). (H) host palpating behavior. (I) Adult T. triozae feeding on second-instar B. cockerelli nymph. (J) T. triozae adults mating. and fifth instars) were parasitized. A possible reason is one optimum foraging performance and may also could be that most parasitoids experienced egg deple- suggest compensation between host feeding and para- tion by parasitizing ideally sized hosts, and nutritional sitism (Heimpel and Collier 1996). balance would have to be regained via other ways of Effects of mutual interference on parasitoid search foraging, such as feeding on a host that was otherwise efficiency become more prominent when more than not ideal for parasitism (Heimpel et al. 1996). one female was confined, and our results with T. triozae Therefore, it is important to distinguish actual parasit- females are consistent with those of another parasitoid ism from host feeding to avoid overestimating parasit- reported previously (Henne and Johnson 2010). Our ism rates. Overall, host stage preference by a parasitoid results clearly showed that female T. triozae searching 422 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 108, no. 2 efficiency was significantly reduced at parasitoid den- This is important to biological programs because the sities greater than one. To our knowledge, T. triozae is fitness of parasitoid raised from alternative host system a solitary ectoparasitoid, where one host can support is expected to be not decreased, better enhanced, as only one parasitoid progeny, and this was also observed compared with its original pest–crop system. Our and confirmed in the current study. First, when three results showed that the lower trophic level (i.e., prey simultaneously ovipositing T. triozae females were con- host plants) did not significantly affect the higher tro- fined in the container, it was often observed that one or phic level (i.e., parasitoid) in parasitism in most of the two females would leave the leaf surface and walk on experiments, suggesting that bell pepper is a sound top of the container, attempting to escape. In addition, candidate for T. triozae augmentation programs in superparasitism of B. cockerelli nymphs by T. triozae B. cockerelli control. Furthermore, previous studies was also observed in our study (Fig. 4G), although no have also demonstrated that bell pepper plants showed previous study have reported this. To some extent, a higher tolerance to B. cockerelli feeding than potato superparasitism behavior is alleviating female parasitoid or tomato (Yang and Liu 2009; Yang et al. 2010). To conspecific competition when there is resource deple- conclude, our study suggests that augmentation of tion or limitation; however, it is still indirectly reducing T. triozae by using bell pepper for B. cockerelli biologi- the search efficiency, referred to as indirect mutual cal control might be an option; however, it will be nec- interference (Visser and Driessen 1991). Our results essary to test the feasibility of the proposed biological and observations suggested that both direct and indi- control strategy under field conditions, especially when rect mutual interference existed when two or more other generalist predators contemporarily exist in the Downloaded from female T. triozae were confined. Notably, superparasi- field. Future studies, including parasitoid host feeding tism is probably rare under field conditions where and parasitism under the field conditions, and func- T. triozae females are less aggregated, given that super- tional response studies on potato and other crops are parasitism rates under field conditions are low (Yang needed. and Henne unpublished data). Except for a lower num- ber of parasitized hosts on bell pepper in the one-pair http://jee.oxfordjournals.org/ mutual interference trial, our results also suggest that, Acknowledgments on a lower trophic level, parasitism of B. cockerelli by We would like to thank the staff of the Subtropical Pest T. triozae on the two host plants tested was not signifi- Management Laboratory at Texas A&M AgriLife Research cantly different. Because all T. triozae adults used for and Extension Center in Weslaco, who provided help in this the experiment were collected from tomato-reared study. Thanks are also extended to Specialty Crop Research B. cockerelli, the preimaginal condition could be one Initiative (2009-51181-20178) and the Texas Department of reason affecting parasitoid behavior or biology (Liu and Agriculture for financial support. Liu 2006).

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