Jeffrey W. Harris

Breeding for resistance to Varroa destructor in North America

Breeding for resistance to Varroa destructor in North America provides the long-term solution to the economic troubles the mite brings. This review reports the development of two breeding successes that have produced honey bees of commercial quality that do not require pesticide treatment to control Varroa, highlights other traits that could be combined to increase resistance and examines the potential uses of marker-assisted selection (MAS) for breeding for Varroa resistance. Breeding work continues with these stocks to enhance their commercial utility. This work requires knowledge of the mechanisms of resistance that can be further developed or improved in selected stocks and studied with molecular techniques as a prelude to MAS.ZusammenfassungDie Zucht auf Resistenz gegen Varroa destructor in Nordamerika bietet die langfristige Lösung für die von der Milbe verursachten wirtschaftlichen Schwierigkeiten. Dieses Review untersucht mehrere potenzielle Mechanismen der Resistenz gegen Varroa und berichtet über die Entwicklung von zwei Zuchterfolgen, aus denen Bienen von wirtschaftlicher Qualität hervorgegangen sind, die weniger Pestizidbehandlungen gegen Varroa benötigen als unselektierte Bienen.Das VSH Zuchtprogramm konzentriert sich auf die Selektion eines spezifischen Resistenzmechanismus, der Varroasensitive Hygiene genannt wird. Das Merkmal VSH wird über den Verkauf von VSH Königinnen, die mit Drohnen bereits vorhandener kommerzieller Linien gepaart wurden, für die Imker verfügbar gemacht. Die größte Resistenz kommt zwar in reinen VSH-Linien vor, die nachhaltigste Verbreitung wird jedoch durch VSH Hybridvölker erzielt. Durch das Auskreuzen reiner VSH Linien mit einer Vielzahl anderer kommerzieller Linien kann die genetische Diversität der Bienenpopulation in den USA auf relativ hohem Niveau gehalten werden. Reine VSH Zuchtköniginnen werden von Glenn Apiaries produziert und an kommerzielle Produzenten von Königinnen verkauft, die ihrerseits ausgekreuzte VSH Königinnen an Imker verkaufen.Das Programm zur Russischen Biene nutzt ein Zuchtschema, das auf einer geschlossenen Population basiert, um gegen Varroa resistente Linien zu verbreiten, die ursprünglich aus dem fernöstlichen Russland stammten. Die Russischen Honigbienen (RHB) des ARS wurden aus 18 importierten Linien durch Geschwistertests über mehrere Jahre hinweg entwickelt. Ihre Varroaresistenz geht auf mehrere Mechanismen zurück, zu denen gegenseitiges Putzen, varroasensitive Hygiene und für die Milbe geringe Attraktivität der Brut gehören. RHB Linien wurden gleichzeitig für Varroaresistenz, gute Honigproduktion und Resistenz gegen Tracheenmilben, Acarapis woodi, selektiert. Die Resistenz gegen Tracheenmilben trägt zu ihrer ausgezeichneten Überwinterungsfähigkeit bei. Der Erfolg der experimentellen RHB Selektion regte eine große kommerzielle Nachfrage an, und RHB werden zurzeit von einer als Russian Queen Breeder’s Association bekannten Züchterkooperative gezüchtet, vermehrt und an die Imker in den USA verbreitet.Die Zucht auf Varroaresistenz wird in der Zukunft wahrscheinlich auch markergestützte Selektion (MAS) mit einbeziehen, in welcher entweder die Expression von mit Resistenz verbundenen Genen (RNA) oder molekulare Marker, die mit Resistenzgenen in Verbindung stehen (DNA), benutzt werden um die Zuchteltern auszuwählen. Das endgültige Ziel ist, die arbeits- und zeitaufwändige Selektion im Feld durch eine Labordiagnose zu ersetzen. Es wird erwartet, dass MAS den Selektionsfortschritt sowohl für Resistenzmerkmale, die schon entwickelt wurden, als auch für Merkmale, für die diese Entwicklung hin zu nutzbaren kommerziellen genetischen Linien noch aussteht, wie z.B. gegenseitiges Putzen und Entfernen von Milben, beschleunigen wird.

Suppressed mite reproduction explained by the behaviour of adult bees

SUMMARY Suppressed mite reproduction (SMR) is a heritable trait of the honey bee (Apis mellifera) that can control the parasitic mite, Varroa destructor. The purpose of this study was to determine whether adult bees with the SMR trait affect mites in brood after cells are capped. Colonies with or without the SMR trait were each given a comb of newly-capped worker brood that was naturally infested with varroa. Each of 7 source colonies provided a comb of brood to at least one SMR (n = 9) and one control colony (n = 8). These combs were removed from their host colonies 8 days later and mite populations evaluated in cells with bee pupae that were >8 days post-capping. Colonies with SMR bees averaged 2.2% of their cells infested with mites; controls averaged 9.0%. Therefore, bees with the SMR trait apparently removed mites from capped cells. Of the mites that remained, the SMR colonies had a much lower rate of reproductive mites, 20% vs. 71%. This suggests that bees with the SMR trait removed reproductive mites more often than they removed non-reproductive mites. When comparing only the number of mites that produced no progeny, the groups were almost identical averaging 1.2 and 1.3 mites per 100 cells of brood. This suggests that the SMR bees did not remove mites from brood cells if the mites did not lay eggs. By targeting the reproductive mites, bees with the SMR trait give the illusion that nearly all of the mites are non-reproductive. Therefore, our selection for a low frequency of reproductive mites may have produced bees that remove reproductive mites from capped brood.

Responses to Varroa by honey bees with different levels of Varroa Sensitive Hygiene

Summary Mite resistance that we had earlier called suppression of mite reproduction (SMR) is a form of hygienic behaviour that we have named Varroa Sensitive Hygiene (VSH). With VSH, adult worker bees (Apis meiiifera) disrupt the reproduction of parasitic mites (Varroa destructor) by removing mite infested bee pupae from their cells. This study determines which brood cells are targeted by bees with VSH and which are not, and describes the relationship between brood removal and the sudden prevalence of sterile mites. We produced 26 colonies with different levels of VSH by backcrossing 14 queens from a high line, H (100% expression of VSH), and 12 queens from a low line, L (no VSH), to drones produced by an HL (high × low) queen. Because each of the 26 queens was mated to one drone, the resulting colonies were expected to represent the complete range of variability (0 to 100% of the alleles for VSH). To estimate brood removal, we measured mite populations in capped worker brood that was 0–3 days postcapping and again 7 days later when the cohort was aged 7–10 days postcapping. We correlated removal of mite-infested brood with the relative presence (at 7–10 days postcapping) of three classes of foundress mites: (1) viable: those with at least one daughter that could mature before emergence of the host bee; (2) nonviable: those with progeny but with no daughters that could reach maturity; and (3) no eggs: those with dead or nonovipositing foundress mites. As the rate of removal increased, both classes 1 and 2 showed significant declines, whereas class 3 was unchanged. Therefore, oviposition of the mite or something associated with mite oviposition provides the stimulus for bees with the VSH trait to remove mite infested pupae.

Resistance to Varroa destructor (Mesostigmata: Varroidae) When Mite-Resistant Queen Honey Bees (Hymenoptera: Apidae) Were Free-Mated with Unselected Drones

Abstract This study demonstrated (1) that honey bees, Apis mellifera L, can express a high level of resistance to Varroa destructor Anderson & Trueman when bees were selected for only one resistant trait (suppression of mite reproduction); and (2) that a significant level of mite-resistance was retained when these queens were free-mated with unselected drones. The test compared the growth of mite populations in colonies of bees that each received one of the following queens: (1) resistant—queens selected for suppression of mite reproduction and artificially inseminated in Baton Rouge with drones from similarly selected stocks; (2) resistant × control—resistant queens, as above, produced and free-mated to unselected drones by one of four commercial queen producers; and (3) control—commercial queens chosen by the same four queen producers and free-mated as above. All colonies started the test with ≈0.9 kg of bees that were naturally infested with ≈650 mites. Colonies with resistant × control queens ended the 115-d test period with significantly fewer mites than did colonies with control queens. This suggests that beekeepers can derive immediate benefit from mite-resistant queens that have been free-mated to unselected drones. Moreover, the production and distribution of these free-mated queens from many commercial sources may be an effective way to insert beneficial genes into our commercial population of honey bees without losing the genetic diversity and the useful beekeeping characteristics of this population.

Bees with Varroa Sensitive Hygiene preferentially remove mite infested pupae aged ≤ five days post capping

Summary Suppressed Mite Reproduction (SMR) is a trait of honey bees that provides resistance to Varroa destructor. The mechanism of resistance in SMR bees is the removal of infested pupae from capped brood, so a better name is VSH bees (acronym for Varroa Sensitive Hygiene). This study compared the removal of infested brood by VSH and control bees to determine whether VSH bees removed infested pupae of different ages at similar rates. A pair of infested combs containing all stages of pupae were transferred into each host colony (six VSH and six control colonies) for 40 hours. VSH bees removed significantly more (55%) infested cells (singly and multiply infested), than controls (13%). They removed significantly more (66%) singly infested pupae aged from one to five days post capping (cohort A) than did controls (16%). The two types did not differ in the removal of singly infested pupae aged five to 10 days post capping (cohort B) (5–22%). Many pupae were found in uncapped cells at the end of the test, and most of the uncapped pupae were infested with mites. None of the uncapped cells contained prepupae, the development stage occurring during the first three days post capping. Thus, removal of infested pupae may be triggered by stimuli in cells with pupae aged 3–5 days post capping.

Variable Population Growth of Varroa destructor (Mesostigmata: Varroidae) in Colonies of Honey Bees (Hymenoptera: Apidae) During a 10-Year Period

Abstract We measured significant variation in the instantaneous growth rates for varroa mites, Varroa destructor (Anderson & Trueman) from 1993 to 2002 in Baton Rouge, LA. Mite population growth was monitored in colonies of honey bees, Apis mellifera L., with queens from miscellaneous U.S. sources that had not been selectively bred for varroa resistance. Mite populations were measured at the beginning and end of short field tests that started in the late spring of each year. Analyses of multiple regression showed that only the first two of the following regressors were linear predictors of r, the instantaneous growth rate: 1) percentage of reproducing female mites, 2) proportion of total mites in capped brood, 3) mortality of mites in brood cells, 4) growth of the bee population, 5) capped brood area at the end of a test, and 6) duration of the test. Analysis of commonality indicated that the percentage of reproducing female mites explained ≈26% of the total variation in r, and the proportion of total mites in capped brood explained 6%. The joint expression of both variables accounted for another 4%. Thus, residual error reflected most of the total variation in r, which suggested possible climatic or environmental effects on mite growth. The lowest growth rates occurred in three consecutive years of drought in Louisiana. Measures of ambient temperature and relative humidity correlated to growth of mite populations among different years. Reduced growth rates were probably the result of diminished reproductive rates by varroa mites during periods of hot and dry weather.

Honey bees (Hymenoptera: Apidae) with the trait of varroa sensitive hygiene remove brood with all reproductive stages of varroa mites (Mesostigmata: Varroidae).

ABSTRACT Varroa sensitive hygiene (VSH) is a trait of honey bees, Apis mellifera L. (Hymenoptera: Apidae), which supports resistance to Varroa destructor Anderson & Trueman. VSH is the hygienic removal of mite-infested pupa. Bees selectively bred for VSH produce colonies in which the fertility of mites decreases over time. In addition, mite fertility decreases after infested brood is exposed to VSH bees for 1 wk. The purpose of this study was to decide whether the reduction in mite fertility is caused by selective removal of mites that produce offspring. Initially, we monitored changes in a small patch of capped brood during exposure to VSH bees at 2-h intervals through 60 h, which provided a reference for the subsequent experiment. The first test showed that VSH bees uncapped, recapped, and began to remove many pupae in ≈2 h. The approach in the second experiment was to compare the percentage of fertile mites from brood exposed to VSH bees for a 3-h period to the percentage of fertile mites in brood that was protected from hygiene by a screen. There were no significant differences in fertility between mites on pupae that were being removed by the bees and mites on protected pupae. These results suggest that neither egg-laying by foundress mites nor mite offspring are the stimuli that trigger hygienic removal of mite-infested pupae by VSH bees. It may be that hygienic activities such as the uncapping of brood cells inhibits or disrupts reproduction by varroa mites.

Simplified methods of evaluating colonies for levels of Varroa Sensitive Hygiene (VSH).

Summary Varroa Sensitive Hygiene (VSH) is a trait of honey bees, Apis mellifera, that supports resistance to Varroa destructor mites. Components of VSH were evaluated to identify simple methods for selection of the trait. Mite population growth was measured in colonies with variable levels of VSH in two field trials using 24 and 16 colonies. Mite population growth was significantly lower in VSH and hybrid colonies than in control (i.e., unselected) colonies. In resident brood with mite infestations below 5%, the percentage of uncapped pupal cells did not differ significantly among VSH, hybrid and control colonies, but the percentage of recapped cells was highest in VSH colonies (P = 0.03). When brood from more highly infested colonies (9–49% of pupae infested) was introduced for forty hours, VSH colonies reduced infestation more than control colonies (P< 0.01) but final mite fertility was similar (P= 0.12). When infested brood was exposed in colonies for one week, VSH colonies reduced both mite fertility (P= 0.05) and mite infestation (P= 0.02). When highly infested brood was exposed to a subset of colonies for two hours, control colonies uncapped no or few cells while uncapping in VSH colonies was variable but on average was much higher. Mite population growth in individual colonies was negatively correlated with reduced infestation after forty hours of brood exposure and with reduced mite fertility after one week. The simpler and shorter-term measures (relative to measuring mite population growth) of uncapping, recapping, and reductions in infestation and mite fertility may facilitate selection of VSH by more bee breeders.

Functionality of Varroa-Resistant Honey Bees (Hymenoptera: Apidae) when used in Migratory Beekeeping for Crop Pollination

ABSTRACT Two types of honey bees, Apis mellifera L. (Hymenoptera: Apidae), bred for resistance to Varroa destructor Anderson & Trueman were evaluated for performance when used in migratory crop pollination. Colonies of Russian honey bees (RHB) and outcrossed bees with Varroa-sensitive hygiene (VSH) were managed without miticide treatments and compared with colonies of Italian honey bees that served as controls. Control colonies were managed as groups which either were treated twice each year against V. destructor (CT) or kept untreated (CU). Totals of 240 and 247 colonies were established initially for trials in 2008 and 2009, respectively. RHB and VSH colonies generally had adult and brood populations similar to those of the standard CT group regarding pollination requirements. For pollination of almonds [Prunus dulcis (Mill.) D.A.Webb] in February, percentages of colonies meeting the required six or more frames of adult bees were 57% (VSH), 56% (CT), 39% (RHB), and 34% (CU). RHB are known to have small colonies in early spring, but this can be overcome with appropriate feeding. For later pollination requirements in May to July, 94–100% of colonies in the four groups met pollination size requirements for apples (Malus domestica Borkh.), cranberries (Vaccinium macrocarpon Aiton), and lowbush blueberries (Vaccinium angustifolium Aiton). Infestations with V. destructor usually were lowest in CT colonies and tended to be lower in VSH colonies than in RHB and CU colonies. This study demonstrates that bees with the VSH trait and pure RHB offer alternatives for beekeepers to use for commercial crop pollination while reducing reliance on miticides. The high frequency of queen loss (only approximately one fourth of original queens survived each year) suggests that frequent requeening is necessary to maintain desired genetics.

Effect of Brood Type on Varroa-Sensitive Hygiene by Worker Honey Bees (Hymenoptera: Apidae)

Abstract Honey bees (Apis mellifera L.) (Hymenoptera: Apidae) have been selectively bred for varroa-sensitive hygiene (VSH), which is the removal of pupae that are infested by Varroa destructor Anderson & Trueman from capped brood cells. This hygienic behavior is a complex interaction of bees and brood in which brood cells are inspected, and then brood is either removed or recapped. Previous work has shown that VSH bees uncap and remove significantly more varroa-infested worker pupae than nonhygienic bees do, but nothing is known about the reactions of VSH bees to mite-infested drone brood. This study compared the reactions of VSH bees with mite-infested worker and drone brood in a laboratory test and a field test. VSH bees inspected brood cells containing mite-infested pupae of both types of brood, but they removed significantly fewer mite-infested drone pupae than mite-infested worker pupae after 1 wk. This result suggests that mite populations in VSH colonies could increase more rapidly when drone brood is available. Additionally, the percentages of uncapped pupae and uncapped mite-infested pupae were positively correlated to the natural infestation rate of brood after a 24-h exposure, but not after an exposure of 1 wk. This result suggests that the rate of uncapping brood by hygienic bees may depend on the infestation rate, which gradually decreases with longer exposures to bees that remove mite-infested pupae from capped brood.