THE SMR TRAIT EXPLAINED BY HYGIENIC BEHAVIOR OF ADULT BEES

Authors: Harris, Jeffrey; Harbo, John

Submitted to: American Bee Research Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 4/15/2005
Publication Date: 5/1/2005
Citation: Harris, J.W., Harbo, J.R. 2005. The SMR Trait Explained by Hygienic Behavior of Adult Bees. American Bee Journal 145(5):430-431

Interpretive Summary: Over the past several years we bred varroa resistant honey bees by selecting colonies that maintain low levels of reproductive varroa mites. We termed the heritable characteristic of bees that produces this effect the “suppression of mite reproduction” (SMR) trait because we thought that the bees increased the infertility of mites. However, recent investigations showed that SMR bees were hygienic and suggested that SMR bees remove varroa-infested pupae from capped brood cells. We tested this possibility by transferring naturally infested combs of capped brood from source colonies into SMR and control colonies. At least one comb from each of 7 source colonies went into each type of recipient colony (9 SMR and 8 control colonies). Brood cells were capped about 1 day before the transfer and examined for mite reproduction about 8 days later. We found 91% fewer pupae with reproductive mites in combs given to SMR colonies than in combs given to control colonies. Similarly, combs from SMR colonies had 58 % fewer pupae with mites that had nonviable offspring (i.e. female offspring that cannot become adults before the host bee emerges from the brood cell). There was no difference in the number of pupae with mites that did not lay eggs between the two types of recipients. Thus, SMR bees selectively removed mites that produced offspring from capped brood cells.

Technical Abstract: Harris, J.W. & J.R. Harbo – The SMR trait explained by hygienic behavior of adult bees - We bred varroa resistant honey bees by selecting colonies with low percentages of reproductive mites (Harbo & Harris, 2001, J Econ Entomol 94: 1319-1323). The trait causing this effect was termed “suppression of mite reproduction” (SMR) because we thought that the bees were increasing mite infertility (Harris & Harbo, 2000, Apidologie 31: 689-699). Ibrahim and Spivak (2004, ABJ 144: 406) found that SMR bees were hygienic and were able to remove varroa-infested pupae from capped brood cells. They suggested that SMR bees may selectively remove pupae with reproductive mites. We tested this hypothesis by transferring combs with naturally infested and recently capped worker brood from 7 source colonies into control and SMR colonies. Combs from source colonies had an average infestation of 12 ' 8 mites per 100 capped worker cells and 71 ' 18 % reproductive mites (mean ± SD) a few weeks before the test. At least 1 comb from each source was transferred into each type of recipient. Immediately prior to this test, the control recipients had 80 ± 10 % reproductive mites, while the SMR recipients had 13 ± 6 % reproductive mites (mean ± SD). The infestation rate and percentage of reproductive mites were measured for 17 transferred combs after 7-9 days in recipient colonies. We defined three types of reproductive success for mites: (1) reproductive mites produced an adult daughter (including unmated daughters) before the host emerged, (2) mites with nonviable offspring may have produced daughters, but no daughter matured before the host emerged, and (3) nonreproductive mites did not lay eggs. Fewer mites were found in combs given to SMR colonies than in combs given to controls (Table). This suggests that SMR bees hygienically removed infested pupae. Combs exposed to SMR bees had > 90% fewer pupae with reproductive mites. They also had 58% fewer pupae with mites that had nonviable offspring. The number of nonreproductive mites was equal in the two groups, which suggests that SMR bees did not remove mites that did not lay eggs. Table – Comparison of varroa mites from combs that were transferred into control and SMR colonies. Newly capped cells of naturally infested brood were transferred, and mite reproductive success was evaluated 7-9 days later by examining cells containing pupae that were 3 days from emergence. Variablea Combs into control colonies (n=8) Combs into SMR colonies (n=9) Comparison of Least Squares Means t dfb Pr > |t| Percentage of reproductive mites 71 ' 5 % 20 ' 5 % 7.78 8.05 < 0.0001 No. reproductive mitesc 6.4 ' 0.7 0.4 ' 0.6 6.62 15 < 0.0001 No. mites with nonviable offspringc 1.2 ' 0.2 0.5 ' 0.2 3.18 15 < 0.007 No. nonreproductive mitesc 1.0 ' 0.2 1.0 ' 0.2 0.24 15 > 0.8 No. dead mitesc, d 0.4 ' 0.1 0.2 ' 0.1 1.60 15 > 0.13 Total infested cellsc 9.0 ' 0.9 2.2 ' 0.8 5.55 15 < 0.0001 a. Least squares mean ± SE for each group of combs. b. Degrees of freedom were estimated using the Kenward-Roger method (Proc Mixed, SAS Institute). The source of brood combs was a random effect in the analysis of variance. c. Values reported as number of mites per 100 capped worker brood cells. We examined 315 ' 35 and 563 ' 180 (mean ' SD) capped worker cells in each comb that had been transferred into control and SMR colonies, respectively. d. All dead foundress mites had no progeny.