Wild Bees as Alternative Pollinators
Posted: September 27, 2011
The economic impacts of pollinator shortages on US specialty crops such as fruit, vegetables and nuts could be considerable. Inadequate pollination can reduce crop quality as well as yield in these crops. In apple or pear, pollination efficiency affects seed set which in turn affects size and quality, and hence the profitability to growers. The most conspicuous consequence of honey bee declines since CCD, has been a dramatic increase in the costs of producing bees which translates into rising costs for bee rentals for specialty crop growers – from $35/hive for Pennsylvania apple growers in 2006 to as high as $100/hive this season. Feral honey bees, which many fruit farms relied on to some extent for pollination, have been almost completely eliminated in the last 5 to 10 years due to these mites and diseases. Rising production costs combined with declining yields or quality would lead to higher prices of U.S. nuts, fruits and vegetables which would reduce exports of major commodities during a record US trade deficit and lead to increased imports of cheaper commodities from foreign markets where CCD is not as much a problem.
There are, however, another 3,500 bee species other than the honey bee which are also important pollinators of most specialty crops in the U.S. These include the many species of bumble bees and what are often referred to as solitary bees. We will refer to both groups henceforth as “pollen bees” because their main value, in relation to people, is not the production of honey, but the collection and transfer of pollen for the fertilization of plants. It is obvious that pollen bees are critical components of food webs associated with wildlife habitats of all types in North America, because almost all of them were here long before honey bees were introduced by Europeans. The value of pollen bees as pollinators in U.S. agriculture is conservatively estimated at $3 billion annually. Because of the popular focus on honey bees, however, the services of pollen bees most often go unrecognized and their value for agriculture and especially for unmanaged ecosystems is probably much higher. For most bee species, the lack of long-term population data and our incomplete knowledge of basic life history and ecological roles make assessing their value and possible declines in some regions very difficult. It is well-known that honey bees are not the best pollinators for all crops. They are generalist foragers easily distracted from target crops like cucurbits, pears, and apples by other species such as dandelions and other better nectar sources.
Many projects are dealing with the various threats to the honey bee industry in the U.S., but to truly address the threats to pollination there should be contingency plans that include the development of alternative pollinators and baseline data to measure future impacts on our native bees. The folly of relying on a single pesticide, tactic or cultivar has been seen repeatedly in the development of IPM programs for specialty crops. In the case of pollinators, a similar reliance on one pollinator such as the honey bee is no less a folly. Developing multiple tactics with multiple pollinator species represents the most robust management approach for a future of uncertain climate, environmental disruptions, and invasive species introductions. We do know, however, that: a) the supply of honey bees in the U.S. will not be able to meet the demand for pollination services in the near future; b) that production costs for apiculturists will go up; and c) that the cost to growers to rent honey bee hives will continue to increase.
The importance of native bees in the pollination of fruits and vegetable crops in the Mid-Atlantic region has probably been underestimated. Our unique landscape ecology of agricultural and non-agricultural lands and a mosaic of diversified fruit and vegetable farms in the Northeast and Mid-Atlantic likely impart unique advantages in pollinator conservation and utilization compared to the monocultures of the Midwest or dry areas of the West. A recent study demonstrated a guild of 46 species of native bees provided full pollination of watermelon on >90% of 23 farms in Pennsylvania and New Jersey. Some of the largest fruit growers in Pennsylvania have relied completely on feral honey bees and wild pollinators for their pollination needs for over 5 years now, with no noticeable loss in fruit quality or yields. These growers still have to pay to chemically or manually thin their crop every year, but with a recommended rate of 1 to 2 hives/acre for apples, they are saving $75-$150/acre in rental fees. As production costs have gone up, it is much more common for fruit growers in the state to use only1 hive to every 5 acres or more. We are actively researching the consequences of this movement away from relying on honey bees for apple pollination through a USDA-Specialty Crop Research Initiative grant that began this spring.
Alternative Pollinator Surveys in Apple
In cooperation with the Pennsylvania Department of Agriculture our surveys of fruit orchards have found over 150 species of bees present sometime during the season, with about half this number actively pollinating both stone and pome fruits during bloom. The other 75 or so species appear to be nesting and utilizing other flowering plants in the ground cover within and adjacent to orchards. This cooperative effort with PDA is the first survey of bees ever undertaken for Pennsylvania, and we have found many species which are state records and have found one or two species new to science. A core group of about 30 bee species appears to be present during bloom in fruit orchards each season, but some of the species vary greatly in abundance each season or at different orchards.
We monitored with colored water pan traps and net collections the bee diversity in the 12 apple orchards from the USDA-RAMP program run by Penn State that developed reduced risk IPM programs from 2002 to 2009. We found that bee diversity does not vary as much with the pesticide programs (reduced risk vs. grower conventional programs), but more on a site to site basis that appears to be linked with orchard size and surrounding habitat. In many orchards, a definite “edge effect” can be seen, with the greatest bee diversity within 150 yards of wooded areas or fence rows. A recent study in Michigan blueberries, found pesticide use patterns as having a strong effect on bee diversity, but most of the important bee species were ground nesting types that may have been nesting within the blueberry plantations. Our bee surveys over the last 3 seasons, indicate that some of most important and abundant pollinators of apple are “cavity nesters” which utilize holes in dead trees or bore nests into old bramble canes and are thus probably not nesting within the orchards currently. These include all of our Osmia species, the Carpenter Bee, Xyocopa virginica, and two small carpenter bees, Ceratina calcarata and C. dupla. The Ceratina species can be especially abundant along the edges of the orchard and nest in old bramble canes, bushes with pithy stems such as elderberry, or artificial nesting bundles of mullein plant seed stalks. Unlike many of the other apple pollinators we have found, Ceratina have multiple generations throughout the season and are commonly found on blackberry and goldenrod blossoms later in the season. The large Carpenter bee (often mistaken for a bumble bee) is a very common pollinator of apple, but can also be a pest boring into people’s decks and barns. They have hairless black abdomens and the males with a yellow patch on their head often aggressively hover in front of people in territorial displays.
As a whole, bee species that nest in the ground make up about 80% of the bee species in the world. From our surveys in about 10 stone fruit orchards, the majority of the pollinating bee species are ground nesting, and mostly of the genus Andrena. Softer insecticide programs (i.e., reduced pyrethroids and OP sprays) might allow some of these bees to nest within orchards. Establishing unsprayed pollinator strips in fruit orchards would allow bees with shorter foraging ranges both habitat and additional pollen resources so that they would not have to fly long distances from natural habitat or fence rows. Most growers incorrectly think that stone fruits do not require bees for pollination, which is probably a reflection of the unrecognized “free” pollination by a number of native bee species and the fact that only a small proportion of stone fruit blossoms need to be set in order to have a good crop. Ground nesting bees are proportionally less abundant in pome fruits, but include at least 4 species of Bumble Bees which often nest in old rodent burrows. Bumble bee colonies in Pennsylvania die in the fall with cold weather, with only the queens overwintering. Only the large queens are present at the early season apple bloom and it is only later in the season that colonies increase enough to be effective pollinators. Other important apple pollinators species that are ground nesting in habit are a dozen species of Andrena of which A. carlini, A. crategi, and A. rugosa are most abundant. The metallic green sweat bee Augochlora pura is often relatively common on apple bloom.
Managed Osmia Pollinators
Mason bees of the genus Osmia are particularly good pollinators of early spring orchard crops because they rarely sting unless handled and concentrate on gathering pollen rather than nectar. The Blue Orchard Bee (BOB), Osmia lignaria, is native to North America, but wild populations seem to be somewhat sporadic in PA orchards. The BOB is popularly used by small organic growers, but are not thought to aggregate in adequate numbers for pollination of eastern orchards. This may change as protocols for managing BOB are being developed for the California almond industry and we develop specific strains of BOB. We have found surprisingly high numbers of O. pumila in apple orchards during our surveys, but they emerge too late for stone fruit pollination and peak only at the end of apple bloom. O. pumila looks like a much smaller version of BOB, and appears to be a good pollinator of blackberries and raspberries later in the season. The large O. bucephala, which looks similar to a small bumble bee, can also be found pollinating apple, but also peaks later during bramble bloom. The European mason bee, O. cornuta, is used extensively to pollinate pears in Europe because honey bees are not attracted to the low sugar nectar of their flowers. O. cornuta is also used for the pollination of fruit crops in urban areas where keeping of honey bees is not allowed for safety reasons. We are working to develop management protocols not only to increase wild populations of our various Osmia species by providing additional nesting sites and floral resources, but also to develop management protocols for growers wishing to keep populations on their own.
The Japanese orchard bee (JOB), O. cornifrons, is used for apple and pear pollination in most of Japan. It was introduced into the U.S. in 1977 by the USDA-ARS lab in Beltsville, MD and most of the early releases and research were made in orchards in Adams County, PA by ARS scientist, Dr. Susan Batra and the late Johnny Miller. JOB is generally more amenable to higher densities for pollination of larger scale fruit orchards than the native BOB. It appears to be well adapted to the environmental conditions of the mid-Atlantic region, and gives effective pollination with only half the bees necessary with BOB. JOB is available commercially and has been used extensively for the pollination of cherries in Michigan and Utah because of its ability to pollinate in temperatures 10oF cooler than the honey bee and because it is not affected by cloudy weather or light rain. Cherry yields in these states have been shown to double using this bee over honey bees in some seasons.
Successful pollination with mason bees does not require a large population of bees. JOB and the European O. cornuta are 80 times more effective in pollinating apple than the honey bee. Only 250 to 500 JOB are required per acre for pollination compared to 50,000 honey bees. A single JOB can visit 15 flowers/min, setting 2,450 apples/day compared to 50 flowers set by a honey bee. This high level of pollination efficiency occurs because mason bees land directly upon the reproductive structures of the fruit tree blossom. The abdomens of foraging female bees are loaded with pollen, and the repeated and direct contact with the anthers and stamens results in higher levels of pollen transfer. Female bees collect pollen while constructing nests to provide food for bee larvae. Therefore, the key to heavy pollination in the orchard is to promote maximum nesting activity in the orchard bee population. Each female JOB will lay approximately 30 eggs if provided adequate pollen and nesting sites so that populations can increase greatly in a single season and can be used in additional sites. Promoting alternative pollinators may be seen as a threat to the honey bee industry, but lower numbers of mason bees can be used to supplement honey bee pollination under adverse weather conditions. A number of beekeepers in the western U.S. now offer the services of both honey and orchard mason bees for almond and fruit pollination.
Wild and managed species of pollen bees can, and unknowingly have, supplemented honey bees for pollination in specialty crops. Under very specific situations, which we are still researching, pollen bees could possibly replace them. Indeed, of a survey of 100 fruit growers at the Mid-Atlantic Fruit and Vegetable Convention meeting early this year, over half indicated that they were no longer renting honey bees, but relying on wild bees for their pollination needs, including some growers with over 100 acres. A similar situation was seen in a larger survey of New York apple growers (http://www.danforthlab.entomology.cornell.edu/pollination-biology.html). Since most pollen bees appear to be flying in from outside the orchards from woodlots and fencerows and because of their limited foraging ranges in comparison to the honey bee, we are advising apple and pear growers to consider the size of each block and the surrounding habitat before they stop relying on honey bees. Small, 5 to 10 acre blocks surrounded by wooded areas are probably adequately pollinated by pollen bees alone, but if they are surrounded by corn or soybeans, they probably won’t be pollinated. Larger blocks without good surrounding bee habitat are especially at risk without honey bees. Even with good bee habitat close by, the centers of these larger blocks could lack sufficient pollination if they are more than 100 yards away from the nesting sites. The goal of our current research is to optimize the numbers and best placement of honey bee hives to fill in pollination gaps left by the short ranged pollen bees so that both species can supplement each other in meeting the pollination needs of fruit growers. Using managed nests of bumble bees and JOB are another way to fill in the pollination needs in these dead zones in orchards too far from the borders for wild bees to fly to. The use of USDA-NRCS pollinator strips could also be used to provide nest sites for bumble bees and JOB that would not only provide supplemental food for summer generations, but also protection from pesticides and disturbance from sprayers. The optimal use for the NRCS pollinator sites for fruit pollination would therefore be as patches or “stepping stones” for pollen bees to move out from the orchard edges and into the orchard centers, instead of being placed next to a woods or fencerow as many currently are.
Fruit growers relying on pollen bees also need to be much more careful with pesticide applications made before, during and after bloom because pollen bees are not moved into the orchards on trucks like honey bees, but are permanently based around the orchards. Eighty percent petal fall, might be ok to spray for plum curculio if you truck the honey bee hives out of the orchard, but for solitary bees, it is still 20% bloom and harsh sprays can still reduce resident populations. Even after bloom, some species such as bumble bees, have multiple generations that may be foraging in the orchard ground cover and could be impacted by pesticide sprays during the rest of the growing season. Border sprays for brown Marmorated stink bugs should also consider that most of the bees (and many beneficial predators & parasitoids) are nesting/living in these borders.