The Role of Pollen Bees in Fruit Tree Pollination and Some New Cautions on Pesticide Use
Posted: August 23, 2013
Left: Fruit diameter and seed set in crabapples associated with various pollinators; Right: Contact toxicity of various neonicotinoids in apple to honey bees in the laboratory.
Worldwide, honeybees (Apis mellifera) have been the most economical and reliable option for pollinating most crops and the most important pollinator for apples in North America. The recent declines in both feral and domestic honeybee colonies due to mites, viruses, and the recent devastating effects of Colony Collapse Disorder (CCD), however, have caused a rise in the cost of honey bee rentals for farmers to pollinate their crops. We have seen an increase from $35 per hive in 2006 to over $100 per hive in 2013 and a noticeable decrease in the vigor for the hives early season due to the increased use of packaged bees to make up for winter losses. The loss of 80% or more of feral honeybee colonies in the mid-Atlantic region along with this rising cost of hive rentals, precipitated the need to investigate potential alternative pollinators for fruit trees and other crops throughout the U.S. Unfortunately, little is known about the basic biology of most native bee species, much less their importance for crop pollination. Some wild bee populations, particularly bumble bees, are also facing declines for unknown reasons. The US Fish and Wildlife Service lists 50 native bee species as threatened or endangered. Thus, it is imperative that we focus our efforts on understanding the role that native bee species can play in crop pollination.
Wild and managed non-honey bee species (hereafter referred to as pollen bees due to their lack of honey production), including both bumble bees and solitary bees, have long supplemented honeybee pollination in fruit orchards, but their efforts have mostly been attributed to the honey bee. In light of the recent decline of honeybee populations, pollen bees will serve an even more integral role in fruit tree pollination and a number of Pennsylvania fruit growers have relied exclusively on pollen bees for pollination for over 5 years with no noticeable loss in fruit quality or yield. This has resulted in significant savings in rental costs, but completely relying on pollen bees for fruit pollination may be risky depending on the size of the orchard blocks and surrounding habitat. This is primarily due to the limited foraging ranges (< 200 yards) of many pollen bees in comparison to the honeybee which will commonly fly several miles. An exception are the bumble bees which are known to forage well over a mile from their nests, but are limited to only low numbers of overwintering queens early season for fruit crops. These queens establish colonies that increase during the summer months with many smaller workers. A recent study found that wild bees were capable of providing full pollination of watermelons on more than 90% of farms in Pennsylvania and New Jersey. Fruit orchard surveys in Pennsylvania conducted by David Biddinger and colleagues in 2007-2009 found a complex of 50 pollen bee species visiting apple blossoms and over 150 species were collected in pan traps placed in apple orchards throughout the season. Thus, apple orchards and the habitat (woodlots, fencerows, streams, etc.) are important sources of food and shelter for many of Pennsylvania’s approximately 450 species of bees.
Orchard bees in the genus Osmia have been shown to be more effective at pollinating apples than honeybees. One species, known as the Japanese orchard bee (JOB), is used for pollinating over 80% of all apples in Japan, and is related to several other species used as commercial pollinators of apple and pear in Europe, and almond in the western U.S. JOB was introduced into Adams County in the 1970s and is now established and abundant in much of the state, nesting in old apple wood and dead trees along the edges of orchards. There are currently at least three other native species of Osmia that contribute to fruit pollination in Pennsylvania. Osmia have been shown to be more effective pollinators than honeybees and are more easily managed in orchards than other native pollen bees because of their nesting habits. However, very little is known about the pollination efficiency of other pollen bee species. Since pollen transfer has a direct effect on fruit set and fruit quality in apples, the identification of the most efficient pollinators will be essential in determining which pollen bee species should be managed in the future in order to offset yield losses associated with declining honeybee populations. Some preliminary data from a block of crabapples (graph on left) show flowers that were bagged to prevent bee visitations had the lowest number of seeds and smaller fruit size, but those flowers left open for natural pollination (multiple visitations possible) or those flowers bagged after only a single visit by a JOB had the best fruit size and the most seeds set per fruit. A single visit by several other species of bees was still more effective than the bagged control flowers, but not as good as open pollination or from single visit by JOB. Keep in mind that only 2 to 8% of the apple flowers available at bloom need to be set by pollination to provide a commercial crop and all other fruit will be removed by chemical or hand thinning to obtain optimal fruit size and to prevent biannual cropping. In general the percentage of blossoms needed for other tree crops are: apricot 20-25%; pear 3-11%; peach 15-20%; and cherry 20-75%, but for berry crops this percentage is generally higher at 75-100%.
The Food, Conservation, and Energy Act of 2008 (otherwise known as the Farm Bill) has acknowledged the great importance of pollen bees for agriculture by providing funding for farmers to increase and protect pollinator habitat on farm land. Farmers are encouraged to seed strips of wildflowers along their property to encourage bee visitation to their crops, or to leave part of their property fallow to increase pollinator habitat. Thus far, little information is known about the efficacy of these wildflower strips in increasing fruit yield, or how far plantings need to be spaced in relation to crop species in order to maximize crop pollination. If pollen bees do not forage for long distances and nest along the borders of orchards, then apple flowers in the interiors of large apple blocks may be pollen limited without the help of the longer range honey bees. If this is the case, honey bee hives, should be placed in the centers of these larger blocks of orchards rather than along the borders or additional nesting sites and food sources in these pollinator strips should be placed in the center of the orchards. Currently, from a USDA-Specialty Crop Research Initiative (SCRI) grant by Biddinger, we do not believe pollen bees are nesting within the orchards due to pesticides and lack of nesting sites and are mostly coming in from the borders. We are currently researching the foraging ranges of some of our most important solitary bees, with preliminary data indicating most species will only fly less than 100 yards into an orchard from nesting sites in the adjacent habitat. Notice that visits by the longer ranged honey bees and bumble bees are only slightly reduced even out to 200 yards from the wooded habitat. On a bee/bee basis, however, JOB has been shown to be at least 80 times as effective in pollinating apple as the honey bee. For those fruit growers relying on wild bees for their pollination needs, removal of this habitat or spraying it with acephate or other insecticides in an attempt to eliminate the Brown Marmorated Stink Bug (BMSB) could almost completely eliminate those populations. Since recent data indicate that BMSB can fly over a mile from wooded areas where they are breeding, spraying the first 10 to 15 yards of woodlots adjacent to orchards gives negligible control of this pest while threatening the most important source of wild bees.
At an international pollinator conference held at Penn State last week, the general consensus was that Colony Collapse Disorder (CCD) of honey bees is caused by multiple factors including: a) viruses and diseases; b) two species of mites; c) poor nutrition caused by foraging in sugar poor crops like cucurbits; d) the stress of interstate travel and e) pesticide exposure. Despite this, however, most of the research presented at the conference concentrated on pesticide exposure with a general call for banning a group of insecticides known as the neonicotinoids. This of course seems to be an easy fix to a complex problem that is still not completely understood, but of course is popular with the public and many ecologists that have never worked with pesticides or IPM. This stance does not take into account the reason these products were developed in the first place which was to replace human toxic OP pesticides and replace them with something safer as mandated by the Food Quality Protection Act. Neonicotinoid insecticides have also proven to be safer to most beneficial insects other than bees and promote the biological control of pests such as San Jose Scale, Woolly Apple Aphid, European Red Mite, leafminers, and leafhoppers to name a few. A general ban of neonicotinoid insecticides would cause a reversion back to OP, carbamate and pyrethroid insecticides which would totally destroy current IPM programs and cause growers an additional $50 to $100+ per acre in secondary pest sprays. Some pests such as the Rosy Apple Aphid are resistant to most alternative products or in the case of plum curculio and apple maggot are best controlled with Calypso or Imidan® now that Guthion® registrations have been cancelled. A new twist on the honey bee decline issue brought up at this conference was that the miticides beekeepers are using in orchards are also strongly impacting the mating and general health of honey bee hives and the most recent product used after a succession of an OP and a pyrethroid, is an old orchard product known as Mitac® (amitraz) that can shut down the immune system of bees without killing them, but can make them more susceptible to pesticides at the next exposure.
We have six types of neonicotinoids registered in apple that include Actara® (thiacloprid), Assail® (acetimiprid), Calypso® (thiacloprid), Provado® (imidacloprid), Scorpion®/Venom® (dinotefuron), and Clutch® (clothianidin), which are mostly used post-bloom with the exception of Assail and Calypso. Both of these products are much more bee safe and often used at pink for control of rosy apple aphid which have become resistant to many OPs including Lorsban® and some pyrethroids. Assail is the only product to attain EPA’s Reduced Risk insecticide status and is allowed to be sprayed during bloom although this is not recommended in Pennsylvania. Not all neonicotinoid insecticides are equally toxic to bees (graph on right) and using Sevin (carbaryl) as a standard it can be seen Assail and Calypso are much safer, but all of the other products are significantly more toxic. From our lab assays with formulated products in water to simulate the situation in orchard applications, we have found an 8 to 10 fold safety margin for the honey bee over the previous lab trials with technical grade pesticide dissolved in acetone. The toxicity of these products to the honey bee, however, does not correlate well with their toxicity to our Japanese orchard bee. JOB was 12 times more susceptible to Assail, but 26 times less susceptible to Provado than the honey bee. In general, however, the products of choice before and immediately after fruit bloom should be the relatively bee-safe neonicotinoid insecticides, Assail and Calypso. We are currently examining movement into the nectar and pollen of these systemic products when applied at the normal pink timing for Rosy Apple Aphid. Preliminary data show only a few parts per billion, which is well below currently acceptable levels, but with additional research on chronic, long-term exposure, those levels may be revised downward.
Precautions acceptable for keeping honey bees safe such as spraying at night or early morning are generally much less effective with this class of insecticides, since they are systemic and move throughout the plant. They can poison bees of all types not just through contact activity of residues on the foliage (which bees generally don’t touch anyway) or direct contact from droplets hitting foragers, but most exposure is thought to be through the ingestion of poisoned nectar by the adults or from contaminated pollen that is a food source to types of bee larvae. From our trials, residues in the pollen have generally been several times higher than that found in the flowers, so pollen bees that eat more pollen as larvae than honey bees may be more susceptible at this stage. Most pesticide research to date has concentrated on adult toxicity, but studies on pesticide impacts on larvae are necessary in the near future. For those fruit growers relying mostly on wild pollen bees for pollination, additional precautions need to be taken. The old definition of petal fall being defined as when the honey bee hives are out of the orchard, no longer applies. Most pollen bees have only a single generation in the spring and spraying a toxic pesticide at 50% petal fall or even 80% petal fall can still greatly reduce populations. We are extensively revising the pollinator section of the tree fruit production guide for the next edition coming out this winter to include this recent information.
Funding was provided by a State Horticultural Association of Pennsylvania grant, an USDA-SCRI Research and Extension grant, an USDA-NRCS Conservation Innovation grant, the USDA-RAMP program and a current regional USDA-SCRI Coordinated Agricultural Project grant.