5 RESULTADOS
5.4 D IFICULTADES DEL ANÁLISIS Y AJUSTE DE CATEGORÍAS
number of patrilines and of genetics re- latedness in honeybee colonies. Proceed- ings of the Royal Society of London, Biological Sciences 258: 1-7
11 Koeniger N., G. Koeniger, and M. Mardan. 1994. Mimicking a honeybee
queen? 9HVSDDI¿QLVLQGRVLQHQVLV (Perez 1910) hunts drones of Apis cerana F. 1793. Ethology 98: 149-153
12 Yue, C., Schrober, M., Bienefeld, K., Genersch, E. 2006. Detection of
viral sequences in semen of honeybees (Apis mellifera): Evidence for vertical transmission of viruses through drones. Journal of Invertebrate Pathology 92: 105-108
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ollowing almonds, the biggest U.S. pollination event, honey bee colonies may go to apples or avocados. Avo- cado culture in the US is concentrated along the lower California coast and in Florida. In CA, avocado trees bloom after almonds so many CA beekeeper colonies are transported directly to pollinate the estimated 57,000 acres of avocados, raised by some 6000 JURZHUV DOO ZLWKLQ PLOHV RI WKH 3DFL¿F Ocean from San Luis Obispo to San Diego.Avocados, once mainly consumed by latino immigrants in the US, is enjoying a worldwide boom in food popularity. Cali- fornia supplies 90% of U.S. avocado con- sumption, a market value worth over 1/3 of a billion dollars annually. However, intense housing pressure (many people desire to live within 5 miles of the southern CA coast) se- verely limits expansion of avocado culture within the U.S. The fruiting process is not tolerant of freezing conditions, a limiting factor as to where avocados can be grown.
Imports are needed to meet the growing U.S., European and Asian market demand. The U.S. imports used to come from Chile, but Chilean growers now seek to supply their growing domestic market and expand- ing nearby South American markets. Mex- ico, the largest European market supplier, too faces intense housing pressure to expand their growing acreage. Peru now is the 3rd
largest avocado exporter (after Mexico and U.S.) in the world.
Avocado (palta in Spanish but called aguacate in Mexico, where it is native), has been grown in South America from be- fore pre-Colombian times. In nature, it is a shrubby tree that blooms in early spring (in California in mid-March, blooming after almonds and about same time as citrus). Avocados, botanical name Persea ameri-
cana, are members of the family Lauraceae,
which includes the plants that produce ed- ible cinnamon, camphor, sassafras and the herb, laurel (bay leaves).
Avocado presents a pollination challenge. $YRFDGRÀRZHUVRSHQIRURQO\GD\V$S- SUR[LPDWHO\WRQHZÀRZHUV open each day/tree; mature trees can pro- GXFHDPLOOLRQÀRZHUVGXULQJD GD\ VSULQJVHDVRQÀRZHULQJSHULRG7KHLQFRQ- VSLFXRXVJUHHQLVK\HOORZÀRZHUVRFFXULQ panicles of several dozens to hundreds of ÀRZHUV
:KHQDQDYRFDGRÀRZHU¿UVWRSHQVLWLV LQWKHIHPDOHSKDVH$VDIHPDOHWKHÀRZHU is open for 2 to 4 hours with the stigma UHFHSWLYH WR SROOHQ 7KHQ WKH ÀRZHU ZLOO FORVH2QWKHVHFRQGGD\WKHVDPHÀRZHU re-opens in the male phase and sheds pollen. 7KLVÀRZHULQJEHKDYLRULVNQRZQDV³V\Q- chronous dichogamy”.
Different avocado varieties have different timing of the male (pollen-producing) and female (stigma-receptive) phases, referred WRDV³$´DQG³%´ÀRZHUW\SHV³$´YDULHW- LHVRSHQDVIHPDOHÀRZHUVRQWKHPRUQLQJ RIWKH¿UVWGD\7KHÀRZHUFORVHVODWHPRUQ- ing or early afternoon, and remains closed until the afternoon of the second day when it re-opens as male.
“B” varieties, in contrast, open as female RQWKHDIWHUQRRQRIWKH¿UVWGD\FORVHLQODWH afternoon and re-open in the male phase the IROORZLQJPRUQLQJ7RWDOÀRZHULQJF\FOHRI Type A is 36 hours and for Type B trees is 24 hours, due to different hours of closure WLPH 7RWDO RSHQ WLPH RI D ÀRZHU PD\ EH as little as a couple of hours upwards to 6 hours, split between the two phases. Flow- ers are open during cool, foggy mornings and with strong afternoon breezes, common spring weather conditions along the coast.
,Q QDWXUH WKH WZR DYRFDGR ÀRZHU W\SHV complement each other. Both are function- DOO\IHPDOHRQWKHLU¿UVWGD\DQGIXQFWLRQ- ally male on their second day, but differ in time of day that they are male and female. A variety of one type produces pollen (func- tionally male) when a variety of the other type is receptive (functionally female). Na- ture has it worked out.
And now the rub. Avocado growers aren’t interested in growing just any avocado. Type A avocado variety ‘Hass’ (also called California avocado, where this variety was developed) commands 80% of the market, thanks to a hard skin that makes transport/ export feasible. ‘Hass’ trees produce me- dium-sized (150–250 g or 5.3–8.8 oz), ovate IUXLWZLWKDEODFNSHEEOHGVNLQ7KHÀHVK KDVDQXWW\ULFKÀDYRUZLWK RLO
Type B varieties (termed “greenskins” – also Florida avocados) are not hard skinned, are blander in taste, have less oil and are
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higher in calories and often larger in size. They do not command the same market value nor are they as easily transported/ex- ported. They are largely consumed locally. Commercial avocado growers seek to opti- mize planting of Type A ‘Hass’, but must also plant pollinizer trees of Type B; ‘Zu- tano’ and ‘Ettinger’ and ‘Fuerte‘ are three of the common greenskin varieties grown of some 100+ varieties
Three types of pollination
There has been considerable differences of opinion as to the pollination needs of av- ocado. Three types of pollination are recog- nized in avocado. Cross-pollination occurs ZKHQSROOHQLVPRYHGIURPPDOHÀRZHUVRI 7\SH$WRIHPDOHÀRZHUVRIDFRPSDWLEOH Type B and vice versa (i.e. Type B to Type A). Growers question if planting a pollinizer 7\SH % WUHH SURYLGHV HQRXJK EHQH¿W WR compensate for loss of a ‘Hass’ production tree it has to replace?
A second pollination, close-pollination, RFFXUV ZKHQ SROOHQ IURP D PDOH ÀRZHU lands on the receptive stigma of a female ÀRZHURIWKHVDPHWUHH7KLVPD\RFFXUGXU- LQJGDLO\RYHUODSRIPDOHDQGIHPDOHÀRZHU- ing stages or between different trees of the same cultivar.
The third pollination mode seen in avo- cado is self-pollination. During the male VWDJHÀRZHUSROOHQPD\IDOORQWRDVWLOOUH- FHSWLYHIHPDOHÀRZHUVWLJPDZLWKLQDVLQJOH ÀRZHU6WLJPDVRIÀRZHUVGU\TXLFNO\RQFH they switch to the male phase and begin to produce pollen, but under more humid mi- cro-conditions there may be some overlap.
So is cross-pollination necessary?
Numerous studies from several countries have shown increased yield when Type A trees are close to Type B. Using isozymes and genetic markers, a 2000 study in Cali- fornia showed a 4 year yield/tree average of 205 fruits of Type A ‘Hass’ when Type B trees were adjacent. When the Type B trees were 5 rows away, the average yield was 90 ‘Hass’ fruits/tree and when 15 rows away, only 54 fruits/tree. A cage study excluding bees resulted in 5 fruits in Type A ‘Hass’ variety and 4 in ‘Zutano’ (Type B). Adding a small nuc to the cage yielded 284 fruit/tree in ‘Hass’ and 120 in ‘Zutano’.
A California study showed close-polli- nation could be important when lower tem- peratures (<70 F -20 C) occur during early VSULQJ ÀRZHULQJ DQG GXULQJ WKH PLGGOH RIWKHGD\ DWWKHWLPHÀRZHUVDUHFKDQJ- ing from female to male). Small amounts
of pollen collected by honey bee foragers IURP PDOH ÀRZHUV FDQ VWD\ RQ WKHLU ERG- LHVDQGSROOLQDWHIHPDOHÀRZHUVRI7\SH% WKHOHVVH[SRUWDEOHYDULHWLHVZKLFKÀRZHU the same day. Pollen on bee bodies is less OLNHO\WRSROOLQDWH7\SH$ÀRZHUVZKHQWKH PDOHÀRZHURSHQVLQDIWHUQRRQEXWIHPDOHV are not open until the following morning. Pollen transfer might also occur within the hive.
Self-pollination may be more of a fac- tor in homeowner backyard fruits where yield is not of utmost importance or (where mostly Type B) avocado are grown under more humid conditions (such as in Florida). 7KH ¿UVW RI WZR IUXLW GURSV WKDW RFFXU LQ avocado is thought to involve mostly self- pollinated fruits.
Thus, the consensus today is cross-pol- lination is necessary to grow exportable quantities of ‘Hass’ variety. New plantings in Peru and Mexico (grower areas similarly DORQJWKH3DFL¿F&RDVWDQGZKLFKDOVRIDFH heavy housing pressure), utilize a cross- pollination planting scheme.
Such an orchard planting scheme seeks to utilize pollinizer (Type B) variety trees, every 6th tree in every 2nd row, adjacent to
solid rows of ‘Hass’. Studies have demon- strated that most individual honey bees tend to forage in a relatively small radius of 1 to 4 trees and there is a “strong preference” to forage only within a single tree, when trees are allowed to expand and grow tall. Mod- ern plantings use a high density planting of trees spaced 2 meters apart, in rows 5 meters apart, 1000 trees per hectare. With pruning, this creates solid tree walls 4.5 meters (10 feet) high.
+RZHYHU SROOLQDWLRQ LV RQO\ WKH ¿UVW phase toward a harvestable fruit. For effec- tive fertilization, studies have demonstrated IHPDOH ÀRZHUV QHHG PRUH WKDQ SROOHQ grains transferred to the female stigma. When foraging honey bees are washed, they show surprisingly few pollen grains. ,QPHDVXULQJLQ¿HOGDGHTXDF\RISROOLQDWRU populations, the estimate of 10 honey bees per tree (1 minute tree walk-around) has been used. In seems that honey bees do not care for avocado nectar, described as high
in potassium and phosphorous, especially when irrigation is used, which is the norm.
Bumble bees may be effective alternatives to honey bees. The role stingless bees, prob- ably their native pollinators in the original avocado growing range of Mexico and Cen- tral America, may play in pollination has not EHHQZHOOGH¿QHGEXWDMRLQW,VUDHOL0H[L- can study showed as many as 5 genera to be FDSDEOHRISROOLQDWLRQ:DVSVDQGÀLHVPD\ contribute minorly to pollination.
Honey Bees as pollinators of avocado
Managed honey bees are usually the pollinator of choice for large plantings of avocado trees. Large numbers of bees can provide effective transfer of pollen in the orchard. Size of colony foraging population matters. A shortage of bees and/or a lack of IHPDOH SKDVH ÀRZHUV DUH XVXDOO\ WKH ¿UVW things to consider in a poor fruit set.
Pollination colonies for California and Florida avocado plantations average 1-2 colonies/acre (5 colonies per hectare). Some Peruvian and Chilian plantations provide double to triple this number, 8 to15 colo- nies/hectare, depending upon the age of the trees.
Colonies are placed throughout the plant- ing in groups of up to 32 colonies. Place- ment has changed from the margins of the plantings to within and placement toward the opening of rows vs facing a row wall is suggested. The strongest colonies are placed in the best yielding blocks. With this quan- tity of colonies, placement may take place over a two week time period.
In Mexico and southward to (but not in- cluding Chile), the pollinating honey bees are Africanized colonies. Along coastal Peru, not all colonies are Africanized, but WKHUHDUHQRZLQVXI¿FLHQWFRORQLHVWRPHHW the growing demand so the only ‘new’ bees available for a rapidly expanding acreage are Africanized.
Colonies pollinating avocado in Califor- nia have had an earlier start in almonds and some colonies of coastal Mexico, Central $PHULFDDQG6RXWK$PHULFDKDYHEHQH¿WRI DQHDUO\VSULQJÀRZIURP(XFDO\SWXV2XW- side of the US, colonies need be transported over the higher Mountain passes from the selva (=jungle) and often on poorer roads with some colonies having as much as an 18-20 hour trip. Such lengthy transport does not improve the Africanized bees defensive nature and absconding, which Africanized bees do excessively, is a big issue
Some bees go directly into avocado fol- ORZLQJWUDQVSRUWDWWKHVWDUWRIÀRZHULQJ Some beekeepers prefer early transport of bees to the general area of the avocados as a means of relieving the stress of transport and for the opportunity to build colonies to VXI¿FLHQWVWUHQJWKIRUSROOLQDWLRQ,Q&$DQG South America, numerous colonies go to av- ocado plantations well in advance (a month or more) of the bloom period as there is no- where else for them to be placed. In Mexico southward, the avocado plantations often have armed guards to protect the growers’ LQWHUHVWVVRWKHEHHVEHQH¿WIURPWKHSURWHF-
dŝŵŝŶŐŽĨĂǀŽĐĂĚŽŇŽǁĞƌŝŶŐĨŽƌ͟͞ĂŶĚ͟͞ŇŽǁĞƌƚLJƉĞƐ͘
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tion, along with the developing fruit. Bees need to remain on site over 2 months (90 days) as avocado bloom is extended.
A shortage of bee colonies means the mean fruit size and yields per tree are less. $ODFNRIIHPDOHSKDVHÀRZHUVUHVXOWVLQD poor fruit set. Flowers and fruit require a lot of resources from the tree and thus excessive ÀRZHULQJ RI WUHHV RU KHDY\ EHDULQJ WUHHV with lots of developing fruit can mean poor shoot growth required for the following years’ crop. A further complication is that in nature, avocado trees (similar to pistachio and apple) tend toward alternate year bearing, one year heavy followed by lighter fruiting the next.
Temperature/water/hormones in avocado culture
Properly managed trees are given plant hormones to promote proper balance of OHD¿QJ DQG IUXLWLQJ 'HSHQGLQJ XSRQ VL]H and age of tree, 200 harvestable 8 oz (229
g) fruit #48s avocados per tree (‘Hass” variety) is ideal (about 100 lbs). Incredibly this represents only about 0.02 percent fruit VHWVLQFHDWUHHSURGXFHVVRPDQ\ÀRZHUV Trees, grown from rootstock, do not ÀRZHUXQWLO\HDUVRIDJHEXWPD\UHPDLQ productive for 25 years. By 5-6 years of growth, pruning becomes critical, especially with top removals to open up the middle of the trees. A wall of fruit 4.5 meters tall is sought.
Water is required to irrigate the avocado trees. Modern plantings utilize hydrometers imbedded in the soil to monitor moisture levels which need be kept in a delicate bal- ance. Too much moisture promotes growth DW H[SHQVH RI ÀRZHULQJ EXW LI WRR OLWWOH moisture is available, fruit maturation may be delayed. Irrigation is supplied via embed- ded drip irrigation, controlled electronically. Most growing regions have little natural forage to sustain the bees when the avocado LVQRWÀRZHULQJ3URJUDPVVXFKDV$SLV0
American Bee Journal
that recommend planting areas of alterna- tive forage (in case of CA almonds, use of mustards) are being eagerly followed to see KRZ PXFK EHQH¿W WKH\ SURYLGH ,Q &$ D %HH%XIIHUSURJUDP www.beebuffer.com) seeks to enlist avocado grower participa- WLRQ *URZHUV HVWDEOLVK SORWV RI WR DFUHVRIÀRZHUVDEHHEXIIHU]RQHIRUWKUHH years and provide annual feedback on the bee buffer performance. In Peru, alternative forage before and during avocado bloom is under active investigation. The conventional ZLVGRPLVFRPSHWLQJEORRPGXULQJÀRZHU- ing of the target crop is undesirable, but this may not be the case with avocado.
Proper bee care on site
%HH FRORQLHV DUH XVXDOO\ SODFHG E\ WKH beekeeper, not the plantation manager and some continue to manage their colo- nies while on site. It is recommended that colonies be fed sugar syrup before avocado bloom. Initially, on arrival, they may receive D PL[WXUHIROORZHGE\ 7KHFRORQLHV are also supplied with protein. In Peru, the beekeeping managers are investigating the feasibility of feeding bees, even a month prior to arrival. It is usually recommended that colonies NOT be moved into the plan- tation prior to bloom, but often are as ex- plained above. In some seasons swarming can be an issue.
Thanks to Francisco Reyes M., Chile’s largest queen breeder and avocado pol- linator, for assistance with this discussion of avocado and to the avocado growers of Peru for allowing me to consult with them as they seek to improve their avo- cado yield.
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