Pests and Diseases of the Mason Bee 

Mason Bees, along with all bee species are in decline, there is evidence that much of this decline is man-made and not from natural causes.  Bees have been in constant decline since the introduction of intensive farming practices in the 1950s through to today's continued advance in chasing maximum growth from small areas.  Add to this the destruction of the hedgerows and woods where the pollinators used to thrive, the persistent use of pesticides, fertilizers and herbicides we are now facing a decimation of the bee population, as never seen previously.


This destruction is not only affecting the mason bee, but there is also plenty of evidence that the decline is happening in other bee species, beetles, bugs, ants, flies, and insects. You can evidence this decline yourself. If you recall your youth, you will remember  the number of bugs and insects that would splatter on the windscreen of your car or school bus as you traveled, now try to recall the last time a bug hit your windscreen in recent years. It seems it is increasingly becoming an extremely rare occurrence.


It is important to understand that YOU can influence the decline in your area. You have the power to substantially boost mason bee populations in your own back yard, and the factors affecting decline can be reduced by careful management of bees and cocoons.


The causes of decline are manifold and we list here the most important, including what we, as individuals, can do to participate in reversing this devastating decline.


Causes of Decline


Although populations of wild mason bees are in decline, current research has not identified any single reason. Declines in mason bees have taken place since the 1950s and a number of factors are to blame, including the change in farming practises in favour of high intensity chemical dependent crops and the loss of naturally occurring habitat

Natural Parasites and Deceases


Within their natural habitat, the mason bee can host a variety of deceases and parasites. These tend to attack individual cocoons or bees and, since the mason bee is not a social insect it will rarely travel to another bee or nest and hence protect the larger bee population. However, the impact of the parasite or disease is greater on the population due to the reduced numbers of egg-laying females who may have succumbed to disease.


Mason bee Parasites 


The pollen mite is often found in damp areas where the bees live. The mite is picked up by the bee whilst in the act of pollination and taken back to the hole where it attaches to the pollen collected by the bee for use by the egg and larva.  The mite will starve the bee larvae by consuming the pollen collected and stored by the female. 


The Sapygid wasp is a family of wasps that also plants its eggs in the mason bee nesting area and will feast on the pollen meant for the developing larvae, with the wasp's larvae feeding on the pollen.


There are several parasitic wasps that attack the mason bees by piercing the larva inside the nest and inserting eggs directly into the body; the wasp larvae consume the bee larva/pupa from the inside. The large yellow and black Leucospis wasp can be a major treat to the mason bee and will lay its eggs inside the larvae. Also the Monodontomerus wasp is extremely populous and can be a serious pest to the northern families of mason bee.  


Mason Bee Diseases


There has been limited research on the impact of the effects of natural diseases on Mason bees. Native and non-native insects, mites, and fungal pathogens are known to cause mortality in immature stages. It is also known that the decrease of honey and bumblebees have been identified as a threat to native wild pollinators. There is also evidence that moving mason bees outside their known areas of living and introducing them to new areas risks the spread of pathogens that could negatively affect the local, native, species. So it is important that you buy your bees locally and keep the local to reduce the impact of diseases.


Man-Made decline


Alongside many other native plant and animal species, habitat loss and degradation due to man's intervention are likely important causes of the Mason bee decline. Because individual bees range over very small areas (up to one hundred meters/yards) and require a varied habitat with features such as trees, dead branches, rocks, and spring-blooming flowers, it is easy to attribute population declines to habitat loss. Bees could theoretically persist in hedgerows between crops or suburban gardens. However, a study of other native bees indicated that crop pollination rates decrease substantially with agricultural intensification and pesticide use.


Since the 1950’s the population of the earth has increased exponentially with the result that the necessity for more food has led to the spreading of the worldwide ecosystem for food production growing to cover 35% of the world's ice-free land. This is now larger in scale than the forest and wooded areas of the world.  The result is that farmers now destroy the hedgerows and small cops and forests to make way for more food production. Add to this the growth in meat consumption and the vast areas of land required for the meat production areas and the unintentional destruction of the ecosystem has resulted in areas of hundreds of square miles/kilometers with no hedgerows and only one or two trees, winds and tornadoes whip across these areas causing devastation as there is nothing to slow them.


Invasive alien plants


Invasive plants are a widespread cause of habitat degradation in Canada, and will often outcompete native plants that provide the nectar and pollen resources needed by mason bees. There is some relief in this invasion, the Mason bee will often forage for nectar and perhaps pollen on these invasive species to the benefit of their survival. The full effect of the introduction of non-native species into our gardens and farms is, as yet, unknown and what its impact on the Mason bee and other pollinators is yet to be assessed. Whatever the answer is it will not be good.


Pesticides —Mason bees inhabiting your garden and agricultural areas are likely to come into contact with pesticides used for insect and weed control on flowers, vegetables, trees, plants, and crops. Because Mason bees are fully active outside of their nests during the spring and very early summer, they are unlikely to be directly affected by spraying that occurs at other times of the year. However, our preventative systemic pesticides sprayed early in the season to prevent caterpillars, greenfly and other pests that become persistent in plants will be transferred to bees via the pollen or nectar they collect and put into the nest with the larvae. Negative impacts on bees from pesticides are a major concern, both within your home and the wider community. This is the single biggest threat to the bees in your area and directly affects your bee condominium or Mason bee house. The threat of pesticides is real and negatively affects all pollinators in your garden. The changes you make in this area are the biggest you can make to encourage the Mason bee into your garden and make it a success. Please see our chapter ‘What you can do to attract Mason bees’ for more information on the positive changes you can make to encourage bees to your garden or farm.

Sprays that are used to specifically target other species of insect or bug can negatively affect the Mason bee.  However, there are some sprays that are not deemed to be a threat at this time. Sprays containing Bacillus thuringiensis (also known as Bt or Btk) are specific to gypsy moth or other spring caterpillars and are unlikely to impact mason bee adults or larvae. 

Use of neonicotinoid pesticides is a growing area of concern worldwide because of their potential lethal and sublethal effects on Mason, and honey bees, native pollinators, and insects in general, in addition to its effects on humans when delivered in large quantities. These pesticides are sprayed on raspberries and fruit trees where they become systemic in the plant and impact the nectar and pollen. Hunting Mason bees encounter these pesticides when visiting the flowers and trees of treated crops. Although the dosages found in the plant will not necessarily affect humans the quantity in nectar and pollen is likely to cause neurological impairment affecting memory and such behaviors as foraging and navigation in the bee, which in turn affects reproductive success (Kessler et al. 2015). In one study performed on mason bees, the neonicotinoids clothianidin and imidacloprid were found to be highly toxic to the bee. Due to the widespread use of these pesticides and the subtle but significant effects on bees, neonicotinoid pesticides pose a threat to mason bees and have been recently shown to reduce or eliminate mason bee nesting under field conditions, possibly due to impaired navigation. Solitary bees, such as Mason bees were found to be more affected than more social species such as Honey bees.


Climate change —Climate change will undoubtedly affect some species of Mason bee. The increasingly warm weather throughout the whole year, after adults develop, can increase pre-winter fat depletion and decrease fitness in spring. The impact of this significant change means that the Mason bee larvae develop quicker, have shorter gestation and therefore cannot perform well on exit from its chamber. The early release of the bee from the chamber affects its navigation and it loses its ng for its chosen nesting site with the result that the nest is never completed and no egg deployed. 

As temperatures rise the Mason bee will either, become extinct, or, at least in the initial stages, move North to more suitable climes with the result that the bees will disappear from their traditional nesting areas. 


An increase in prolonged droughts - across the world will result in a reduction of pollinators and affect the production of crops and their pollination.


Intrinsic Threats and Vulnerabilities


Intrinsic vulnerability —Mason bees have three significant vulnerabilites that affect their production of teh next generation.

Firstly, the female lays very few eggs in their lifetime. The avearge female lays 25-30 eggs and with the known threats outlined elsewhere in this chapter, there is every likelyhood that many of these will not survive the winter.

Secondly, the female only lays in one cycle per year and this is completed in a very short period, March to June, resulting in nine months of the year where the next lays undefended and vulnable.

Finally, the Mason bee has a very short range, (100m/100yds), resulting in them veing vulnable to threats such as severe weather changes and the application of pesticides.