The native lace bug, Leptoypha mutica Say (Hemiptera: Tingidae), has demonstrated potential as an insect biological control agent of invasive Chinese privet (Ligustrum sinense Lour). To better evaluate its potential to establish on a new host, developmental biology on Chinese privet was studied at temperatures of 20°C, 24°C, 28°C, and 32°C. The complete duration of development of L. mutica from egg deposition through five instars on this previously unreported host ranged from 24.4 to 57.1 d. Estimated threshold temperatures and calculated thermal unit requirements for egg development were 11.0°C and 211.9 degree-days (DD); for nymphal development, 9.9°C and 326.8 DD; and for complete development, 10.5°C and 527.4 DD. Results of this study indicate that L. mutica will oviposit and can develop successfully on Chinese privet over a range of temperatures suggesting the potential for multiple generations to occur in a single season.
Chinese privet (Oleaceae), Ligustrum sinense Lour, was introduced into the United States in 1852 as an ornamental shrub (Dirr 1983) and escaped cultivation by 1932 (Small 1933). Because of its adaptations to a range of environments and habitats, Chinese privet has become a widespread exotic plant throughout the southeastern United States. Even though the plant species has tropical origins, it can endure a range of light, temperature, and soil conditions (Brown and Pezeshki 2000). Surveys show that of the approximately 7.1 million ha where the privet occurs, it densely covers approximately 1 million ha (Miller et al. 2008, Rudis et al. 2006). In these areas, privet depletes native plant diversity by competing for water and nutrients in the soil and by competing for light by producing a shade cover over forest understory (Merriam and Feil 2002, Morris et al. 2002, U.S. Fish and Wildlife Service 1992). Other problems are decreased invertebrate communities and toxic effects on humans and animals (Biesmeijer et al. 2006, Crisp et al. 1998, Kerr et al. 1999).
Once the privet is established, it is difficult to remove and large-scale removal becomes labor intensive and requires the use of herbicides (Hanula et al. 2009). To help alleviate its spread, a potential solution is using lace bugs (Hemiptera: Tingidae), as a biological control agent. A U.S.–China cooperative project on biological control of Chinese privet was initiated in 2005. In China, a survey of insects feeding on Chinese privet found Leptoypha hospita Drake et Poor (Hemiptera: Tingidae) to be a potential biocontrol agent because its common feeding on the privet sometimes resulted in plant dieback (Zhang et al. 2008, 2011). The potential of L. hospita as a control agent initiated further examination of the native, congeneric lace bug species, Leptoypha mutica Say, currently utilizing swamp privet (Oleaceae), Foresteria acuminata Michx (Mead 1975). Other hosts of L. mutica include fringe trees, Chionanthus virginicus L., and green ash, Fraxinus pennsylvanica Marsh, both in the family Oleaceae (Drake 1918).
In some cases of classical weed control, just one or a combination of host specific biological control agents are released (McFadyen 2003), but only a small proportion of multiple releases may prove to be effective (Denoth et al. 2002, McEvoy and Coombs 2000,Myers 1985). In efforts to prevent futile introductions, the candidate of interest should be extensively evaluated. Thus, the objective of this study was to establish the relationship between temperature and development of L. mutica on Chinese privet. Understanding this relationship and developing a degree-day (DD) model can be used as prediction of occurrence and sustainability of the lace bug. The interrelationship between insects and invasive plants may also lead to clues of why L. mutica has not already taken advantage of Chinese privet as a host.
Materials and Methods
Leptoypha mutica adults used for this study were collected from fringe trees (C. virginicus) in Spalding Co., GA (N 33°11.6638′, W 84°8.7993′), and green ash (F.pennsylvanica) and swamp privet (F. acuminata) from the Montezuma Bluff Wildlife Management Area (Macon, Co., GA, N 32°20.274′, W 84°1.2245′). Lace bugs were reared in the laboratory on fringe tree leaves in plastic petri dishes at a 14:10 (light:dark) photoperiod at approx. 23°C on the University of Georgia Griffin Campus.
Cuttings of Chinese privet were obtained from potted greenhouse plants grown from field-collected seedlings. Cuttings were placed in cages constructed from 32-ml plastic cups (Jetware PC-100, Bio Serv, Frenchtown, NJ). Cages consisted of a water-filled cup with a lid with a small hole. A piece of Tygon tubing was placed in that hole, and the plant cutting was inserted into the tube and secured with Parafilm wrap (Fisher Scientific, Hampton, NH). Several male and female lace bugs (approximately 10 total) were added to each plant cutting, and another cup with a screen bottom was placed on top and secured with Parafilm. Cages were placed in Percival environmental growth chambers (Model: I-36VL, Percival Scientific, Inc., Perry, IA) at four separate temperatures (± 0.5°C): 20°C, 24°C, 28°C, and 32°C. Females were allowed to oviposit for 24 h at each temperature, then all lace bugs on the cuttings were moved to new cages with fresh cuttings of Chinese privet, and the process was repeated.
To determine the development time of each life stage, all cages were monitored twice a day (10 a.m. and 4 p.m.) under a dissecting microscope for the emergence of nymphs or molted individuals. First instars were transferred individually onto separate caged cuttings of Chinese privet and monitored for development. Nymphal molts were evidenced by the presence of exuviae. The nymphs were reared and examined until adults emerged, and adult sex ratio was then determined and any gender influence on duration of development was examined.
The mean duration was calculated for each life stage for total nymphal and complete development. The effect of gender on development was compared at each temperature using the t test procedure (PROC TTest; SAS Institute 2010).
Least squares linear regression was used to establish the relationship between developmental rate (days) and temperature (°C) over the linear portion of development curves. The lower temperature threshold (T0) was estimated by using the regression procedure, PROC REG (SAS institute 2010), on the developmental means to calculate the slope and intercept of the regression equation Y = a + bX. The equation was solved using the slope and intercept at no development (Y= 0), which produced the T0 value. The mean thermal unit requirement (Kt)for each life stage was calculated using the means at each temperature of all stages of development with the following equation, Kt = (T − T0) × Dt , where T was 20°C, 24°C, 28°C, or 32°C; T0 was the lower temperature threshold of a stage of interest and, Dt was the mean development time (days) for a stage at temperature T. Results for each temperature at each life stage were then averaged to produce the final mean thermal unit requirements.
Eggs of L. mutica were tubular, slightly curved, and yellow. The cephalic pole was characterized by an oval-shaped operculum, which was easily visible when the eggs were inserted into the plant material. Eggs were found inserted on both the dorsal and ventral sides of leaves and commonly along the midrib. Eggs were occasionally found in the petiole and stems as well. This occurred mostly in new growth where the plant tissue was still soft. Eggs developed successfully at all four temperatures. Eggs developed between 10.0 to 24.6 d with decreasing temperatures (Table 1). Less than half of the total development occurred in the egg stage.
There are five nymphal instars of L. mutica. The nymphs were oblong and flat. Each instar could be identified by the presence of exuviae on the plant material after molting. Newly emerged nymphal instars were reddish in color with red eyes, but turned pale yellow/brown in color with black eyes after approximately 1 d. The nymphs had short setae on their dorsal surface. They fed on the ventral and dorsal epidermis of the Chinese privet leaves almost immediately after hatching. Their feeding on the mesophyll tissue produced chlorotic, bleached-appearing leaves. Total nymphal development ranged from 14.7 to 33.9 d as temperatures decreased (Table 1).
The adult lace bugs were elongate and light brown/cream with red eyes upon emergence. Their color changed to a deeper brown color and black eyes within approximately 1 d. Males could be distinguished by their narrow abdomens, whereas females had a broader abdomen with an ovipositor present. The duration of complete development to adulthood ranged from 24.4 to 57.1 d with decreasing temperature (Table 1). The lace bugs were able to develop at all temperatures tested, but had the least success at 20°C, with only 7.7% survival (n = 26). The optimal temperature for complete development was 28°C, with 52.0% survival (n = 102) to the adult stage. Female and male developmental times did not differ significantly at any temperature for any life stage (α > 0.05). The male:female sex ratio for 20°C was 1:1; at 24°C it was 1.3:1; at 28°C, 0.9:1; and at 32°C, 1.1:1.
The T0 value was 11.0°C for egg development, 6.2°C for first instar development, 14.9°C for second instar development,13.8°C for third instar development, 13.0°C for fourth instar development, 9.8°C for fifth instar development, 9.9°C for total nymphal development, and 10.5°C complete development. Mean thermal unit requirement (Kt) for egg development was 211.9 degree-days (DD), 90.0 DD for first instars, 39.8 DD for second instars, 40.8 DD for third instars, 49.6 DD for fourth instars, 86.9 DD for fifth instars, 326.8 DD for total nymphal development, and 527.4 DD for complete development. Backward extrapolation of the linear portion of the temperature/development curves predicted the threshold values (Table 2; Fig. 1).
Studying the basic biology of L. mutica is useful not only to better understand the behavior of the lace bug, but also to analyze its full potential as a biological control agent. The lace bugs were able to develop at all temperatures tested, but were least successful at surviving at 20°C; however, other factors could have contributed to mortality (i.e., handling). The regression equations for the reciprocal of developmental times on temperature for each life stage and values for T0 and K were used to predict at what temperature each life stage would cease to develop, and it was estimated that complete development would cease below 10.5°C.
This relatively low threshold, and having a broad range over which development occurs, suggests the potential utility of this insect under a variety of environmental conditions. Currently in the United States, Chinese privet occurs from Texas to Florida and northward to the New England states (USDA–Natural Resources Conservation Service 2002). Leptoypha mutica is found in a similar range, having been reported from Maine, west to North Dakota, and south to Florida and Texas (Mead 1975).
Development of the Asian lace bug, L. hospita, at 24–26°C indicates total development occurred in 24.77 d on Chinese privet (Zhang et al. 2011) in comparison to the L. mutica at 34.2 d when development times at 24°C and 28°C are averaged. Barber and Weiss (1922) indicated that L. mutica probably has two generations per year and overwinters as an adult. If a host shift occurred, their relatively short generation time and multiple broods would aid in their ability to potentially colonize Chinese privet.