PHYSIOLOGICAL,BIOCHEMISTRY, AND TOXICOLOGY Temperature-Dependent Development, Survival, and Potential Distribution of variegatus (: ), a Herbivore of West Indian Marsh Grass

1,2 1 3 4 5 RODRIGO DIAZ, WILLIAM A. OVERHOLT, J. P. CUDA, PAUL D. PRATT, AND ALISON FOX

Ann. Entomol. Soc. Am. 101(3): 604Ð612 (2008) ABSTRACT The bug (Signoret) (Hemiptera: Blissidae) is an adventive herbivore, native to South America that feeds in the invasive grass Hymenachne amplexicaulis (Rudge) Nees (). This grass is a problematic weed in Florida and Australia, but it is a highly valued forage in Mexico, Cuba, and Venezuela. We studied the inßuence of nine constant temperatures (8Ð38ЊC) on the developmental time and survival of I. variegatus. Complete egg and nymphal mortality occurred at temperatures Յ20.5ЊC and at 38ЊC. Developmental time decreased linearly with tem- perature until 28Ð30ЊC and then increased at 33ЊC. Mortality of Þrst, second, and third instars was high across all temperatures. Developmental time across all temperatures was greatest for eggs, Þrst and Þfth instars compared with other stages. Linear and Brie`re-1 nonlinear models were used to determine the lower temperature threshold at which the developmental rate (1/D) approached zero. The lower thresholds to complete development (egg to adult) estimated with the linear and nonlinear model were 14.6 and 17.4ЊC, respectively. The total degree-days required to complete development estimated by the linear model was 588. Using temperature data from Florida, a map was generated to project a prediction grid of I. variegatus generations per yr. Based on these predictions, the can complete three to Þve generations per year in areas currently invaded in Florida. Results of this study will be used to understand the potential distribution and population growth of I. variegatus in H. amplexicaulis infested regions.

KEY WORDS Blissidae, Poaceae, developmental rate, degree-days, biological control

Hymenachne amplexicaulis (Rudge) Nees (Poaceae) high production of stolons and perhaps the absence of (West Indian marsh grass) is a robust, stoloniferous, effective natural enemies. Once the grass invades a semiaquatic, perennial grass, native to the Neotropics. wetland, it forms monotypic stands 2Ð3 m in height, The timing and pathway of introduction of this plant with complete canopy cover. At the end of the grow- into Florida are unknown, but its quality as forage ing season, this results in a massive accumulation of suggests that the introduction may not have been biomass. The grass disperses by seeds, which are pro- accidental. The grass also is established in Indonesia duced in large quantities, and broken stolons, both of (Holm et al. 1979) and in Australia (Csurches et al. which can travel great distances during ßooding 1999) where it is considered a weed of national sig- events. Negative impacts of the grass in Australia affect niÞcance. The Florida Exotic Pest Plant Council listed the sugarcane industry, water resources, Þsheries, and the grass as a category I species, which are invasive ecotourism (Csurches et al. 1999). Plant managers in exotics that are altering native plant communities by Florida and Australia Þnd it challenging to control this displacing native species, changing community struc- grass with herbicides due to regrowth from below tures or ecological functions, or hybridizing with na- ground stolons (Csurches et al. 1999). tives (FLEPPC 2005). The aggressive growth of H. H. amplexicaulis is considered a valuable forage amplexicaulis is due in part to rapid adaptation to grass in the Neotropics, particularly in Mexico, Cuba, changes in water levels (Kibbler and Bahnisch 1999), and Venezuela. Important forage characteristics of this grass include high digestibility, high nitrogen con- tent, and adaptation to changes in water levels (Antel 1 Biological Control Research and Containment Laboratory, Uni- et al. 1998, Kibbler and Bahnisch 1999). In the Bra- versity of Florida, Fort Pierce, FL 34945. 2 Corresponding author, e-mail, rrdg@uß.edu. zilian pantanal, H. amplexicaulis occurs within four 3 Department of Entomology and Nematology, University of Flor- habitats: marsh ponds, waterlogged basins, tall grass- ida, Gainesville, FL 32611. lands, and forest edges (Pinder and Ross 1998). Ob- 4 USDAÐARS, Invasive Plant Research Laboratory, Ft. Lauderdale, servations in marshes at Myakka River State Park, FL 33314. Њ Њ 5 Department of Agronomy, University of Florida, Gainesville, FL Sarasota Co., FL (27.2 N, 82.2 W) suggest that when 32611. subject to inundation, H. amplexicaulis is capable of

0013-8746/08/0604Ð0612$04.00/0 ᭧ 2008 Entomological Society of America May 2008 DIAZ ET AL.: DEVELOPMENT OF I. variegatus 605 rapid stem elongation, increase in foliage volume and of the plant and the insect were deposited in the rapid nodal adventitious root production (R.D., per- Florida herbarium (accession no. 208823) and the sonal observation). Kibbler and Bahnisch (1999) dem- Florida State Collection of (accession no. onstrated that rapid elongation of the stem maintains E2002-6139), respectively. the leaves above the water allowing emergent leaves Laboratory Studies. The lengths of eggs, nymphs, to function at full photosynthetic capacity. In Vene- and adults were measured from randomly collected zuela, Tejos (1978) found a positive relationship be- individuals from the insect colony. Pictures of indi- tween H. amplexicaulis growth and depth of ßooding viduals placed in a sealed petri dish were taken and that biomass production ranged from 5,911 to through a microscope using a digital camera Þtted with 18,162 t/ha/yr during the ßood period and from 5,553 Automontage software (Synchroscopy, Frederick, to 7,836 t/ha/yr during the dry season. MD) and measured with National Institutes of Health In 2000, the Neotropical bug Ischnodemus variegatus ImageJ software (http://rsb.info.nih.gov/ij/). Length (Signoret) (Hemiptera: Blissidae) was discovered of eggs was measured along the longest axis. Nymphs feeding and causing severe damage to H. amplexicaulis and adults were measured from the tip of the rostrum at Myakka River State Park. Scientists from the Florida to the most distal point of the abdomen. Behavioral Department of Agriculture and Consumer Services observations were described from insect colonies and (FDACS) identiÞed I. variegatus as a new record for in Þeld settings. Observations were performed only the continental United States (Halbert 2000). The during the day at least once every 2 wk for a 2-yr native distribution of I. variegatus includes Central period. and South America and collection records indicate H. Developmental Time and Survival. Temperature amplexicaulis as the only host (Baranowski 1979, Slater development studies of I. variegatus were conducted 1987). Like other species in the Blissidae family in environmental chambers at 10 constant tempera- (Hemiptera classiÞcation follows Henry 1997), Isch- tures (8 Ϯ 0.5, 13 Ϯ 0.5, 18 Ϯ 0.5, 20.5 Ϯ 0.5, 23 Ϯ 0.5, nodemus feeds on the sap of monocotyledonous plants 25.5 Ϯ 0.5, 28 Ϯ 0.5, 30.5 Ϯ 0.5, 33 Ϯ 0.5, and 38 Ϯ (Slater 1976). Population outbreaks of this insect in 0.5ЊC). Relative humidity and photoperiod were kept central and south Florida occur from August to No- constant at 80% and 14:10 (L:D) h, respectively. En- vember. Feeding effects of I. variegatus diminish car- vironmental variables were conÞrmed with HOBO bon dioxide assimilation, growth rate and biomass of data loggers (Onset Computer, Bourne, MA) placed in H. amplexicaulis (Overholt et al. 2004). Despite its each chamber. Ten adult couples were placed in a potential as a fortuitous biological control agent of H. small cage (0.3 by 0.3 by 0.3 m) and given stems of H. amplexicaulis, there are no studies that address the amplexicaulis for feeding and oviposition. Fresh eggs basic biology of I. variegatus or its host range. The (Ϸ1 d old) were collected from the stems and trans- purpose of this study was to determine temperature- ferred individually to small (5-cm) petri dishes con- dependent developmental times and survival, and taining moist Þlter paper. Fifty eggs were placed at with this information generate a map depicting the each temperature treatment. Egg development was predicted number of generations per yr of I. variegatus monitored daily and hatching dates recorded. across Florida. This study is an initial step toward Eggs collected from the adult colony were moni- understanding the thermal requirements for I. varie- tored daily and newly hatched nymphs were used for gatus establishment and population growth. the nymphal development study. First instars were placed singly in 250-cm3 vials containing a small sec- tion of H. amplexicaulis whorl that was placed upright Materials and Methods in wet sand. The center of the vial lid was removed in Source of I. variegatus and H. amplexicaulis. I. var- a circular shape and replaced by Þne mesh to allow gas iegatus and H. amplexicaulis were collected in Myakka exchange. Nymphs were transferred to fresh plant River State Park, and Fisheating Creek, Glades Co., FL material every2dbyusing a Þne brush. Fifty indi- (26.5Њ N, 81.7Њ W) and maintained at the Biological viduals were exposed to each temperature treatment. Control Research and Containment Laboratory Nymphal molts, conÞrmed by the presence of exuviae, (BCRCL), Fort Pierce, FL. Stolons of H. amplexicaulis and survival were recorded between 8 and 11 a.m. were planted in two liter pots and placed in large trays every other day until the last individual molted to the Þlled with water to maintain permanent ßooding con- adult stage. ditions. Pots received 14g of Osmocote (Scotts, Marys- Developmental Rate and Degree-Day Require- ville, OH) after transplanting and weekly applications ment. Developmental time at different temperatures of Miracle-Grow water soluble fertilizer (Scotts). Pot- was analyzed using the general linear model proce- ted plants were placed in small, mesh screened cages dure (PROC GLM, SAS Institute 1999) for each instar (0.90 by 0.90 by 0.90 m) located within a walk-in separately as well as the total immature stages com- rearing room maintained at 25Ð30ЊC, 50Ð80% RH, and bined. Whenever signiÞcant (P Ͻ 0.05) F values were a photoperiod of 14:10 (L:D) h. Field-collected I. obtained, means were separated using the Student- variegatus were released in these cages and monitored Newman-Keuls (SNK) test (SAS Institute 1999). every other day for nymphal survival and colonization. Linear Model. For the egg, nymphal and total im- The maintenance of I. variegatus genetic variability mature stages (egg and nymphal stages combined), was ensured by addition of Þeld-collected individuals the linear portion (20Ð33ЊC) of the developmental to the colony three times a year. Voucher specimens rate curve [R(T) ϭ a ϩ bT] was modeled using the 606 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 101, no. 3 least squares linear regression (PROC GLM, SAS In- per station by K, 588.24, required by I. variegatus stitute 1999), where T was temperature, and a and b immature stages to complete development. were estimates of the intercept and slope, respec- Generation of GIS Map for Prediction of I. varie- tively. The temperatures 18 and 38ЊC were not in- gatus Generations in Florida. Weather station name, cluded in the regression analysis, because their values latitude, longitude and number of I. variegatus gen- were not part of the linear portion of the curve. The erations were tabulated in a Microsoft Excel spread- base temperature threshold was estimated by the in- sheet, saved as IV dBase Þle and then imported into ϭ ϭ tersection of the regression line at R(T) 0, T0 ArcGis 9.0 (ESRI Inc., Redlands, CA). The imported Ϫa/b. Degree-day requirements for each stage were Þle was converted to shapeÞle using the ADD X-Y calculated using the inverse slope of the Þtted linear DATA function followed by the selection of the State regression line (Campbell et al. 1974). Plane Projection. A shapeÞle of Florida was obtained Nonlinear Model. The nonlinear relationship be- from AWhere Continental database (AWHERE, Inc., tween developmental rate r(T) and temperature T Denver, CO) to delineate the range of predictions. was Þtted to the Brie`re model, which allows the es- The ArcGis Geostatistical Analyst function (ESRI timation of the upper and lower developmental Inc.) was used to generate prediction grids of I. var- thresholds (Brie`re et al. 1999). The Brie`re-1 model is iegatus generations across Florida. Prediction values in ϭ Ϫ Ϫ 1/2 deÞned as R(T) a T(T T0)(TL T) ; where unsampled locations were obtained by surface inter- R is the rate of development and is a positive function polation of sampled locations. The inverse distance of the rearing temperature T, T0 is the base temper- weighted (IDW) deterministic method was used, ature threshold, TL is the lethal (upper) temperature whereby predictions are made from mathematical for- threshold, and a is an empirical constant (Brie`re et al. mulas that generate weighted averages of nearby 1999). The developmental rate of I. variegatus was known values. The IDW method gives closer points modeled using the Marquardt algorithm of PROC more inßuence on the predicted value than points that NLIN (SAS Institute 1999), which determines param- are farther away (hence the name inverse distance eter estimates through partial derivations. Tempera- weighted). This method was used by Pilkington and ture data used in the nonlinear model were from 18 to Hoddle (2006) to predict the number of generations 33ЊC. Initial model parameters were calculated by the of an egg parasitoid in California. The parameters used grid search method (SAS Institute 1999), with T0 and in the IDW analysis were as follows: Њ Њ TL set between 13 and 18 C and 32 and 38 C, respec- 1. The number of stations used for interpolation was tively. set to 15 and with a minimum of 10. Due to the Weather Data from Florida. Daily minimum and large number of weather stations, there were al- maximum temperatures from Florida were obtained ways 15 stations available for interpolation. from 98 weather stations recorded by the Applied 2. The Power Optimization option was selected gen- Climate Information System (Climate Information for erating a Power value p ϭ 1.7021. This means that Management and Operational Decisions [CLIMOD], the weights of each weather station are propor- Southeast Regional Climate Center; http://acis.dnr. tional to the inverse distance raised to the power sc.gov/Climod/). Daily minimum and maximum tem- value p. Therefore, as the distance from the station peratures were averaged for the last 5Ð11 yr depending increases, the weights decrease rapidly. on the availability of data, which provided 365 values 3. The search neighborhood shape was circular be- for each temperature and station. The maximum pe- cause there were no directional inßuences on the riod of weather data were from 1 January 1996 to 31 weighting of number of generations per station; December 2006. thus, equal weight was given to each sample point Calculation of Degree-Days and Number of Gen- regardless of the direction from the prediction lo- erations for Geographic Information System (GIS) cation (ESRI Inc.). Ellipse parameters were set to Analysis. Accumulated degree-days for I. variegatus the default values: angle, 0; major and minor semi- were obtained from DegDay version 1.01, which is an axis, 2.3878. Excel (Microsft, Redmond, WA) application devel- opedbyUniversityofCalifornia-Davis(http://biomet. Results ucdavis.edu/). This application uses the upper and lower temperature threshold for an organism, and Size and Behavior of Stages. Description of I. var- daily average of minimum and maximum temperatures iegatus eggs and nymphs was reported by Baranowski to calculate the accumulated degree-days by using the (1979) and Slater (1987). The size, color, location, and single sine method (Baskerville and Emin 1969). The behavior of the different stages found in our studies upper and lower temperature thresholds for I. varie- are given below. gatus immature stages (egg and nymphal stages) were Eggs. Length is 2.97 Ϯ 0.13 mm (mean Ϯ SD) (n ϭ estimated from the Brie`re-1 nonlinear model as 17.38 25) (Fig. 1A). Eggs are laid in masses (12 eggs per and 35.08ЊC, respectively. Degree-day requirements mass, range 1Ð38) between the leaf sheath and culm for I. variegatus were calculated from the Þtted linear preferentially near the node. Newly deposited eggs regression of the developmental rate function (0Ð5 d) are white and older eggs (6Ð10 d) turn bright [R(T) ϭ a ϩ bT] as K ϭ 1/b (Campbell et al. 1974). red. An egg parasitoid, Eumicrosoma sp. (Hymenop- The prediction of the number of generations per year tera: Scelionidae) was found in Myakka River State was calculated by dividing the cumulative degree-days Park and later identiÞed as a possible introduced spe- May 2008 DIAZ ET AL.: DEVELOPMENT OF I. variegatus 607

Fig. 1. Life stages of I. variegatus and length in mm (mean Ϯ SD). (A) Egg mass on H. amplexicaulis culm, 2.97 Ϯ 0.13 (n ϭ 25). (B) First instar, 1.45 Ϯ 0.28 (n ϭ 23). (C) Second instar, 2.70 Ϯ 0.39 (n ϭ 47). (D) Third instar, 3.06 Ϯ 0.31 (n ϭ 42). (E) Fourth instar, 3.95 Ϯ 0.32 (n ϭ 53). (F) Fifth instar, 5.45 Ϯ 0.43 (n ϭ 46). (G) Female, 7.23 Ϯ 0.56 (n ϭ 28); male, 6.05 Ϯ 0.22 (n ϭ 49). (H) Female sclerites at ventral tip of abdomen. (I) Male sclerites at ventral tip of abdomen. (J) Scent glands in thorax of adult. cies for North America (T. Nuhn, personal commu- and adults can be observed exploring as individuals. If nication. This parasitoid attacks young and old eggs nymphs or adults are disturbed, they secrete a strong (R.D., unpublished data), and its presence can be odor from the scent glands located in the thorax (Fig. detected by the black coloration of the eggs. Because 1J) and abdomen. After molting, the cuticles of eggs are immobile and take longer to develop than nymphs and adults are bright red and delicate, but other stages, it seems to be the most vulnerable stage after a couple of hours they darken and harden. for parasitization or predation. The impact of Eumi- Adults. Females are larger than males (Fig. 1) and crosoma sp. on I. variegatus population is unknown but both genders have a distinctive “M” pattern at the base previous studies have demonstrated that egg parasit- of the hemelytra. The sclerites at the ventral tip of the ism on hemipterans play an important role on regu- abdomen of females are triangular in shape whereas in lation of populations (Buschman and Whitcomb 1980, males the last sclerites are rounded (Fig. 1H and I). Irvin and Hoddle 2007). Adults become highly active during the hottest part of Nymphs. The length of each immature stage is the day and mating individuals can be found in ag- shown in Fig. 1. Upon hatching Þrst instars are bright gregations. Despite having fully developed wings, red (Fig. 1B), and they remain aggregated near the adult ßying was restricted to short hops of a few meters eggs and then migrate to tightly oppressed spaces or less. Gravid females mostly walked, possibly due the between leaves and stems. Feeding and resting occurs larger size of their abdomens. Oviposition behavior in tight spaces between the leaf sheath and culm, and can be described as follows: females locate the fold of in the inner whorl. Fourth and Þfth instars are darker the leaf sheath by walking around the stem while than early instars (Fig. 1E and F). Laboratory and Þeld performing antennation of the surface. Once a fold is observations showed the Þrst to fourth instars are located, the proboscis is extended and brießy inserted more often found in aggregations, whereas Þfth instars at the site for probing; if the site is accepted then the 608 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 101, no. 3

Table 2. Linear regression parameter estimates describing the relationship between temperature and developmental rates (1/D) of I. variegatus stages

Threshold Degree- Stage Intercept Slope R2 n (ЊC) daysa Egg Ϫ0.144 0.0087 0.9507 4 16.55 114.94 Nymph Ϫ0.028 0.002 0.9498 5 14.00 500.00 Egg to adult Ϫ0.025 0.0017 0.9478 5 14.70 588.24

a Total degree-day to complete development.

nymphal stages combined) (Table 1). Mean develop- mental time from egg to adult was longest, 122 d, at 20.5ЊC and shortest, 40 d, at 30.5ЊC. Egg, Þrst, and Þfth instars had the longest developmental times at each temperature indicating critical stages for I. variegatus survival. Developmental time of each stage signiÞ- Fig. 2. Proportion survival of I. variegatus stages at con- Њ Њ cantly decreased between 20.5 and 30.5 C (Table 1). stant temperatures ( C). At 33ЊC, there was a slight increase in developmental time, which could indicate the initiation of stressful female extends and inserts the ovipositor at the site conditions. and starts laying eggs. Both linear and nonlinear models were used to Immature Survival and Developmental Time. The determine the relationship between developmental survival of I. variegatus nymphs varied with temper- rate (1/D) and temperature (T). Developmental rates ature (Fig. 2). Nymphs could not complete develop- of each stage and total immature stages (egg and ment at extreme low and high temperatures (8, 13, and nymphs) were estimated between 20.5 and 30.5ЊC 38ЊC) and died before molting to the second instar where the relationship with temperature was approx- (Fig. 2). At 8, 13, and 18ЊC, Þrst instars typically sur- imately linear. Table 2 shows the lower threshold vived several weeks before dying, whereas at 38ЊC temperature and total degree-days required to com- they usually died a few days after eclosion (Table 1). plete development of each immature stage. The linear At 18ЊC, a few nymphs molted to the second instar, but model estimated that the lower temperature threshold none survived to the third instar. First instar survival for all stages ranged from 14 to 16.55ЊC and total increased between 20.5 and 33ЊC and it was the high- degree-days required for immature development was est, 84%, at 25.5ЊC (Fig. 2). Nymphal survival de- 588. creased up to the third instar, after which survival The parameter estimates for the Brie`re-1 nonlinear stabilized. The percentage of nymphs that molted to model are shown in Table 3. The lower temperature the adult stage was highest, 42%, at 30.5ЊC and lowest, threshold predicted for the immature stages ranged 16%, at 20.5ЊC. from 16.8 to 17.9ЊC (Table 3) which may be slightly Temperature affected the developmental time for low, because laboratory studies showed that nymphs eggs, nymphs, and total immature stages (egg and did not complete development at 18ЊC. The upper

Table 1. Mean developmental time in days (mean ؎ SE) of immature I. variegatus stages at 10 constant temperatures

Temp (ЊC) Stagea 8 13 18 20.5 23 25.5 28 30.5 33 38 Egg 35.7 Ϯ 0.21a 15.51 Ϯ 0.32b 12.94 Ϯ 0.10c 10.41 Ϯ 0.20d 11.43 Ϯ 0.07e 14.15 Ϯ 0.23f 80 80 80 80 80 80 80 80 80 80 N1 2Ð19 3Ð55 38.33 Ϯ 5.75a 21.79 Ϯ 1.80b 13.40 Ϯ 1.57c 11.81 Ϯ 0.56c 7.52 Ϯ 0.48d 5.83 Ϯ 0.46d 8.32 Ϯ 0.37e 5Ð7 50 50 6 24 20 42 25 30 22 50 N2 29.67 Ϯ 8.17a 19.13 Ϯ 2.57b 11.31 Ϯ 1.25c 10.76 Ϯ 1.24c 5.80 Ϯ 0.54c 4.85 Ϯ 0.40c 5.78 Ϯ 0.48c 3161325202618 N3 12.50 Ϯ 2.00a 7.83 Ϯ 0.80b 7.32 Ϯ 0.95b 5.25 Ϯ 0.46b 4.96 Ϯ 0.43b 3.56 Ϯ 0.30c 10 12 19 20 23 16 N4 10.89 Ϯ 0.73a 7.08 Ϯ 1.11b 5.53 Ϯ 0.48b 5.15 Ϯ 0.55b 4.87 Ϯ 0.37b 5.00 Ϯ 0.35b 91217202314 N5 21.75 Ϯ 1.33a 10.75 Ϯ 1.11b 8.59 Ϯ 0.53c 7.47 Ϯ 0.38c 8.29 Ϯ 0.40c 7.82 Ϯ 0.50c 81217192111 Only nymphal 71.38 Ϯ 6.20a 47.83 Ϯ 2.40b 46.12 Ϯ 2.97b 31.58 Ϯ 1.07c 28.81 Ϯ 1.01c 30.09 Ϯ 0.76c stage Egg to adult 121.76 65.88 56.95 41.6 40.23 44.63

Means within a row followed by different letters are signiÞcantly different (P Ͻ 0.05; SNK). Analysis of variance of eggs (F ϭ 2204.04; df ϭ 4, 395; P ϭ 0.0001), N1 (F ϭ 46.88; df ϭ 6, 162; P Ͻ 0.0001), N2 (F ϭ 21.37; df ϭ 6, 114; P Ͻ 0.0001), N3 (F ϭ 12.17; df ϭ 5, 94; P Ͻ 0.0001), N4 (F ϭ 10.91; df ϭ 5, 89; P Ͻ 0.0001), N5 (F ϭ 46.79; df ϭ 5, 82; P Ͻ 0.0001), and nymphal stage (F ϭ 41.95; df ϭ 5, 82; P Ͻ 0.0001). N1, nymphal Þrst instar; N2, second instar; N3, third instar; N4, fourth instar; and N5, Þfth instar. May 2008 DIAZ ET AL.: DEVELOPMENT OF I. variegatus 609

a Table 3. Parameter estimates (a, T0,TL) for the Brie`re-1 nonlinear model describing the relationship between temperature and developmental rate (1/D) of I. variegatus stages

Parameters estimates (ЊC)b Stage 95% 95% aT T R2 0 conÞdence L conÞdence Egg 0.00016 17.9000 16.74Ð19.06 32.5171 31.12Ð33.87 0.9900 Nymph 0.00004 16.7993 13.71Ð19.89 36.1051 32.41Ð39.80 0.9527 Egg to 0.00003 17.3837 15.84Ð18.93 35.0794 33.70Ð36.46 0.9820 adult

a a, empirical constant; T0, lower temperature threshold; TL, upper temperature threshold. b Degrees centigrade except for a. temperature threshold for immatures was predicted to Њ be between 32.5 and 36.1 C (Fig. 3). The rate of Fig. 4. Geographical information system map showing development increased with temperature until the the predicted number of generations of I. variegatus in curve reached an optimum and then decreased rapidly Florida. as temperatures reached the upper temperature threshold (Fig. 3). GIS Mapping of I. variegatus Generations in Flor- Discussion ida. A grid map indicating the predicted number of I. Developmental time and survival of eggs as well as variegatus generations was generated for Florida (Fig. immature stages were affected by temperature. No 4). Overall, the predicted grids followed a thermal survivorship was observed at extreme low and high gradient across the state. Predicted number of gener- temperatures (Table 1). Nymphs died within a few ations ranged from 2.36 to 4.84 in Florida counties. days at 38ЊC and after weeks at lower extreme tem- Florida counties located below Lake Okeechobee had peratures, suggesting that I. variegatus has a broader the highest number of generations per year ranging lower temperature threshold compared with the up- from 3.52 to 4.84. Florida counties located between per threshold. A wider range of lower lethal temper- Orlando and Lake Okeechobee had fewer generations ature threshold is common for (Heinrich 1981, per year (3.21Ð3.52). Counties where the average max- Bayoh and Lindsay 2004). The overall high mortality imum temperature in January was below 17ЊC were observed in the Þrst three instars may have been due excluded, because laboratory studies and nonlinear to the rearing of I. variegatus as individuals in our models predicted high I. variegatus mortality at con- experiments, as opposed to typical aggregations ob- stant low temperatures. served in the Þeld. Harrington (1972) observed that Ischnodemus species were strongly gregarious and

Fig. 3. Developmental rates (1/D) of I. variegatus at different temperatures (ЊC). Linear regression of eggs to adult stages and observed and predicted values by Brie`re-1 nonlinear model. 610 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 101, no. 3 nymphs reared in isolation died sooner than nymphs nymphal lower thresholds ranged from 16.6 to 19.1ЊC reared in groups. The beneÞts of aggregations on sur- and from 13.7 to 19.9ЊC, respectively. This indicates a vival in early instars is unclear but may be related to greater susceptibility of eggs to lower temperatures an increase in humidity as shown for German than nymphs. Although nymphs can move and locate cockroach, Blatella germanica (L.) (Dambach and microclimates suitable for development (e.g., plant Goehlen 1999) and the southern green stink bug, Nez- structures, conspeciÞc aggregations), eggs are immo- ara viridula (L.) (Lockwood and Story 1986). Another bile and successful development depends on local explanation for the high mortality at extreme temper- conditions. This greater resistance to lower tempera- atures could be constant conditions at which the in- tures of nymphs compared with eggs also has been sects were exposed. Extreme temperatures such as 8, observed in other heteropteran insects (He et al. 2003, 13, and 38ЊC are typically present for only a few hours Bommireddy et al. 2004). a day in the subtropics. During certain hours in winter The degree-days (588) required to complete devel- and summer, temperatures in I. variegatus infested opment from egg to adult could be underestimated, regions in Florida could reach 0 and 40ЊC, respectively because the lower threshold is probably higher than (CLIMOD 2007). Despite these conditions, Þeld sam- 14.7ЊC (Table 2). The lower threshold for develop- pling conÞrms that I. variegatus is present throughout ment of I. variegatus, 18Ð20.5ЊC, explains its mostly the year (R.D., unpublished data) demonstrating that tropical and partially subtropical distribution. The this insect can survive extremes under existing envi- preoviposition period of I. variegatus is Ϸ7dat28ЊC ronmental variability. Eggs, Þrst, and Þfth instars took (R.D., unpublished data), and it was not included in longer to develop than other stages, indicating their the calculations of degree-days. Therefore, the importance for I. variegatus development. The length present model probably overestimates the number of of time spent during the Þfth instar could be explained I. variegatus generations. A future model could be by the larger amount of food that insects require improved by including the preoviposition period at during the last immature stage before reproduction different temperatures to accurately predict the de- (Scriber and Slansky 1981, He at al. 2003, Bommireddy gree-day requirement to complete one generation. et al. 2004). Longer developmental time in stages be- Outside of Florida, I. variegatus has been reported fore reproduction also was found in Ischnodemus fali- from as far north as Honduras in Central America, as cus (Say) and Ischnodemus slossoni Van Duzee (Har- far east as the Dominican Republic and Trinidad in the rington 1972), which have temperate distributions. West Indies and as far south as northern Argentina and Developmental time from egg to adult was 3 times less Uruguay (Slater and Wilcox 1969, Baranowski 1979, at 30.5ЊC (40 d) compared with 20.5ЊC (122 d), dem- Slater 1987, Baranowski and Slater 2005). onstrating clearly the inßuence of temperature on The current distribution of H. amplexicaulis and I. development. The range of temperatures where de- variegatus in the continental United States is limited to velopment was fastest occurred between 28 and 33ЊC central and south Florida (Wunderlin and Hansen (Table 1; Fig. 3), which is in agreement with immature 2004, University of Florida Herbarium 2007). Further survival (Fig. 1). These ideal conditions for I. varie- studies on cold tolerance of H. amplexicaulis and I. gatus development are typical in central Florida from variegatus would provide a better understanding of April to October. Developmental rates of I. variegatus the potential distribution on United States. Prediction increased almost linearly with temperature until of the potential range and population growth of her- reaching an optimum at 28Ð30ЊC and then decreasing bivores could decrease some of the uncertainty about rapidly (Fig. 3). This pattern has been observed in potential ranges of introduced biological control other hemipteran (Scott and Yeoh 1999), and non- agents. This study estimated the number of I. varie- hemipteran insects (Ponsonby and Copland 1996, gatus generations based on long-term data of 98 Mazzei et al. 1999, Herrera et al. 2005). Our results of weather stations across Florida and the degree-days developmental rates were obtained at constant tem- required to complete egg-to-adult development. The peratures; however, there is a possibility that our val- GIS map shows spatially the areas across Florida ues are underestimated, because some insects develop where I. variegatus could establish and the potential faster at variable temperatures (Worner 1992). number of generations. Both linear and nonlinear models overestimated the The use of degree-days for mapping insect voltinism lower temperature threshold, because laboratory re- has been used recently for insect conservation sults indicated that eggs and nymphs did not complete (nymphalids butterßies, Bryant et al. 2002), pest man- development below 20.5ЊC. The partial nymphal de- agement (western corn rootworm, Hemerik et al. velopment at 18ЊC could be an indication that I. var- 2004), and biological control (egg parasitoid, Pilking- iegatus can develop at this temperature for short pe- ton and Hoddle 2006). The current northern and riods making the prediction of an absolute lower southern invasion fronts of H. amplexicaulis are the St. threshold not possible (Herrera et al. 2005). However, Johns River (28.08Њ N, 80.75Њ W, Brevard Co.) and Big because there is a narrow temperature range between Cypress National Park (25.92Њ N, 81.3Њ W, Collier Co.), 18 (no development) to 20.5ЊC (complete develop- respectively. The lower winter temperatures in north ment), we can safely predict that the lower threshold Florida could be a climatic barrier for the invasion of of I. variegatus occurs within this range. The predic- H. amplexicaulis and I. variegatus, which are mostly tions of both models for the lower threshold for eggs restricted to the tropics. Degree-day accumulation in and nymphs were different (Tables 2 and 3). Egg and central and especially in south Florida, provides ideal May 2008 DIAZ ET AL.: DEVELOPMENT OF I. variegatus 611 conditions to sustain nearly Þve I. variegatus genera- Bayoh, M. N., and S. W. Lindsay. 2004. Temperature-related tions per year (Fig. 4), which could facilitate its es- duration of aquatic stages of the Afrotropical malaria tablishment in case H. amplexicaulis invades the Ev- vector mosquito Anopheles gambiae in the laboratory. erglades National Park. If I. variegatus arrives in Med. Vet. Entomol. 18: 174Ð179. Australia, the tropical climate of the northern regions Bommireddy, P. L., M. N. Parajulee, and D. O. Porter. 2004. would likely provide ideal conditions for its develop- Inßuence of constant temperatures on life history of im- ment and population growth. Other countries where mature Lygus elisus (Hemiptera: ). Environ. En- tomol. 33: 1549Ð1553. climatic conditions for I. variegatus may be ideal in- Brie`re, J. F., P. Pracos, A. Y. Le Roux, and J. S. Pierre. 1999. clude Mexico, Puerto Rico, Venezuela and Cuba, A novel model of temperature-development for arthro- where H. amplexicaulis is highly valued as forage. Eval- pods. Environ. Entomol. 28: 22Ð29. uation of herbivores for weed biological control pro- Bryant, R. S., C. D. Thomas, and J. S. Bale. 2002. The inßu- grams includes studies on climate matching between ence of thermal ecology on the distribution of three native and adventive range, host speciÞcity of the nymphalid butterßies. J. Appl. Ecol. 39: 43Ð55. agent, and effectiveness in reducing weed density. Buschman, L. L., and W. H. Whitcomb. 1980. Parasites of TemperatureÐdevelopment studies of weed biological Nezara viridula (Hemiptera: ) and other control agents provide baseline knowledge that facil- Hemiptera in Florida. Fla. Entomol. 63: 154Ð162. itates agent rearing, colonization, and prediction of Campbell, A., B. D. Frazier, N. Gilbert, A. P. Gutierrez, and population growth. Temperature experiments re- M. Mackauer. 1974. Temperature requirements of some vealed that optimal conditions for I. variegatus ranged and their parasites. J. Appl. Ecol. 11: 431Ð438. [CLIMOD] Climate Information for Management and Oper- from 28 to 30ЊC, which explains the occurrence of ational Decisions. 2007. Southeast Regional Climate Cen- outbreaks late in summer in Florida (R.D., unpub- ter (http://acis.dnr.sc.gov/Climod/). lished data) and its subtropical to tropical distribution. Csurches, S. M., A. P. Mackey, and L. Fitzsimons. 1999. Ongoing studies on population dynamics, host range Hymenachne in Queensland. Pest Status Review Series. testing and impacts to H. amplexicaulis will elucidate Queensland Government-Natural Resources and Mines. the importance of I. variegatus as a fortuitous biolog- (http://www.nrm.qld.gov.au/). ical control agent in Florida. Dambach, M., and B. Goehlen. 1999. Aggregation density and longevity correlate with humidity in Þrst instar nymphs of the cockroach (Blattella germanica L., Dic- Acknowledgments tyoptera). J. Insect Physiol. 45: 423Ð429. [FLEPPC] Florida Exotic Pest Plant Council. 2005. Florida We extend our gratitude to the following people who Exotic Pest Plant CouncilÕs 2005 list of invasive species. helped facilitate this project. Diana Cordeau, Jackie Markle, Brittany Evans, Yordana Valenzuela, Brianne Schobert, (http://www.ßeppc.org/list/05List.htm). Freddy Soza, Douglas Gonzalez, Veronica Manrique, Fabian Halbert, S. E. 2000. Entomology section. Trilogy 39(5) (http:// Diaz, and Larry Markle provided great support during the www.doacs.state.ß.us/pi/enpp/00-sep-oct.html#ent). data collection. Julieta Brambila (USDAÐAPHIS) conÞrmed Harrington, J. E. 1972. Notes on the biology of Ischnodemus the identiÞcation of I. variegatus. Terry Nuhn (Hymenoptera species of America North of Mexico (Hemiptera: Lyg- specialist; Systematic Entomology Laboratory, USDAÐARS) aeidae: Blissinae). Occ. Pap. Univ. Conn., Biol. Sci. Ser. 2. identiÞed the egg parasitoid. Paul Benshoff and Diana Don- 6: 47Ð56. aghy from Myakka River State Park provided support during He, X., Q. Wang, and A. Carpenter. 2003. Thermal require- the development of this project. Mariana Soza sent voucher ments for the developmental and reproduction of Nysius specimens of I. variegatus collected in northern Argentina. hottuni White (: ). J. Econ. Ento- Two anonymous reviewers provided helpful suggestions to mol. 96: 1119Ð1125. improve an earlier version of this work. The Southeast Re- Heinrich, B. 1981. Ecological and evolutionary perspec- gional Climate Center (http://www.sercc.com) provided tives, pp. 236Ð392. In B. Heinrich [ed.], Insect thermo- free access to weather from Florida. 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