Some honey bees (Apis mellifera) perform a behavior that consists of uncapping cells containing dead, sick or damaged brood and removing this brood from the colony. Rothenbuhler (1964) termed this behavior hygienic behavior, and he was the first to study the genetics of this trait. He determined that hygienic behavior is regulated by two pairs of recessive genes; the uncapping gene ”u” (which controls the opening of brood cells) and the removing gene “r” (which controls the removal of the affected brood). Moritz (1988) suggested that the genetic determination of the hygienic behavior probably is more complex, because removing brood from the cells would be controlled by more than two loci, perhaps three. Gramacho (1999) presented a new hypothesis in which the control of this behavior could be explained by three recessive genes (d1/d1, d2/d2 = uncapping and r/r = removal). However, neither hypothesis has been tested. Recently, Lapidge et al. (2002), using molecular techniques, have suggested that seven loci are involved in hygienic behavior.
Many authors have demonstrated that hygienic behavior is a natural mechanism of resistance to American foulbrood (Rothenbuhler, 1964; Taber, 1982; Spivak and Reuter, 2001) and chalkbrood diseases (Milne Jr., 1983; Gilliam et al., 1989; Spivak and Gilliam, 1993). Some breeding programs have selected for this trait to increase the frequency of the behavior in honey bee populations (Rothenbuhler, 1964; Taber, 1982; Spivak and Gilliam, 1993; Palacio et al., 2000).
Most research on hygienic behavior has recorded the percentage of dead brood uncapped and removed by bees after a certain time. Assays for this behavior are often tested in full-size colonies and results are evaluated after one or two days. Two main methods have been used to elicit hygienic behavior: brood killed by freezing (Gonçalves and Kerr, 1970; Spivak and Reuter, 1998) and brood killed by a pin or “pin-kill test” (Newton and Ostasiewski, 1986; Gonçalves and Gramacho, 1999; Palacio et al., 2000). Both methods have been tested by Gramacho (1995) and Palacio et al. (1996) and have been considered efficient to test this behavior. However, the pin-kill test is both practical and cheaper.
Gramacho (1999) studied the different stages of hygienic behavior in full-size colonies at intervals of 2 h after killing the brood by the pin-kill test over 48 h, and recorded the following variables: number of capped cells, empty cells, punctured cells, uncapped cells, and cells with partially removed brood. She suggested that cell uncapping was preceded by the bees poking a hole through the cell capping, and found that hygienic (H) and non-hygienic (NH) colonies performed the stages of uncapping and removing at different rates. It became apparent that shorter intervals can provide more discriminatory evaluations of hygienic behavior than the normal evaluations made after 24 or 48 h. Nevertheless, opening the colonies every 2 h can disturb the bees, and these manipulations may affect the results of the evaluations. Consequently, we evaluated the time that H and NH honeybees take to uncap and to remove dead brood in observation hives after the pin-killing test.
MATERIAL AND METHODS
One hundred and eight colonies were evaluated for their total hygienic behavior (THB) (Palacio et al., 2000) using the pin-killed test (Newton and Ostasiewsky, 1986, modified by Palacio et al., 2000). All the capped cells present in an area of 10 cm x 5 cm were counted (x). Later, capped brood cells were perforated using a pin to kill the brood. The treated comb was replaced in the original colony and after 24 h the number of uncapped cells with dead brood inside (z) and the number of cells that remained capped (y) were recorded. THB was calculated as the number of cells of dead brood that were removed by the honey bees divided by the total number of cells of brood that had been killed
In order to select the H and NH colonies for this experiment, 108 colonies were tested three times at 15-day intervals and the four colonies with the most extreme values were designated: H colonies (H1 and H2) and NH colonies (NH1 and NH2). The H colonies had an average THB score of 100% and the NH colonies had an average score of 40%.
A standard deep, Langstroth size comb containing sealed worker brood covered by honey bees was removed from each colony and was placed inside an observation hive. A new queen was introduced in each hive and observations began when the queen was free and the colony appeared normal.
Modified observation hives were used to avoid disturbing the honey bees while perforating the brood. Sixty-five holes (35 mm in diameter) were made in one of the walls of the observation hive and they were covered by small plastic sheets. The plastic sheets were fixed by two sides to the colony wall and they were used as little windows. In this way, it was possible to reach the brood through these windows and handle the colony with minimum disturbance (Figures 1 and 2).
When uncapping and removal were considered together, the distribution frequency was similar for both colonies within groups (H and NH) but differed between groups. Six hours after perforation, all the H colonies had uncapped all the cells (Figure 5) and about 70% of the brood had already been removed (Figure 6). At this time the NH colonies had uncapped about 70% of the cells (Figure 5) and removed 30% of the brood (Figure 6).
This indicates that NH colonies not only uncapped and removed less perforated brood cells than did H colonies, but they took more time to do the job. Differences between distribution frequencies of brood removal were also found to be significant during the first 6 h.
Hygienic colonies uncapped and removed a significantly higher percentage of pin-killed brood than did NH colonies in observation hives. This difference was detected 1 h after the pin-kill test and it was significant for all the time periods. In addition, the time spent by NH colonies to uncap and remove the dead and damaged brood was longer than in H colonies. It is possible to discriminate between colonies that show extremes in hygienic and non-hygienic behaviors within 1 h using observation hives.
The mean percentage of totally removed dead or damaged brood 6 h after perforation of the brood by H colonies was 64%, while in NH colonies it was only 26%. Gramacho (1999) recorded values between 1 and 31.6% for removed dead brood 6 h in four different colonies. We found higher rates of removal for both H and NH colonies.
When we consider the data obtained after 24 h, NH colonies uncapped 90% of the dead or damaged brood but they only removed 53%. At this time H colonies had uncapped 100% of dead brood and removed 99% of them. Though all differences (uncapped and removed cells) were significant between H and NH colonies for each hour, differences were more obvious when the rate of removal was considered. Palacio et al. (2000) also registered a significant increase in total hygienic behavior (which involves uncapping and removal of dead brood) but not in partial hygienic behavior (which involves uncapping and partial removal of dead brood) in a population selected for this behavior. Gramacho (1999) also found that after 2 h all the colonies tested had some cells uncapped and that maximum values for this variable were detected between 4 and 6 h after killing the brood.
Many authors have studied the relation between the expression of hygienic behavior and colony strength. Jones and Rothenbuhler (1964) observed that workers take the same time to remove 2000 or 100 dead larvae from the colony and suggested that honey bees patrolling inside the hive perform cleaning jobs whenever necessary. Spivak and Gilliam (1993) evaluated hygienic behavior in Langstroth hives and in observation hives (containing two frames covered by honey bees). No significant differences were detected in percentages of removed brood in Langstroth hives and observation hives when NH colonies were evaluated, but H colonies removed less brood when they were in observation hives. These authors suggested that hygienic behavior expression was affected by colony strength. We used observation hives with one frame and H colonies removed 99% of dead brood after 24 h while NH colonies had low removal values (53%).
Up to now no conclusive evidence exists about the stimuli that release hygienic behavior. Gramacho et al. (1997) indicated that differences in temperature registered between dead and live brood are recognized by workers. Masterman et al. (1998, 2000) detected differences in olfactory discrimination between H and NH honey bees using the proboscis extension reflex. Gramacho et al. (2003) postulated that sensilla placodea have an important role in enabling worker bees to sense sick brood. They studied the number of these structures in H and NH honey bees, but did not find significant differences.
We conclude that honey bees are able to detect affected brood during the first hour after killing with the pin-kill test. It is possible that when brood cell is experimentally perforated dead brood odor is more easily detected by hygienic bees, than dead or dying brood with unperforated cappings. We agree with Gramacho (1999) who concluded that it is not necessary to wait 48 h to test the hygienic behavior of a colony because 24 h after the pin-kill test all the punctured brood are already removed. When the pin-killing test is used in observation hives it is possible to evaluate hygienic behavior within a few hours after perforation.
Research supported by the Argentine National Agency of Scientific and Technological Promotion and International Foundation for Science (Grant No. 2756/1). We thank Claudia Forte for her collaboration.
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