University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Entomology Papers from Other Sources Entomology Collections, Miscellaneous 1998 Herbivore Adaptations to a Low-Nutrient Food: Weed Biological Control Specialist Spodoptera pectinicornis (Lepidoptera: Noctuidae) Fed the Floating Aquatic Plant Pistia stratiotes G. S. Wheeler Aquatic Weed Research Unit, USDA-ARS, 3205 College Avenue, Ft. Lauderdale, FL 33314 T. K. Van Aquatic Weed Research Unit, USDA-ARS, 3205 College Avenue, Ft. Lauderdale, FL 33314 T. D. Center Aquatic Weed Research Unit, USDA-ARS, 3205 College Avenue, Ft. Lauderdale, FL 33314 Follow this and additional works at: https://digitalcommons.unl.edu/entomologyother Part of the Entomology Commons Wheeler, G. S.; Van, T. K.; and Center, T. D., "Herbivore Adaptations to a Low-Nutrient Food: Weed Biological Control Specialist Spodoptera pectinicornis (Lepidoptera: Noctuidae) Fed the Floating Aquatic Plant Pistia stratiotes" (1998). Entomology Papers from Other Sources. 88. https://digitalcommons.unl.edu/entomologyother/88 This Article is brought to you for free and open access by the Entomology Collections, Miscellaneous at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Entomology Papers from Other Sources by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. BIOLOGICAL CONTROL Herbivore Adaptations to a Low-Nutrient Food: Weed Biological Control Specialist Spodoptera pectinicornis (Lepidoptera: Noctuidae) Fed the Floating Aquatic Plant Pistia stratiotes G. s. vVHEELER, T. K. VAN, AND T. D. CENTER Aquatic Weed Research Unit, USDA-ARS, 3205 College Avenue, Ft. Lauderdale, FL 33314 Environ. Entomol. 27(3): 993-1000 (1998) ABSTRACT Performance of the specialist herbivore Spodoptera pectinicomis (Hampson) was studied when fed the floating aquatic plant waterlettuce, Pistia stratiotes L. (Araceae). Plants were either collected from 6 populations in southern Florida or from plants grown with low or high fertilizer levels. Consumption of leaves with increasing toughness resulted in increased larval mortality (>80%); most mortality occurred during the first 2 instars. Larvae compensated for low-nitrogen leaves by increasing fresh weight consumption 3-fold. Both developmental time and biomass gain were effected by the source of the plants but these performance parameters were not directly related to either leaf toughness or nitrogen levels. These results are useful in understanding the adaptations and limitations of specialist herbivores to low-nutrient foods. Additionally, they assist in the selection of suitable release sites for this weed biological control agent. Finally, they improve our mass-rearing techniques for augmentative releases of this biological control species. KEY WORDS Pistia stratiotes, Spodoptera pectinicomis, weed biological control, nitrogen, leaf toughness, compensatory feeding AQUATIC WEEDS CONSTITUTE significant threats to navi­ control agent of P. stratiotes in Thailand (Napompeth gable waterways, flood control activities, and natural 1982) and has been cleared for release in the United area biotic diversity, and are potential health threats States (Habeck and Thompson 1994). Despite releases (Holm et al. 1977, Lounibos and Escher 1985, Orr and at numerous sites, field populations of this species may Resh 1992). One of the most widely spread and im­ be only sporadically established and its general status portant floating aquatic weeds in tropical and sub­ at the remaining sites is unknown (Center 1994). tropical regions of the world, including the southeast­ Knowledge of the factors that limit the performance ern United States, is waterlettuce, Pistia stratiotes L. (i.e., growth, development, and fecundity) of S. pec­ (Araceae) (Cook et al. 1974, Holm et al. 1977). Wa­ tinicomis may improve our success in the establish­ terlettuce is a floating perennial that consists of a ment of this species by providing the information rosette of leaf blades that arise from a central meris­ needed to select the most conducive release site (s) for tern; a very short stem axis; and long feathery roots. rapid population growth. An important factor that Leaves are prominently veined below, covered with frequently limits growth and development of herbiv­ velvety hairs, and produced sequentially from the cen­ orous insects is the nutritional quality of plants, espe­ ter of the rosette. The plant reproduces both vegeta­ cially percentage of nitrogen (Mattson 1980) and leaf tively by stoloniferous daughter plants and by seeds toughness (Coley and Barone 1996). The importan('e (Dray and Center 1989). of plant nutritional factors for weed biological control Research to develop biological control of P. stra­ has been shown for a number of other aquatic weeds, tiotes has been conducted in Florida for over a decade including alligatorweed, Altemanthera philoxeroides (Center 1994). These activities have resulted in the (Mart.) Griseb. (Amaranthaceae) (Maddox and release and establishment of a leaf-feeding weevil, Rhyne 1975); salvinia, Salvinia molesta Mitchell Neohydronomous affinis Hustache, from South Amer­ (Salviniaceae) (Taylor 1984, 1988; Room 1990); wa­ ica (Dray et al.1990). The weevil is widely distributed terhyacinth, Eichhomia crassipes (Mart.) (Center throughout the state and at some locations has had a 1994); and hydrilla, Hydrilla verlicillata (L. f.) Royle significant impact on P. stratiotes populations, at best (Hydrocharitaceae) (Wheeler and Center 1996, reducing the weed coverage to 5% of its previous level 1997). As with hydrilla, P. stratiotes may range widely (Dray and Center 1992, Center 1994). However, the at different sites in both the levels of nitrogen and leaf weed continues to be a problem as this level of control toughness. Therefore, some plant populations may be does not occur at all sites nor at all times. Therefore, better suited than others for herbivore establishment additional biological control agents could supplement and population increases. The goal of this study was to the control exerted by the weevil. The noctuid Spo­ determine the range of nitrogen and leaf toughness at doptera pectinicomis (Hampson) is a very effective various sites in Florida and to determine their impact 994 ENVIRONMENTAL ENTOMOLOGY Vol. 27, no. 4 on the survival, growth, and development of the P. required to puncture leaf tissues (Wheeler and Center stratiotes biological control agent S. pectinicomis. Ad­ 1996, 1997). Measurements were recorded for each ditionally, we compare these results with those from leaf position (1-4) and from 4 locations on each leaf: outdoor tank studies where larvae were fed leaves the tip, apical quarter, halfway point where the leaf from plants grown at known fertilizer levels. begins to enlarge, and the base. Toughness measure­ ments of each leaf position and location within the leaf were replicated 40 times per site. Leaf percentage of Materials and Methods dry weight (n = 20) was determined gravimetrically Plant Collections. P. stratiotes was collected from 6 by weighing each leaf fresh and after drying for 48 h potential release sites in southern Florida from No­ at 60°C. vember 1995 to January 1996. The sites, which con­ Larval Survival, Growth, and Development. N eo­ sisted of small lakes or impoundments, included Tor­ nate S. pectinicomis larvae (n = 40) were collected rey Island, Palm Beach County; Corkscrew Swamp from a laboratory colony that had been in culture for Sanctuary, Collier County; Pioneer Park, Palm Beach 6 mo (=6 generations) without infusion of wild ge­ County; Loxahatchee site 1, Palm Beach County; notypes. This colony had been cultured continuously Loxahatchee site 2, Palm Beach County; and Christ­ in outdoor tanks on live plants. Each neonate was mas Park, Orange County. Collected plants were re­ transferred to a leaf and reared through to pupation. turned to our laboratory where the youngest 4 ex­ The leaves fed to the larvae were selected randomly panded leaves (counting from the central-most from among leaf positions 1-4. The larvae were reared expanded leaf outward) were excised and refrigerated individually in plastic petri dishes (15 by 3 cm) lined in moist plastic shoe boxes at lOoC until ready for with moistened filter paper and sealed with Parafilm. feeding tests (= 2 d). All rearing was conducted at 28°C, 50% RH, and a The effect of fertilizer on insect performance was photoperiod ofl4:10 (L:D) h. The final location of the determined using P. stratiotes plants (4-5 leaves per larvae was recorded, namely whether the larvae were plant) grown outdoors in 64-liter pots (0.16-m2 surface located internally or externally, on the leaf apical or area) inside 3 m long by 2 m wide by 0.75 m deep tanks basal half, and on the upper or lower leaf surface. Data at relatively high (high; 5 ppm nitrogen with Peter's also were collected on larval survival, consumption, [W. R. Grace, Fogelsville, PAl 15-5-15 and 2 ppm Fe, growth and developmental time to the pupal stage. changed weekly) and low fertilizer rates (low; 0.25 Leaf consumption was estimated gravimetrically on an ppm nitrogen with Peter's 15-5-15 and 0.1 ppm Fe, electronic balance (Mettler AC-I00, ± 0.1 mg) ac­ changed weekly). Each tank was screened with or­ cording to the following method. Each leaf was cut gandy cloth to prevent infestations by herbivores. This lengthwise and each half was weighed fresh. One half procedure reduced sunlight penetration by 45 ± 1.0% served as the control leaf and was dried (60°C for 48 h) (mean ± SE). Three or 4 plants were grown in each directly to estimate the initial percentage of dry pot for the high and low fertilizer treatments, respec­ weight of the entire leaf and the other half was fed to tively, and each treatment was replicated 4 times. By a larva. Leaves were replaced when >60% of the leaf growing the plants at different initial densities we half had been consumed. Generally, each larva was avoided the morphological changes observed with dif­ refed only once. The uneaten portion of this leaf half ferent crowding and fertilizer conditions (Tucker was dried and weighed. With the estimate of initial dry 1981).