171). Another is the regular risk of freeze damage during the critical spring pollination window. NeSmith et al. (1999) chilled plants of
‘Brightwell’ and ‘Tifblue’ at 32, 30.2, 26.6, or 23.9°F (0, 21, 23, or 24.5°C) for one hour during flowering and then exposed the plants to pollination by either bumble bees only or to bees plus the growth reg- ulator gibberellic acid (GA3). Fruit-set in the bee pollination treatment dropped sharply in plants chilled to 30.2°F (21°C) or lower, but fruit- set in the bees plus GA3 treated plants did not decline significantly from non-chilled controls until temperatures dropped below 26.6°F (23°C). Thus, GA3 confers some degree of compensation for chill- induced pollination deficiencies. GA3has become widely used in the southern US, but it should not be thought of as a replacement for pol- lination. Compared to hand pollinated plants, fruit weight was lower and time to ripening longer in ‘Beckyblue’ rabbiteye plants treated with GA3only, and fruit weight was maximized when GA3 was used in conjunction with pollination (Cano-Medrano and Darnell, 1998).
Thus, GA3treatment is best thought of as a pollination supplement or compensation for periods of suboptimal pollination conditions.
the relative efficiency of single bee visits depends largely on whether that species can buzz-pollinate. But even non-buzz-pollinating species, such as honey bees, can pollinate blueberry when they are present in sufficient numbers and visit flowers legitimately.
Lowbush blueberry pollinators
Honey bees and many species of non-managed bees visit lowbush blueberry (Table 24.2). Some wild bees are locally numerous and probably important pollinators. This seems to be the case for Andrena carlini, A. carolina, A. vicina, Evylaeus quebecense [sic], Dialictus viridatus, Bombus perplexus, B. ternarius, and B. terricola in Nova Scotia (Finnamore and Neary, 1978) and for D. pilosus pilosus, E. que- becensis, B. ternarius, and B. terricola in Sagamie, Québec (Morrissette et al., 1985). Populations of wild bees are generally too small or unpredictable to support commercial pollination needs in lowbush blueberry (Morrissette et al., 1985).
Wild pollinators can be supplemented with honey bees. Yield in Newfoundland lowbush blueberry fields stocked with honey bees at 0.7 colonies acre21 (1.7 ha21) was 54% higher than in fields without supplemental honey bee hives (Lomond and Larson, 1983). In Québec, 500 honey bee hives were introduced during bloom at one end of a large lowbush blueberry farm, and researchers measured bee densities and fruit characteristics at regular intervals in the field for up to 3.1 miles (5 km) from the hives (Aras et al., 1996). Honey bee densities decreased as distance from the hives increased. As honey bee densi- ties decreased, there was a corresponding drop in seed set, fruit-set, berry weight, and speed of fruit ripening. Densities of wild non-honey bees were uniform across the field; thus, these results demonstrate the contribution of honey bees to lowbush blueberry pollination.
Positive results from importing honey bees may stem in part from the particular mixture of lowbush species Vaccinium angustifolium and V. myrtilloides growing in a field. When V. angustifolium and V.
myrtilloidesoccur together, incompatibility problems can limit maxi- mum fruit-set to about 50% (Hall and Aalders, 1961). But where V.
angustifoliumpredominates, incompatibility is not a problem and the fruit-set potential and the potential value of honey bees are much higher (Free, 1993).
Managed leafcutting bees and orchard bees may be potential polli- nators of lowbush blueberry. Alfalfa leafcutting bees (Chapter 11) and the orchard bee O. ribifloris(Chapter 12) were released in a lowbush blueberry orchard in Maine. Both bee species visited the flowering crop and collected blueberry pollen. However, O. ribifloris collected blueberry pollen exclusively and adults emerged in good synchrony
Blueberry 1771
178 Chapter 24
Table 24.2. Non-honey bee pollinators of lowbush blueberry (after Finnamore and Neary, 1978 and Morrissette et al., 1985).
Colletidae
Colletes consors mesocopus Swenk Hylaeus modestus modestus Say Colletes inaequalis Say
Andrenidae
Andrena algida Smith Andrena kalmia Atwood
Andrena bipunctata Cresson Andrena lata Viereck
Andrena bradleyi Viereck Andrena miserabilis bipunctata Cresson
Andrena carlini Cockerell Andrena nivalis Smith
Andrena carolina Viereck Andrena planida placida Smith Andrena ceanothi Viereck Andrena regularis Malloch Andrena clarkella (Kirby) Andrena rufosignata Cockerell Andrena crataegi Robertson Andrena sigmundi Cockerell Andrena cressonii Robertson Andrena thaspii Graenicher Andrena frigida Smith Andrena vicina Smith Andrena grandior Cockerell Andrena wilkella (Kirby)
Halictidae
Halictus confusus Smith Dialictus cressonii (Robertson) Halictus rubicundus (Christ) Dialictus disabanci Knerer & Atwood Lasioglossum athabascence (Sandhouse) Dialictus imitatus (Smith)
Lasioglossum forbesii (Robertson) Dialictus pilosus pilosus (Smith) Evylaeus arcuatus (Robertson) Dialictus viridatus (Lovell) Evylaeus cinctipes (Provancher) Augochlora pura pura (Say) Evylaeus comagenensis Knerer and Atwood Augochlorella striata (Provancher) Evylaeus divergens (Lovell) Sphecodes cressoni (Robertson) Evylaeus foxii Robertson Sphecodes persimilis Lovell &
Cockerell
Evylaeus macoupinensis Robertson Sphecodes ranunculi Robertson Evylaeus quebecensis (Crawford) Sphecodes solinis Graenicher
Megachilidae
Megachile melanophoea Smith Osmia inspergens Lovell & Cockerell Osmia atriventris Cresson Osmia proxima Cresson
Osmia inermis (Zetterstedt) Osmia tersula Cockerell Anthophoridae (see Roig-Alsina and Michener, 1993) Nomada cressonii Robertson Nomada lepida Cresson
Apidae: Bombinae
Bombus borealis Kirby Bombus terricola Kirby Bombus fervidus (Fabricius) Bombus vagans vagans Smith Bombus rufocinctus Cresson Psithyrus ashtoni (Cresson) Bombus perplexus Cresson Psithyrus fernaldae Franklin Bombus sandersoni Franklin Psithyrus insularis (Smith) Bombus ternarius Say
with crop bloom. Leafcutting bees visited the crop, but they were more likely to visit competing bloom (Stubbs et al., 1994).
A separate study demonstrated promising performance of alfalfa leafcutting bees in lowbush blueberry grown in Maine (Stubbs and Drummond, 1997). Fruit-set was increased significantly in most cases in plots nearer to the bee shelters. Alfalfa leafcutting bees increased fruit-set as much as 30% over background pollination provided by honey bees and wild bees. Rate of reproduction, however, was less than 0.2 loose cells per loose cell incubated. If these bees are to be used for commercial pollination of lowbush blueberry, it is probable that cells would have to be purchased annually.
Highbush blueberry pollinators
Although northern and southern highbush varieties range from some- what self-fertile to highly self-fertile, cross-pollination between many varieties improves fruit size, seed count, and earliness of ripening, all of which are important economic considerations (Lang and Danka, 1991). Large bee populations help transport large quantities of pollen and cross-pollinate highbush blueberry. In one study, honey bee visi- tation to ‘Gulfcoast’ highbush variety increased speed of fruit ripening by five days and berry weight by 28%. These effects did not depend on the type of pollen the bees were carrying (selfing with same vari- ety, crossing with different variety, or crossing with a rabbiteye vari- ety), so this study confirms the value of honey bees as pollinators of southern highbush blueberries (Danka et al., 1993b).
With the northern highbush ‘Bluecrop’, increasing honey bee visi- tation with a synthetic queen mandibular pheromone (QMP) attractant increased fruit yield by at least 6% and farmgate revenue an average of US$364 acre21(US$900 ha21) (Currie et al., 1992a).
Solitary bees may be helpful highbush blueberry pollinators. The orchard bee O. ribifloris (Chapter 12) is an efficient pollinator in California highbush blueberry. As few as 300 nesting O. ribifloris females can pollinate 1 acre of highbush blueberry (741 ha21) (Torchio, 1990b). The solitary soil nester Colletes validus visits blue- berry (many species, including highbush) in Maryland, works the blossoms legitimately, and nests in synchrony with crop bloom. It is a prime candidate for conservation efforts (Batra, 1980).
Rabbiteye blueberry pollinators
Southeastern blueberry bees and bumble bees are the most efficient pollinators of rabbiteye blueberry, based on single-bee flower visits
Blueberry 1791
(Cane and Payne, 1990). The southeastern blueberry bee (Habropoda laboriosa) (Chapter 10) emerges, mates, and nests in February–April in close synchrony with blueberry bloom in the southeastern US.
These bees appear to be near-obligate specialists on blueberry, and they are sometimes present in blueberry orchards in the southeastern US at economically important pollinating levels (Cane, 1993, 1994;
Cane and Payne, 1988, 1991, 1993).
Bumble bees (Chapter 8), especially nesting queens, are naturally plentiful in some rabbiteye orchards in the southeastern US. However, their populations in early spring when rabbiteye is blooming are limit- ed to nesting queens or to very small, young colonies with only a few workers. Thus, the species’ maximum forager potential is not realized in time for rabbiteye bloom in that area. Some growers buy commer- cially-reared mature colonies with ≥80 workers each (see Chapter 8, page 82) and disperse them throughout the orchards. However, the efficacy of this practice is untested.
Carpenter bees (Chapter 13) rob rabbiteye blueberry flowers by cutting slits in the side of the flower to reach nectar without touching the anthers or stigma. The robbery holes can then attract other bee species which would otherwise legitimately visit the flower. As few as one carpenter bee per 25 bushes or 4% incidence of slitted flowers can make 80–90% of the honey bees switch to robbing (Cane and Payne, 1991). Whenever possible, blueberry growers should control carpenter bee populations. It may help to remove any piles of lumber that are near the orchard that could provide nest sites for carpenter
180 Chapter 24
Table 24.3. Recommended bee densities for blueberry.
No. of honey bee hives acre21(ha21) Reference 1, 5, 10 (2.5, 12, 25) McGregor (1976)
2–5 (5–12) McCutcheon (1983)
1 (2.5) Krewer et al. (1986)
3–4 (7.4–10) Levin (1986)
2 (5) Kevan (1988)
0.5, 0.8, 2 (1.2, 2, 5) Free (1993)
4 (10) Williams (1994)
1–4 (2.5–10) Scott-Dupree et al. (1995)
3 (7.5) Literature average
Other bees
300 O. ribifloris females acre21(741 ha21) Torchio (1990b) 1–4 bumble bees or southeastern blueberry
bees per bush Cane (1993)
20,000 alfalfa leafcutting bees acre21
(49,420 ha21) Stubbs and Drummond (1997)
bees. If bees are nesting in wooden sheds, one can control them by injecting an approved insecticide inside individual tunnels, plugging the holes, and painting the wood surfaces.
Honey bees are the most numerous bee visitor in blooming rabbit- eye blueberry in south Georgia, USA, followed in descending order by bumble bee queens, bumble bee workers, carpenter bees, and south- eastern blueberry bees. Percentage of bees collecting pollen is highest with bumble bee workers (76.3%), followed by southeastern blueberry bees (60%), bumble bee queens (38%), honey bees (3.2%), and car- penter bees (1%). Bumble bee queens and workers carry the highest percentage (70.2%) of Vaccinium spp. pollen on their bodies, fol- lowed by honey bees (67.7%), southeastern blueberry bees (58.1%), and carpenter bees (29.5%). Thus, bumble bees and southeastern blue- berry bees are more diligent pollen collectors in rabbiteye blueberry, but even honey bees and carpenter bees can carry Vaccinium spp.
pollen on their bodies (Delaplane, 1995).
Blueberry 1811
This chapter covers the crucifers (Brassica oleracea) or cole crops – cabbage, cauliflower, broccoli, Brussels sprouts, kohlrabi, and the kales. Other Brassica species, oilseed rape or canola, are covered in Chapter 26. Cole crops are large-leaved, low-growing plants until the time at which they begin flowering. Most are biennial, with the excep- tion of cauliflower which is annual.
Flowering
The flowering stem elongates after leafy growth stops. The stem has many branches, small leaves, and bright yellow or white flowers. Each flower has four petals, !s–1 in (1.3–2.5 cm) long, that form a cross, hence the family name Cruciferae (‘cross bearing’). Each flower has six stamens; two are shorter than the style and four are longer. There is a single stigma at the tip of the style (Fig. 25.1). The flower opens in the morning, but the anthers do not release pollen until a few hours later.
Nectar is excreted at the bases of the short stamens and ovary. The flower is open for three days, and bees visit the blossoms for nectar and pollen.