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© 2010 Plant Management Network.
Accepted for publication 31 December 2009. Published 12 March 2010.

Seasonal and Diurnal Activity of Larra bicolor (Hymenoptera: Crabronidae) and Potential Ornamental Plants as Nectar Sources

Cheri M. Abraham, Department of Entomology, University of Georgia, 417 Biological Sciences Bldg., Athens, GA 30602; David W. Held, Department of Entomology and Plant Pathology, Auburn University, 301 Funchess Hall, Auburn, AL 36849; and Corey Wheeler, Coastal Research and Extension Center, Mississippi State University, 1815 Popps Ferry Road, Biloxi, MS 39532

Corresponding author: David W. Held. david.held@auburn.edu

Abraham, C. M., Held, D. W., and Wheeler, C. 2010. Seasonal and diurnal activity of Larra bicolor (Hymenoptera: Crabronidae) and potential ornamental plants as nectar sources. Online. Applied Turfgrass Science doi:10.1094/ATS-2010-0312-01-RS.

Abstract

Larra bicolor was introduced to Florida for biological control of mole crickets (Scapteriscus spp.) and has spread to adjacent states. The objectives of this study were to determine the seasonal and diurnal activity of L. bicolor in its expanded range and to compare ornamental plants as nectar sources. Experiments with colored, pan-type traps and sugar solutions to monitor for L. bicolor were unsuccessful and direct observations of wasps feeding on flowers or hunting in turfgrass were used to monitor activity. Seasonal activity of L. bicolor was monitored on four sites in coastal Mississippi from 2006-2008. Wasps are active from June to October and were active later in the season on flowering plants. Diurnal activity on flowering plants was monitored during August and October. Activity began at 0700 h in August and 0900 h in October and continued to near dusk. Populations on flowers were typically male-biased with numbers of females on flowers being greatest in the afternoon (about 1300 h). Sixteen plant taxa representing nine genera and seven families were evaluated in common garden experiments during 2007 and 2008. Of these, wasps only fed on Pentas lanceolata and Spermacoce verticillata. However, pentas, a popular ornamental, may be a more aesthetic and less problematic alternative to S. verticillata, a weed in turfgrass, as a nectar source for L. bicolor in urban landscapes.

Introduction

Three natural enemies; the red-eyed fly Ormia depleta Wiedemann (Diptera: Tachinidae), the beneficial nematode Steinernema scapterisci Nguyen and Smart, and the digger wasp Larra bicolor F. (Fig. 1), were imported and successfully introduced in Florida for biological control of mole crickets (Scapteriscus spp., Orthoptera: Gryllotalpidae) (7). Acoustic trap captures of Scapteriscus mole crickets over a 25-year period on the same sites near Gainesville, FL, indicate a 95% reduction in trap captures in the years following the release of these natural enemies (7). Of these, L. bicolor has naturally expanded its range into Alabama (1) and Mississippi (10) and has been introduced into Georgia and Louisiana (9).

Fig. 1. Adult L. bicolor feeding on flowers of Pentas lanceolata.

Larra bicolor (Fig. 1) females hunt in pasture and managed grasses infested with mole crickets. Females enter the burrow forcing the host to emerge. Once on the surface, the mole cricket is stung which induces a short paralysis to allow oviposition (4). The egg requires 6 to 7 days to hatch. The larva develops as an ectoparasite (Fig. 2) through five larval instars in about 11 days (4). Pupation occurs in the soil with adults emerging about 6 to 8 week later. Between bouts of hunting, females are found on flowering plants (2,4) and the presence of nectar sources are important to the establishment of L. bicolor (11).

Fig. 2. Larra bicolor larvae attached to a mole cricket host. Larvae feed on hemolymph by piercing the exoskeleton of the host.

Most of the published research on the biology and ecology of L. bicolor has been conducted in South America and Puerto Rico, and Florida for the North American populations. In Florida, L. bicolor has two to three asynchronous generations per year (6,9). Activity of L. bicolor is typically determined by physical observations of wasps on turfgrass or flowering plants (2,11). Wasps foraging on flowers or hunting in turfgrass are easily disturbed further complicating detection [(5), authors observations]. Traps baited with phenylacetaldehyde, a floral attractant, collect L. bicolor as a non-target species (12) indicating passive detection may be possible. Adult Tiphia spp., parasitoids of white grubs, were recruited to sugar solutions applied to the grass or to foliage on low hanging limbs of trees and collected at pan-type traps (16). Identification of alternative monitoring methods may enable researchers and extension personnel to better document the presence and continued spread of L. bicolor.

Arévelo and Frank (2) compared attractiveness of five flowering wildflowers to L. bicolor; however, all were visited less frequently than S. verticillata (Rubiaceae). Spermacoce verticillata is a non-native weed (13) and readily reproduces from seeds and overwinters in the landscape in coastal Mississippi (authors observations). Populations of L. bicolor on flowers of S. verticillata are reportedly male biased (6) but there are apparently no data to support these claims.

In 2006, CMA observed L. bicolor feeding on P. lanceolata (Rubiaceae) in the landscape in coastal Mississippi. This suggests that other flowering ornamental plants or other plants in the Madder family may substitute as nectar sources for L. bicolor especially where there are concerns about the pernicious nature of S. verticillata. Pentas are common in southern landscapes for their heat tolerance and attractiveness to butterflies (3). The objectives of this study were to compare the recruitment of adult L. bicolor to ornamental plants, to evaluate potential passive monitoring techniques, and to determine the seasonal and diurnal activity of L. bicolor.

Are Certain Flowering Ornamental Plants as Attractive to L. bicolor as S. verticillata?

In May 2007 and in 2008, a garden of potential nectar sources (Table 1) was planted on the grounds of the Coastal Research and Extension Center (REC), Biloxi. Plants were chosen based on observations of flower-feeding by L. bicolor (e.g., pentas), those reported to benefit other parasitic wasps or bees (e.g., fennel and sweet hyssop), members of the Madder family (i.e., Ixora), or ornamental plants with small, shallow flower that consistently flower all summer (e.g., verbena). Plants for this study were grown in plastic pots (15 cm tall) at the South Mississippi Branch Experiment Station in Poplarville, MS, or purchased from local retailers. Each plant species or cultivar was established in four replicated plots, 0.5 × 0.5 m. Plants in production and after planting were fertilized with 10-50-10 (Schultz Co, Bridgeton, MO) monthly to sustain flowering. The garden was watered through drip irrigation lines and covered with 5 cm of ground hardwood mulch. In 2008, annual plant taxa (i.e., pentas, hybrid and trailing verbenas, fennel, and jungle geraniums) were replanted in May.

Table 1. Flowering plants evaluated as nectar sources for Larra bicolor

Beginning in mid-May each year, plants were monitored for L. bicolor. Data were collected when wasps were consistently observed in the garden. Numbers of wasps on flowering plants were recorded between 1100 to 1200 h each day with a 15-min observation period for each replicate. Plants species visited and the number of foraging wasps was recorded. During these observations, all plants were in flower. Numbers of wasps (dependent variable) from 6 days in 2007 and 10 days in 2008 were pooled, log transformed. Since these are counts, Poisson regression (2) was performed (PROC GLIMMIX, SAS Institute Inc., Cary, NC) using the difference of least squares means procedure (t tests) to determine the fixed effect of flowering plant taxa (independent variable).

In both years, L. bicolor wasps were only observed on flowers of pentas and S. verticillata (Fig. 3, F = 12.1, P < 0.0001). In both years, the number of wasps found on S. verticillata and white-flowered pentas was double the number found on either red- or pink-flowered pentas (P < 0.01).

Fig. 3. Numbers of adult L. bicolor recorded on flowers over 6 days in 2007 and 10 days in 2008. Differences among plant taxa were significant (Poisson, F = 12.1, P = 0.0001, df = 3, 27). Numbers of wasps recorded on white-flowered pentas were not significantly different from S. verticillata (t = 0.27, P = 0.79). Numbers of wasps recorded on red- or pink-flowered pentas were not significantly different (t = 1.7, P = 0.1), but both were significantly different from whiteflowered pentas (t = 3.6 and 4.65, P < 0.01, respectively).

Monitoring L. bicolor Using Traps

Monitoring techniques reported by Rogers and Potter (16) were evaluated for effectiveness to monitor L. bicolor. Experiments with sugar sprays were conducted on two golf courses, President Broadwater (Biloxi, MS) and Great Southern (Gulfport, MS), where L. bicolor were active. Three trials, one each week, were conducted on each golf course from 30 June to 4 August 2005. Three plots, 1 × 1 m, of hybrid bermudagrass [Cynodon dactylon (L.) Pers × C. transvaalensis Burtt-Davy] were selected in three out-of-play areas on each course. Using a handheld, low volume sprayer, the grass in each plot was sprayed to runoff with a 10% sugar solution (16). In addition, foliage on a lower branch on the closest tree to each set of three plots was also sprayed. Treatments were applied around 1000 h and then monitored hourly for a 3-h period immediately after treatment. Numbers of L. bicolor present in each sprayed plot and the number of wasps on the treated branch were recorded.

Another experiment, conducted on 6 November 2008, evaluated the effectiveness of pan-type traps. Four sets of 355 ml plastic bowls (Solo, Highland Park, IL) of three colors (white, yellow, and clear) (16) were placed 0.5 m apart around the edge of the previously mentioned flower garden at 0800 h. Sets of bowls were placed such that they were adjacent to the four plots of flowering S. verticillata. Each bowl was filled with water and about 10 ml of liquid soap to reduce surface tension. At 0900, 1200, and 1500 h, the number of adult L. bicolor captured in each bowl and the number of wasps counted in 2 min in each plot of flowering plants were recorded.

No wasps were observed in the patches of grass treated with sugar water or on the foliage of trees sprayed with sugar water. Adult L. bicolor were active on both golf courses during these experiments but most activity was >1 m from sprayed plots. Likewise, no wasps were captured in any of the bowls despite an average of 0.75, 4.75, and 6.75 wasps at 0900, 1200, and 1500 h respectively, on nearby flowers.

Seasonal and Diurnal Activity

The seasonal abundance of L. bicolor has been reported from Florida (11), so this experiment was conducted to determine the seasonal activity of L. bicolor in Mississippi. From April to July 2006, eight golf courses in coastal Mississippi (Harrison and Hancock Co.) were surveyed for activity of L. bicolor. Many of these courses include locations where L. bicolor was originally discovered (10). Of these, three that had consistent activity were selected for seasonal monitoring of L. bicolor. On each golf course, two sites that had activity of L. bicolor and an active infestation of mole crickets were selected for monitoring. Observations were also made of wasps foraging on flowers of S. verticillata and P. lanceolata in the landscape at the Coastal REC. Since these wasps are noted nectar feeders, monitoring flowering plants would gauge the seasonal foraging activity of L. bicolor in addition to the hunting activity. From April to December 2006 to 2008, each site was scouted for 15 to 20 min on clear days, between 1000 to 1500 h, every 2 week.

Diurnal activity was studied by monitoring foraging activity of L. bicolor on flowering S. verticillata and white flowered pentas in the previously mentioned garden. The number and sex of wasps on flowering plants in eight plots, four plots of each species, were taken hourly from 0500 to 2000 h each day on 7-9 August, 2007 and 9-11 October, 2008. Observations were made for 5 min in each plot. Wasps were counted regardless of feeding activity. On two days in August, plots of flowering plants were observed every 2 h through the overnight period to determine if and when wasps abandoned nectar sources. Data were collected despite changes in weather such as rain or overcast conditions. Air and soil temperatures were recorded by a mini-weather station (Model #450, Spectrum Technologies, Plainfield, IL) adjacent to the flower garden. Data from August 2007 and October 2008 were analyzed separately.

Across all sites, wasps were active from June through October (Fig. 4) with extended activity in the fall of 2007. This seasonal activity of L. bicolor is similar to that reported from the Florida’s Panhandle (Gadsden Co.) (11). One site, St. Andrews Golf Course, had a population of L. bicolor in 2004 (10) and August 2006, but subsequent surveys failed to find wasps there. An increased and more aggressive mole cricket management program at that site may have reduced host density below a critical threshold, although unknown, or perhaps the insecticides had a direct impact on the wasp population. The impact of turfgrass insecticides on L. bicolor has not been studied, but certain insecticides cause significantly mortality of parasitoids (e.g., Tiphia spp.) foraging in treated turfgrass (15).

Fig. 4. Seasonal occurrence of Larra bicolor as indicated by presence or absence of wasps hunting in turfgrass infested with mole crickets (Scapteriscus spp.) on three golf course sites (Gulf Hills, St. Andrews, Great Southern) or foraging on flowers (Coastal REC) in Mississippi.

Wasps were consistently active one month later on flowers (Coastal REC) than in turfgrass. Wasps were observed on flowers from about 13 h (0700 to 1900 h CST) in August and 9 h (0900 to 1700 h CST) in October (Fig. 5). Adults of L. bicolor are active between 0830 and 1530 h (Brazil) and 0645 h to near 1800 h (EDT) in Puerto Rico (May to June) (5). During this study, sunrise and sunset were 0520 h and 1845 h (August), and 0600 and 1730 h (October), respectively. Wasps were first reported active when air temperatures were 27.2 and 20°C in August and October, respectively (Fig. 5). Wasps typically leave the plants around dusk despite warm air temperatures. Dusk or overcast conditions may trigger wasps to seek refuge sites (5,14). Attempts to follow individuals to refuge sites were unsuccessful. Wasps may also leave flowering plants before the constraints (e.g., thermal or visual) of night prevent activity (14). A few wasps remained overnight underneath foliage on pentas. The following morning (0600 to 0700 h), these individuals began grooming themselves before abandoning the plant. Wasps were commonly observed in the shaded interior of the flowering plants during the day. Lack of activity before sunrise or after sunset, and movement into the shade may indicate additional thermal or visual constraints on the activity of L. bicolor.

Fig. 5. Daily activity (0600 to 2000 CST) of Larra bicolor (indicated by bars) over 3 days in August 2007 (upper) and October 2008 (lower) in relation to air (TA) and soil (Ts) temperature.

A maximum of 14 females and 31 males were observed at one time on flowers in August versus 3 and 25, respectively, in October. Although fewer total wasps and fewer females were observed in October, these data were from two different years and may not reflect population dynamics in a single season. Males outnumbered females on flowers, which is consistent with previous observations (6). Fewer females later in the season may also explain why wasps were still observed on flowers when none were observed hunting later in the season (Fig. 4).

Males were almost always present on flowers during the hourly observations. Females, however, were observed on flowers for distinct periods (3 to 9 h) each day. The timing of this period, however, was inconsistent from day to day. In Puerto Rico, females hunt early in the day (0645 to 1200 h EDT), about 1 h after sunrise, and forage on flowers later (0745 to 1600 h) from May to June (5). In this study, females were often present once wasps became active in the morning, but the daily maximum number of females usually occurred later, about 1300 h in August and October. Females likely need nectar early in the morning before hunting, returning later in the day to feed or rest as temperatures increase. Although flowers are logical rendezvous sites (14), males do not approach females on flowers [author’s observations, (4)]. Mating, although rarely observed and poorly understood, reportedly occurs on the ground near hunting sites (1,4).

Conclusions and Recommendations

Attempts to monitor L. bicolor using methods successful with other solitary wasps (Tiphia spp.) that forage in turfgrass (16) were unsuccessful. Wasps were not attracted to pan-type traps in the field nor were they recruited to sugar water sprays on the grass or nearby trees. Tiphia spp. are noted as honeydew feeders (16) which likely explains their attraction to supplemental sugar sprays, whereas L. bicolor has not been reported to feed on honeydew. The wasps may use floral volatiles (12) or a combination of volatile and visual cues to find nectar sources. Future field experiments are planned to evaluate floral extracts of flowers of pentas and spermacoce as attractants for L. bicolor. This may produce a reliable passive monitoring tactic or enable mass collection of wasps.

Observations of L. bicolor on flowers are the most reliable indicator of activity (2,11). Spermacoce verticillata has been the primary nectar source used to sustain L. bicolor in Florida (9). This paper is the first to report use of P. lanceolata as a nectar source by L. bicolor. In the garden experiments, wasps were only recorded on mainly the white flowers of pentas and S. verticilllata. However, wasps were never recorded on the white-flowered verbena nor were they collected in the white or yellow pan traps. White or yellow are highly reflective colors commonly used to attract day-active wasps. Larra bicolor has been observed or reported to feed on flowering wildflowers (2) or weeds such as dollarweed (W. Hudson, personal communication) that vary considerably in flower color and flower size. Despite this, this study and others (2) show a preference by this wasp for S. verticillata. As previously noted, more research on volatile attractants, a comparison of nectar composition, and possible interactions of these two factors may be required to better understand this affinity for certain flowers. Nectar from flowering plants such as S. verticillata are reportedly important to sustain populations of L. bicolor (9,11). Pentas is already widely used in landscapes across the southeast. Even in coastal Mississippi, this plant is an annual and doesn’t volunteer from seeds. Spermacoce verticillata, also an annual, readily volunteered from seeds during this study and is reported as a weed in managed turfgrass (13). Given this and the data presented here, flowering pentas may be a less problematic alternative to S. verticillata as a nectar source for L. bicolor in urban landscapes.

Acknowledgements

We thank P. Gonsiska (UW-Madison), P. Broussard, L. Easterly, T. Knight (Coastal REC, Biloxi, MS) and D. Murchison and D. Lee (South MS Branch Exp. Station, Poplarville, MS) for technical assistance. Thanks to the course superintendents and staff at Great Southern, St. Andrews, President-Broadwater, and Gulf Hills Golf courses. Thanks to D. Rowe (MS State University) for assistance with the statistical analysis. Voucher specimens have been placed in the museums at Mississippi State and Auburn universities. Funding for this project was provided by a grant from the United States Golf Association.

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