Heat Retention During Incubation in Nests of the Alfalfa Leafcutting Bee (Hymenoptera: Megachilidae)

J. Econ. Entomol. 87:345-349

To receive a reprint of this article, please request from Stephen Peterson:
steve@pollinationsystems.ca

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Abstract

Temperatures within polystyrene, paper and wooden nests of the alfalfa leafcutting bee, Megachile rotundata (F.), were monitored during incubation at 30.0°C. In the polystyrene and paper nests, temperatures began to increase 10 d after the start of incubation and continued to rise until adults emerged. Temperatures reached 35.1°C in the polystyrene and 37.2°C in the paper, while the wood nests never exceeded 30.6°C. Compared with air temperature degree-days, cells in the center of the polystyrene nests accumulated up to 28 more degree-days after 20 d, while those in the paper accumulated 32 more degree-days. In polystyrene nests, heat units increased towards the center holes, reaching a maximum at the fifth row towards the center. Emergence was also monitorred from polystyrene blocks, wooden boards and loose cells. Females emerged 2.1 days later from polystyrene blocks compared with wooden boards and the emergence period was lengthened by 2.5 d. Because of the increased temperatures and the prolonged emergence period experienced with these materials, we recommend that filled polystyrene and paper nests not be incubated, but stripped of their cocoons, as they were designed.

Key words: Insecta, Megachile rotundata (F.), nest materials, incubation

Introduction

The alfalfa leafcutting bee, Megachile rotundata (F.), is the primary pollinator of alfalfa seed in the Pacific Northwest (Washington, Oregon, Nevada, Idaho, and Montana) and Canada. This bee was first recognized as a manageable pollinator over 30 years ago (Stephen 1961). The alfalfa leafcutting bee is a solitary bee though it nests gregariously. This bee has one to two generations per year in the Pacific Northwest, depending on the climate, and undergoes a faculatative larval diapause. In the spring, approximately 21 d prior to alfalfa bloom, the bees are incubated at 30°C until adults emerge. In 1991 the recommended leafcutting bee population cost $500 per ha (Peterson et al. 1992), which is approximately 25% of the typical input for alfalfa seed production in Idaho. Assuming an optimum yield, the net return can be as high as $3750 per ha.

Two systems for leafcutting bee management are currently employed. In the solid board system, drilled wooden boards are used for nesting and the diapausing larvae are kept in the boards. The boards containing larvae are placed in incubators in the spring and taken to the fields when the first adults emerge. The loose cell system involves removing the bee cocoons from their nests, usually in the fall or winter. This system improves the control of insect predators and nest destroyers and also allows better synchronization of bee emergence with the beginning of flower bloom. (Bohart 1972, Richards 1983). The solid board system is used by the majority of seed producers in the United States.

Several nest materials are available for use in the loose cell system, including various polystyrene and paper nests (Richards 1978, Parker et al. 1983). These materials were designed to facilitate removal of the cocoons from the nests. Some of these nests, such as those made of paper, are not designed to be reused, while most of the others can be reused following sterilization.

Recently, seed producers have found that the polystyrene and paper nest materials used in the loose cell system are light-weight, inexpensive, readily available. Therefore, producers have begun to use these nest materials as they use wooden boards, without removing the cocoons in the fall. Consequently, reports of poor emergence from these blocks have become common.

During incubation, bees ecdyse twice (prepupa to pupa and pupa to adult). The heat released by this process has been estimated to reach 0.15 g cal/h for immature leafcutting bees. We hypothesized that the metabolic heat produced by developing bees, when held in insulating nest materials such as paper and polystyrene, would raise the temperature within the nest. The purpose of these studies was to measure the temperatures in various nest materials during incubation, to determine the importance of location within the nest on heat unit accumulation, and to compare the emergence patterns of bees from differing nest types.

Materials and Methods

Experiment 1. Filled polystyrene, wood and rolled paper nest materials were obtained in 1991 and refrigerated at 5°C for 8 mo. Three types of polystyrene were tested: (1) Beaver Megablock (9.53 cm deep)(Beaver Plastics Ltd., Edmonton, Alberta), (2) Wolf Block (9.53 cm)(Wolf Farms Ltd., Carrot River, Saskatchewan) and (3) Dalziel Block (7.62 cm)(Dalziel Enterprises Ltd., White Fox, Saskatchewan). A filled Rol-A-Board (Pan Agro, Logan, UT), made of rolled fluted paper and a standard wooden board were also tested. Three replicates of each nest material were tested. The polystyrene and wooden nests were cut into 12 by 12 hole replicates, whereas the paper nest was kept intact but divided into four quadrants. Three of the four quadrants were monitorred in this trial. Sample nests were placed in a temperature controlled growth room at 30 ± 0.5°C in constant darkness. The polystyrene and wooden nests were arranged on the benchtop in randomized complete blocks.

Temperatures were monitored at two positions and two depths in each replicate. For the wood and polystyrene nests, position 1 (P1) was a corner hole and position 2 (P2) was one of the four centermost holes. For the paper nest, position 1 was located in the outer row of holes and position 2 was located in the tenth row towards the center (21 rows total). Depth 1 (D1) was 25% of the length of the hole from the entrance, and depth 2 (D2) was 50% of the length of the hole. Teflon insulated thermocouple wire (30 gauge, 0.55 by 0.96 mm) with welded tips were inserted into the nests along the leafcutting bee cells.

A pilot study indicated heat accumulation in polystyrene depended on the position of the hole in the block. Therefore, a series of positions at a common depth (50%) were monitored. Each Dalziel and wooden nest was monitorred with thermocouples installed in six holes along a diagonal from the corner to the center of the sample block. The outermost hole was designated row 1.

Temperatures were recorded daily with a handheld digital thermometer with a precision of ±0.1°C. Degree-days were calculated using a base temperature of 15.7°C (Richards & Whitfield 1988). The upper threshold for development is between 32° and 35°C (Richards & Whitfield 1988), therefore, we truncated degree-day accumulations at 33°C. The degree-day data were analyzed using a factorial analysis of variance with factors of nest material, depth and position. The protected least significant difference method was used to separate means (Steel & Torrie 1980).

Experiment 2. In 1993 a study was conducted to compare the emergence periods among loose cells, wooden boards and polystyrene blocks. A filled wooden board and a 9.5 cm deep Dalziel block were cut into 12 X 12 hole sample blocks. Four samples were cut from each and four 47 g lots of loose cells, were also tested. Samples were placed in plexiglass boxes, 30 X 17 X 13 cm, with the 17 X 13 cm side on the bottom. A 2 cm diameter hole in the bottom opened into a 0.47 l glass jar with ca. 100 ml of soapy water. To allow air circulation, three sides of each box had a 2 cm diameter hole, covered with aluminum window screen. The loose cells were held in a tray, 1.5 cm deep. The boxes were placed in a walk-in incubator and maintained at 30°C in constant darkness. Adult bees were attracted to the bottom of the cage by a 10 watt ultraviolet light, positioned below and in front of the cages. Once bees began to emerge, the jars were collected daily and male and female bees were counted.

Results

Experiment 1. Temperatures were monitored until 20 d after the start of incubation. Temperature readings from the wooden blocks remained close to the air temperature throughout the trial (Fig. 1). The highest temperature recorded in wood was 30.6°C on day 20. All three of the polystyrene types showed increases in temperature by day 10 in the interior positions (Fig. 1). Temperatures continued to rise throughout the remainder of the study. The highest temperature in a polystyrene nest was 35.1°C in the Wolf block on day 20. The paper nest showed a temperature pattern similar to the polystyrene, although more extreme (Fig. 1). The paper nest reached a peak of 37.2°C on day 19.

Analysis of variance on the heat unit accumulations indicated a significant interaction between position and depth (F = 4.95; df = 1,38; P < 0.05) and between position and material (F = 33.15; df = 4,38; P < 0.0001)(Fig. 2). Thus, the effect of position varied among materials and depths. At position 1, depth had no effect on heat units (F = 0.81; df = 1,18; P > 0.30). However, at position 2, significantly more degree-days were accumulated at the 50% depth compared with the 25% depth (F = 15.93; df = 1,18; P < 0.001). While there was no effect of depth or position in the wood, the three polystyrene types and the paper showed increased heat units in the interior positions.

At both positions, significant differences among materials were observed (P1, F = 35.75; df = 4,18; P < 0.0001; P2, F = 116.12; df = 4,18; P < 0.0001). Degree-day accumulations at position 2 ranked: paper > Wolf and Beaver > Dalziel > wood. At position 1 , a similar pattern was found: paper > Wolf, Beaver and Dalziel; Wolf and Beaver > wood.

After all adults had emerged from the blocks, the paper and polystyrene nests were examined for dead larvae. Only mature larvae, not killed by chalkbrood, were considered “dead larvae.” Therefore, larvae killed before incubation began were not included. Five interior and five exterior holes from each replicate of the non-wood nest materials were dissected. No dead larvae were found in any of the polystyrene nests. Four dead larvae were found in the 30 holes examined in the paper nests.

For the position series, a significant material by position interaction was observed (F = 21.64; df = 5,22; P < 0.0001). Again, the wooden nests showed no effect of position (F = 2.55; df = 5,10; P > 0.05). In the polystyrene, position had a significant effect on heat units, (F = 36.22; df = 5,10; P < 0.0001), increasing up to the fifth row of holes towards the center (Fig. 3).

Experiment 2. A total of 631 ± 31 (mean ± sem) bees emerged from the polystyrene, 391 ± 67 from the wood, and 365 ± 15 from the loose cells. The polystyrene blocks reached a maximum of 33.6°C, while the wooden boards peaked at 31.0°C, and the loose cells reached 30.9°C. The number of days for 50% of the females to emerge was significantly different among the groups (F = 21.1; df = 2,6; P < 0.002). Female bees from polystrene nests emerged 2.1 d later than those from the wooden boards (Table 1). The length of female emergence, not including days when

Discussion

Respiration in developing leafcutting bees increases when prepupae ecdyse to pupae, and continues to increase until adults emerge (W.P. Stephen, Department of Entomology, Oregon State University, Corvallis, OR, unpublished data). We found that temperatures inside insulating nest materials show a similar pattern. Our study indicates that filled polystyrene and rolled paper nests are better heat insulators than filled wooden nests. Paper nests probably show the highest temperatures because of the arrangement of the holes. Only a thin wall of paper separates each hole in a paper nest, resulting densely packed larvae. For example, a rolled paper nest can contain up to 4.2 larvae/cm3, while a polystyrene nest typically contains 1.3 larvae/cm3 and a wooden board contains 1.6 larvae/cm3. Higher larval density undoubtedly results in a greater heat build up.

Little mortality was observed in the study; however, the most dead larvae were found in the paper nests. Incubating leafcutting bees at 40°C produces complete mortality (Stephen & Osgood 1965). Survival drops from 91% at 30°C, to 77% at 35°C, and to 14% at 37°C (Richards & Whitfield 1988). Thus, the mortality in the paper nests could easily be explained by the temperatures observed (up to 37.2°C). Undurraga & Stephen (1980) showed that lightly melanized pupae are more sensitive to heat shock than prepupae, melanized pupae or adults. Female leafcutting bees reach the lightly melanized pupal stage after 17 to 18 d of incubation. Thus, our study shows that temperatures are indeed elevated in polystyrene and paper at the time when the developing bees are the most sensitive to heat shocks.

Bees that are incubated in polystyrene and paper nests experience uneven heat unit accumulation. For example, after 20 d the center of the polystyrene blocks had accumulated 28 more degree-days than the air. This is 2.0 d ahead of the expected schedule at 30°C. The center of the paper nests accumulated 32 degree-days more than the air, or 2.2 d ahead of schedule. With increased degree day accumulations, we might expect adult bees to emerge sooner. However, we found that bees in polystyrene emerged 2.1 d later than similarly treated bees in wood. This finding is supported by Richards & Whitfield (1988), who showed that leafcutting bee prepupae exposed to 35°C and 37°C emerged later than bees incubated at 30°C.

In polystyrene nests, bees emerge over a longer period of time compared with wooden boards. In addition, fewer bees emerge at the peak in polystyrene nests and the average emergence is later than found in wooden boards. Thus, the placement of these bees in the field will be more difficult to synchronize with peak bloom. Based on our findings, we recommend that alfalfa seed producers always remove cocoons from polystyrene and paper nests, and incubate these cocoons loose. These nest materials were not designed to be incubated with larvae in situ and are not well suited for this use.

If growers insist upon incubating filled polystrene nests, it may be beneficial to lower the incubation temperature. An incubation temperature of 25°C may not produce nest temperatures as high as the 30°C incubation temperature. Development would be slower, however, and adjustments in timing would be needed. Further studies are needed to examine the practicality of incubating filled nests at lower temperatures.

Acknowledgments

We thank Carolyn Nyberg for assistance. We also thank George Hoffman and Joseph J. Peterson for assistance with building a thermocouple welder. Joe McCaffrey and William Stephen provided helpful suggestions on the manuscript. Two anonymous reviewers also helped to improve the manuscript. This research was funded by grants from USDA-ARS, USDA-APHIS, FMC Corporation, and the Idaho Alfalfa Seed Commission. Scientific paper no. 92764, University of Idaho Experiment Station.

References

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Parker, F. D., R. Teranishi & A. C. Olson. 1983. Influence of attractants on nest establishment by the alfalfa leafcutting bee (Hymenoptera: Megachilidae) in styrofoam and rolled paper. J. Kansas Entomol Soc. 56:477-482.

Peterson, S. S., C. R. Baird & R. M. Bitner. 1992. Current Status of the alfalfa leafcutting bee, Megachile rotundata, as a pollinator of alfalfa seed. Bee Science 2:122-129.

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Undurraga, J. M. & W. P. Stephen. 1980. Effect of temperature on development and survival in post-diapausing alfalfa leafcutting bee prepupae and pupae (Megachile rotundata (F.): (Hymenoptera: Megachilidae). I. High temperatures. J. Kansas Entomol. Soc. 53:669-676.

Table 1. The number of incubation days, number of days of emergence, and the peak emergence level for female alfalfa leafcutting bees emerging from loose cells, polystyrene blocks, and solid wooden boards

Source	Days to 50% Emergence (d � SEM)	Days of adult Emergence (d � SEM)	Highest level of Emergence (% � SEM)
Loose Cells	21.4 � 0.3 a	6.50 � 0.30 a	25.5 � 4.0 a
Polystyrene	23.8 � 0.3 b	8.25 � 0.30 b	17.1 � 4.0 a
Wooden Board	21.7 � 0.3 a	5.75 � 0.30 a	40.6 � 4.0 b