PHYSIOLOGICAL,BIOCHEMISTRY, AND TOXICOLOGY Temperature-Dependent Development, Survival, and Potential Distribution of Ischnodemus variegatus (Hemiptera: Blissidae), 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 Ischnodemus variegatus (Signoret) (Hemiptera: Blissidae) is an adventive herbivore, native to South America that feeds in the invasive grass Hymenachne amplexicaulis (Rudge) Nees (Poaceae). 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 insect 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 Arthropods (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