Mr. Rémy Vandame has just published an eloquent thesis dated 18 December 1996 at the Université Claude Bernard-Lyon 1, Institut d’Analyse des Systèmes Biologiques et Socio-économiques, Lyon France. It is entitled: The Importance of Hybridization in Host-Parasite Tolerance (my translation of “Importance de l’hybridation de l’hote dans la tolérance à un parasite”). The work is subtitled “the case of the parasite Varroa jacobsoni in European and Africanized bee races in the tropical humid climate of Mexico,” (again, my translation).
This thesis is extremely well written and fully referenced. Its conclusions are provocative; they help us focus on some of the conundrums that still exist concerning European and Africanized honey bees in the Americas and their relationships with Varroa mites. I had the pleasure of talking to Mr. Vandame in English, Spanish and French here in Aix-en-Provence. He has now returned to Mexico to continue his studies.
Mr. Vandame’s full dissertation is 112 pages long in French, but it has both English and Spanish summaries. I do not have room to go into much detail and do not want to compromise the publishability of the work in scientific journals. Thus, I will emphasize the major conclusions and hypotheses only.
According to Mr. Vandame, honey bees may be tolerant to Varroa for several reasons found in the literature. He lists them to show that tolerance may be conferred by one or all, and discusses the controversy that exists about their relative importance:
1. Attractiveness of brood–Only drone brood is parasitized in the original host, Apis cerana, but in Apis mellifera both drone and worker brood are susceptible. The relative attractivity of the brood, therefore, may determine the level of parasitization.
2. Brood postcapping time–The longer brood is capped, the more mites produced. Apis cerana has a shorter postcapping time than does Apis mellifera, conferring tolerance. However, Mr. Vandame says that at least two studies provide evidence that breeding bees for a short postcapping period is not a useful strategy.
3. Brood temperature–Although not proven to everyone’s satisfaction, lower brood temperatures, Mr. Vandame says, might reduce Varroa reproduction in tolerant bees. However, this low temperature also increases bee development time, and so the results may be offsetting.
4. Varroa female infertility–Lower mite female fertility has been found in Brazil, New Guinea, Japan and Vietnam; this is probably the best known and most recognized factor in all cases of tolerance found to date. This could mean different Varroa subspecies might exist, some more adapted to reproducing on Apis mellifera than others.
5. Grooming behavior–It is known that Apis cerana has both a strong self and social grooming behavior which reduces the mite load. Apis mellifera from various areas (Brazil, Slovenia) also has demonstrated variability in this character. Unfortunately, groomed mites don’t necessarily die and may get right back on another bee, reducing the effectiveness of this factor.
6. Removing parasitized brood–This hygienic behavior is presumably responsible for resistance to other diseases (American foulbrood and Chalkbrood). Results from Brazil and other areas show variability in this trait.
[Editor’s note: I discussed with Mr. Vandame, another possible reason that tolerance is observed, the capacity of AHB to abscond as a migratory swarm, a kind of self-nest-cleansing behavior.]
Mr. Vandame asks two questions in his thesis: (1) Are Africanized honey bees in Mexico tolerant to Varroa? and (2) if so, are some of the same mechanisms seen in Brazil and elsewhere responsible? It is important to recognize the difference between “resistant” and “tolerant,” according to Mr. Vandame. The original host for Varroa, Apis cerana, tolerates a modest population of mites and, therefore, this is the preferred term.
[Editor’s note: This appears to be an academic and semantic issue. Increasingly, however, the term tolerant is used over resistant in agreement with the current trend in parasitology literature and vocabulary.]
Mr. Vandame carried out his experiments in 1995 at the Colegio de Postgraduados, Córdoba, Veracruz, Mexico. He chose a total of twenty colonies, ten Africanized (AHB) and ten European (EHB), selected using morphometrics from some forty on the campus that had not been treated for Varroa for one year. In the course of the study, three of the AHB colonies absconded in August during the rainy season and were replaced by colonies with the same degree of Africanization and Varroa infestation. Several EHB colonies, instead of becoming simply weaker during the slowest period of the year in January, actually were in danger of collapsing due to Varroa infestation, and had to be reinforced with brood from other colonies.
Mr. Vandame found that AHB occupied an average of 23,686 brood cells throughout the year (plus or minus 6,342) with about 28,423 bees/colony, whereas EHB occupied 16,575 cells (plus or minus 7319) with 19,891 bees/colony. Besides superiority in numbers, AHB colonies also stored almost twice as much pollen and honey. The Varroa brood infestation rate for EHB was almost twice that of AHB. Mr. Vandame also measured the number of adult Varroa (phoretics). EHB averaged 2,835 mites/19,891 bees (14.2 percent) while on AHB there was an average of 1,513 mites/28,423 bees (5.3 percent). The difference between the two populations’ development, according to Mr. Vandame, is most probably related to their responses to Varroa infestation. The data reveal a greater tolerance in the AHB population than the EHB one in Mexico. He cautions that to show differences in other factors (population buildup, honey production), experiments must be carried out on both populations, after being treated for Varroa.
Other results found by Mr. Vandame include a higher natural mite mortality in AHB brood and the fact that female mites find EHB brood twice as attractive as that of AHB. The possible reasons for the tolerance found in Mr. Vandame’s study with reference to those listed above, he summarizes as follows: fertility : no; postcapping duration : no; brood attractivity : maybe; grooming behavior: no; removal behavior : perhaps). He says it is remarkable that both AHB in Brazil and Apis cerana have the same Varroa tolerance level of two mites/100 bees (two percent) and suggests that the Mexican AHB may also finally reach this equilibrium point in the future. He concludes this could happen as it did in South America through the hybridization process and awaits the outcome with anticipation.
[Editor’s note: The amount of hybridization that actually takes place has always been clouded by the controversial issue of how Africanized bees are identified. Dr. Glenn Hall’s DNA work at the University of Florida has suggested that hybridization does not occur as readily as might be supposed from other identification techniques. One piece of evidence to support this is that Brazilian mitochondrial DNA has been found in many Central American and Mexican mother lines. Another is that metabolic measurements have indicated a possible incompatibility in the hybridization process. Identification using only morphometrics, the official and most used method, however, cannot recognize as components of the process either metabolic rate or DNA makeup. Finally, the amount of natural hybridization attained will be very much be influenced by the active human beekeeping population. The Mexican government and private queen producers are helping beekeepers to re-queen regularly, keeping much of their stock European in nature.