predefaecating stage of 4–6 days, the body appears bloated and brown faeces are visible inside the larva. Larvae defaecate by smearing faeces on the sides of the cell wall over 2–3 days. The prepupa is opaque white, has a sharp angle between the head and the thorax, and has prominent humps on the back (Fig. 9.4). This is the overwintering dia- pause stage of the alkali bee which lasts 10–11 months. Prepupae are first found in cells from about 3–9 July.
Increasing soil temperature in spring breaks diapause and the prepupa changes to the pupa around the end of May. This stage lasts 15–20 days, with the pupa gradually becoming coloured more like an adult. The pupa changes to the adult which remains in the cell for a short time before digging to the surface. This generally occurs during June. Individual females live 4–6 weeks. Females will be pre- sent and active in a given area for about 60 days under good weather conditions.
time, average seed yields jumped from 560 lb acre21(627 kg ha21) in 1960 to 950 lb acre21(1064 kg ha21) in 1971. But in 1970/71 alkali bee populations declined sharply; possible causes were insecticide appli- cations in the area, insecticide residues in the water used to dampen bee beds, and bee parasites. Lucerne yield and acreage declined rapid- ly thereafter, and lucerne acreage has not yet recovered to pre-1971 levels. Thus, we see the rise and fall of a local lucerne industry coinci- dental with the presence of alkali bees.
Today alkali bees are largely supplanted as commercial pollina- tors by the exotic alfalfa leafcutting bee. However, for two or more decades theirs was a remarkable case history of ‘a new pollinator brought up from the ranks of native bees’ and ‘domesticated’ for commercial pollination purposes (Buchmann and Nabhan, 1996).
Moreover, alkali bees are the only soil-nesting bee native to North America for which practical, large-scale management practices have been developed. Thus, alkali bees are a valuable natural resource for agriculture in the western US. In the interest of preserving this nat- ural resource we include in this chapter detailed instructions for their management.
Recommended Bee Densities
The number of foraging female bees required in a lucerne field is not fully known, but it should probably exceed 3000 acre21(7410 ha21). It is easier to measure bee densities by assessing nest concentration. A good natural nesting site will average about 1 million nests acre21(2.5 million ha21). One acre (0.4 ha) of bed with this number of nests should provide for 1000 lb of clean seed acre21(1120 kg ha21) on 200 acres (81 ha) of lucerne. The maximum population in artificial beds is about 5!s million nests acre21 (13.6 million ha21). Estimates of nest concentration can be made on the basis that 23, 46, and 69 nest holes ft22 represent 1, 2, and 3 million nests acre21, respectively (2.5, 4.9, and 7.4 holes dm22 represent 2.5, 4.9, and 7.4 million nests ha21, respectively).
There is a simple sampling method that works reasonably well for estimating the value of a bee bed. While walking along a line across the bed, an observer takes ten or more random counts of the number of nests per square foot or square decimetre. The more counts, the bet- ter the estimate. After finding the average number per square foot, it is multiplied by ten to derive the average number of nests 10 ft22. To find average number of nests per square metre, the number per square decimetre is multiplied by 100. This figure is inserted into the for- mula below to determine a Pollination Index (PI).
88 Chapter 9
An index value of 230 indicates an adequate number of pollina- tors; values above 230 are good to excellent, and values below this are fair to poor.
If one is planning a semi-artificial or artificial bee bed (see pages 92–93), there is a different form of this formula useful for estimating the required size of a bed for a given area of lucerne. Using a recom- mended PI value (say, at least 230), a recommended target density of nest holes per ten square feet (say 230) or square metre (say 250), and the known area of lucerne to be served – these values can be inserted in the following formula to determine the required size of bee bed.
It takes time for new nesting sites to fill up with a full comple- ment of larvae. A new bed in southeastern Washington had a mass fly- in of nesting females in 1976 which provided up to 70 nest entrances per square foot (7.5 dm22) the first season. However, there were only one-fourth as many progeny as in older sites. New sites usually require at least three favourable seasons to develop good production.
Artificial beds often produce two to three times as many progeny per unit soil volume as do managed natural sites. However, they are more variable and do not maintain peak populations without periodic reno- vation or expansion.
Three hundred healthy prepupae per cubic foot (106 dm23) is excellent production and 200 (71 dm23) is good. Since more males than females are produced, the local increase in females is compara- tively slower than that for males. It takes time for alkali bees to repop- ulate an area following a natural or man-made disaster. In 1973, several beds in southeastern Washington were reduced to about ten prepupae per cubic foot (3.5 dm23) after an insecticide poisoning. By
Alkali Bees 89
PI = no. nests ft acres of bed
acres served
10 -2 ´ ´ 200
PI = no. nests m ha of bed
ha served
-2 ´ ´ 186
or
Required size of bee bed (acres) = PI acres served target nest ft
´
´ -
200 10 2
Required size of bee bed (ha) = PI ha served target nest m
´
´ -
186 2
or
the end of the 1974 season, they had increased a little over four-fold.
With poor weather for reproduction in 1979 they remained about the same – 17–45% of a good population.