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To receive a reprint of this article, please request from Stephen Peterson:
steve@pollination.com
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 <1% of the
total emerged, also varied significantly among the groups (F = 18.2; df
= 2,6; P < 0.003). Females emerged for 2.5 d longer in the polystyrene
blocks compared with wooden boards. The peak emergence for females also
differed significantly among groups (F = 9.12; df = 2,6; P < 0.015).
While 40.6% of the females emerged from the wood on the peak day, only
17.1% of the females emerged from the polystyrene on the peak day.
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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Ann. Rev. Entomol. 17:287-312.
Lambden, T. L., W. J. D. Shaw & G. H. Green. 1973. Heat release from
alfalfa leaf-cutter bees. Canadian Agric. Eng. 15:110-112.
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.
Richards, K. W. 1978. Comparisons of nesting materials used for the alfalfa
leafcutter bee, Megachile pacifica (Hymenoptera: Megachilidae). Can. Ent.
110:841-846.
Richards, K. W. 1983. Leafcutter bees revive alfalfa seed production.
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leafcutter bees, Megachile rotundata, held at constant incubation temperatures
(Hymenoptera: Megachilidae). J. Apicultural Res. 27:197-204.
Steel, R. G. D. & J. H. Torrie. 1980. Principles and procedures of
statistics, 2nd ed. McGraw-Hill Book Company, New York.
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the leafcutter bee, Megachile (Eutricharaea) rotundata, for alfalfa pollination.
J. Econ. Entomol. 54:989-993.
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J. Econ. Entomol. 58:284-286.
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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 |
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