Pollination in Seedless Watermelons and Honey Bee Placement, Bumble Bees as Pollinators

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

A female watermelon flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a minimum of 8 visits by a honey bee for seeded watermelons. In seedless watermelon more visits will be required. The pollen produced by seedless watermelons is not viable. To fertilize seedless watermelon, pollen must be transferred from viable male flowers in standard or special pollinizer seeded types to triploid seedless female flowers. Because bees foraging in seedless watermelon plantings carry a mix of viable and non-viable pollen, more pollination visits (16 to 24) by honey bees are needed to set fruit.

First planted watermelons are now flowering in Delaware and Maryland. Honey bees should be placed when the first female flowers appear to achieve good crown sets without defects (i.e. prominent lobes or hollow heart). Placement should be made before 10% of plants are in bloom.

The crown set in watermelon is fruit that set on one of the first 8 nodes of the plant. This is often the most profitable, especially early in the season. Poor crown sets in watermelon can occur when there is poor weather during early flowering. Honey bee flights are reduced significantly in rain and when winds are 15 mph or greater. Cloudy weather also reduces bee activity. Honey Bees also do not fly much below 55°F, so on cold mornings, as we often have in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon.

In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives. As the temperature rises or the weather improves, the bees will forage further from the hive. This means that in bad weather watermelons closest to the hives will have the best set and furthest from the hives will have reduced set.

Another problem that causes crown set reduction is the loss of pollenizer plants due to unfavorable weather conditions during or after planting. This means that pollen will be limiting. Research has shown that were pollen is limiting, fruit numbers will be reduced with distance from a pollen source. In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing.

Watermelon growers can manage crops for improved pollination and fruit set with honey bees by:

  • Increasing the number of honey bee hives for early watermelon crops. A minimum of one strong hive per acre is recommended in general and 2 hives per acre can be justified for early planted fields.
  • Placing hives in several locations in a field rather than just on one edge. While bees will fly over a mile, the best pollination activity is closest to the hives. Hives placed within the field will provide more bee visits to the crop compared to edge placements. Place hives in groups of 4-8 in good locations throughout the field to have even distribution of bees.
  • Having ample sources of pollen by planting pollenizers at a minimum ratio of one pollenizer per every 3 seedless plants. Use the most effective pollenizers as shown by local trials. In-row pollenizers should have limited competitiveness with the seedless melons.

Bumble Bees
Compared to a honey bee, bumble bees are about 10 times more efficient as a pollinator due to their size, the speed at which they transfer pollen, the efficiency with which they gather pollen within various crops, and their increased endurance to fly in adverse weather for longer periods of time. The bumble bee also has the ability to buzz pollinate the flower for pollen, a pollination technique not seen in honey bees. Buzz pollination occurs by bumble bees vibrating the flower by pumping their wings at a certain frequency, to dislodge pollen. Bumble bee foraging activity starts earlier and ends later in the day than managed honey bees and they forage in lower temperatures. Because of these characteristics, fewer bees are needed to achieve the same crop pollination and commercial colonies only have about 200 bees each (800 per quad).

When assessing bumble bee activity, flag out 10 areas in your field and observe each area on three different days during bloom. These observations should last one minute under sunny, windless conditions, between 9 a.m. and noon. Approach each plot with care so as not to disturb the foraging bees. Stand about three feet from the crop to avoid blocking the flight path of the bees. Count and record the number of bumble bees at each flag, then calculate the average for your observations. You should an average one bumble bee per ten flags (0.1 bees per flag) to have adequate pollination.

Bumble bee colonies should be shaded and can be placed along shaded field edges. However, if there are other wild flowers nearby, they will also work in those areas, reducing their field effectiveness. Therefore, when placing bumble bees in watermelons or other flowering vegetable or fruit fields needing pollination, it is recommended that bumble bee quads be placed in the field middles under a shade canopy to have more foraging in the target field. Bumble bees should be placed far from honey bee hives to avoid honey bee pollen theft from bumble bee nests.






Problems with Pollination in High Tunnel Tomatoes

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

Some mid-Atlantic growers are seeing excessive tomato blossom drop in their high tunnels (HTs). Flowers are forming but then abscising from the plant (Fig. 1). In a few of the high tunnels bumblebees were used to pollinate the flowers (Fig. 2) and growers thought the bees were a little too aggressive in their pollination enthusiasm and that they were damaging the flowers (Fig. 3) to the point they would abort. But I think it is more likely environmental factors are causing the flower drop.

Figure 1. Flower abortion on tomato plant.

So, I’ll start with a quick recap as to how tomato flowers are pollinated and fertilized. Tomatoes are self-pollinated at the frequency of around 96% of the time. Tomato flowers are complete flowers that have both male (stamen) and female (pistil) parts within the same flower. The yellow anthers (pollen producing parts) of the stamen wrap around the pistil which is in the center of the flower. The style with the stigma on its end is the part of the pistil that extends above the anthers. Tomato pollen is heavy and sticky and needs to be jostled loose from the male to fall onto the female. This ‘jostling’ can include wind or insect visits. Once pollen is shed onto the stigma of the flower, fertilization can take place. Without pollination the pedicle turns yellow, the flower dies and then drops. Tomato flowers must be pollinated within 50 hours of forming or they will abort. Pollination usually occurs between 10 a.m. and 4 p.m.

Unfortunately, there are numerous factors that can cause tomato plants to drop their blooms. One of the main ones is temperature. Tomato plants will drop their flowers when daytime temperatures are above 85°F or when nighttime temperatures are above 70°F. Obviously this can and does occur during mid-summer. In the early part of the season low nighttime temperatures below 55°F can interfere with the growth of pollen tubes or cause the pollen to become sterile, preventing normal fertilization and causing flower drop. Fruit will not set until nighttime temperatures are above 55°F for at least two consecutive nights. The relative humidity (RH) also can play a role in poor pollination and fruit set. The best RH for tomato development is between 40% and 70%. Low RH (<40%) can dry pollen out making it unable to stick to the stigma. A high RH (>70%) can prevent the pollen from being shed properly. While there are other factors that have been found to influence pollination in tomato such as levels of nitrogen that are either too high or too low, too high or low soil moisture, a heavy fruit set, excessive wind that can desiccate flowers, and the lack of sufficient light these are minor factors compared with temperature.

Bumblebees pollinate tomato flowers by sonication or buzz pollination. They will fly up to a flower and grasp the anthers with their mouth parts and hold tightly. They then vibrate their wing muscles which causes pollen to drop from the anthers onto the stigma causing pollination and at the same time the bumblebee gets to collect some of the pollen (Fig. 2). This grasping of the tomato flower by the bee leaves a mark on the flower (Fig. 2) and can cause flower damage if visited too many times (Fig. 3). When there are fewer flowers than what would normally be expected fewer bumblebees should be released to prevent overzealous bee visits.

Figure 2. Bumblebee visiting tomato flower results in pollination. Arrow shows marks by other bee visitors.

Figure 3 These tomato flowers may have been visited too many times by bumblebees.

This year we have had an unusually cool spring and even in high tunnels the temperatures, especially at night, were not conducive for flower pollination and fertilization. Some growers who used bumblebees did have higher levels than usual of bee love as the bumblebees repeatedly visited the few flowers that were forming causing some of the flowers to abort. But this was a very small amount compared with what the cooler temperatures were doing to tomato pollination. Reports out of the Midwest and the Northeast say similar things, poor fruit set in high tunnel tomatoes up to this point in time and most of these high tunnels did not use bumblebees. Besides the direct effect of cooler temperatures on tomato pollination, the cooler than normal temperatures and often overcast skies also caused growers to not ventilate their HTs as much, reducing the probability of wind pollination of their tomato plants. Most growers depend on wind pollination for tomato pollination even in high tunnels. An excellent source for further description of problems with tomato pollination can be found at: Blossom Drop, Reduced Fruit Set, and Post-Pollination Disorders in Tomato by Monica Ozores-Hampton and Gene McAvoy, University of Florida HS1195: http://edis.ifas.ufl.edu/hs1195 .

New EPA Bee Advisory Box

Steve Carter, Agricultural Specialist, Delaware Department of Agriculture; steved.carter@state.de.us

EPA has new pesticide label requirements regarding pollinator protection. The graphic below highlights some of the features of the new label (click on the graphic for a larger version.) Additional information on how to protect insect pollinators from pesticides is online at: http://pesticidestewardship.org/pollinatorprotection/Pages/default.aspx


Best Management Practices for Using Commercial Bumble Bees on Horticultural Crops in Delaware

Jacquelyn Marchese, Graduate Student, Department of Entomology & Wildlife Ecology marchese@udel.edu; Deborah Delaney, Assistant Professor Entomology & Wildlife Ecology, dadelane@udel.edu; Gordon Johnson, Extension Vegetable & Fruit Specialist, gcjohn@udel.edu

A common native pollinator in Delaware is the Common Eastern Bumble Bee (Bombus impatiens). The behavior, physiology and morphology of bumble bees make them ideal pollinators because of the speed at which they transfer pollen, the efficiency with which they gather pollen within various crops, and the increased endurance to fly in adverse weather for longer periods of time. The bumble bee also has the ability to buzz pollinate the flower for pollen, a pollination technique not seen in honey bees. Buzz pollination occurs by bumble bees vibrating the flower by pumping their wings at a certain frequency, to dislodge pollen. Bumble bee foraging activity starts earlier and ends later in the day than managed honey bees and they forage in lower temperatures.

In the early 1980s, commercial rearing of bumble bees for pollination services was developed in The Netherlands. By the 1990s commercial bumble bee production made its way to the United States. In the United States, there are two companies that distribute commercial nests of B. impatiens, Koppert Biological Supply Company and Biobest®. Bumble bee colonies are reared and placed within plastic boxes, equipped with enough pollen and a sugar substance to sustain bumble bee during shipping. Two holes are engineered into each plastic box that allows the grower to control the activity of their bumble bee nest. These entrance holes are controlled by a plastic flap that either opens or closes all or one entrance hole (Figure 1).

Bumblebee1 Figure 1. The colony entrance reducing tab, closed, one open hole, two open holes. The one open hole allows for bees to enter the colony but prevents bees from escaping, a useful tool to keep bees protected from pesticide sprays.

Bumblebee2Figure 2. Pictures of the different components of a commercial bumble bee quad. Top left, inside the individual colony unit. Top right, the whole individual colony unit. Bottom left, inside the quad with three out of the four colony units. Bottom right, the full quad.

Bumble bees differ greatly from their more popular, honey bee, counterpart and thus have different biological requirements in order to optimize their use to growers in the field.

Bumble bees were studied in 3 crops in Delaware: Strawberries, watermelons, and pickling cucumbers. We found that bumblebees are a viable alternative pollinator for strawberry and watermelon and can be considered for those crops. During our two year study that looked at bumble bees in strawberry and watermelon field crops, bumble bees were constantly and consistently detected in the fields throughout both seasons. Although we determined that strawberry and watermelon pollen were not the main pollen sources being brought into the colonies, foragers were seen with enough frequency and abundance that we are confident in their ability to pollinate these crops. The bumble bees observed on strawberry and watermelon blooms were most likely nectar foragers, but if foragers are strictly on the crops for the intention of gathering nectar, transfer of pollen should still be occurring.

Bumblebees did not perform well in pickling cucumbers in our studies. In two years of sampling pickling cucumber fields with commercial bumble bees, we found that they made up at most 8% of all pollinators collected and they were frequently seen on weedy forage such as morning glory, ragweed, horse nettle and other common flowers. Honey bees were the most abundant pollinator found in these fields, followed by native sweat bees and pollinating hover flies. Therefore, at this time, without additional research, we cannot recommend bumblebees in pickling cucumber crops. Growers should continue to rely upon and use honey bees in pickling cucumber plantings.

Place bumble bees in the field after crops have begun to bloom. Like honey bees, bumble bees need access to forage to sustain themselves. Bees that have found unintended forage in the beginning of the season are likely to continue to forage on this unintended source, especially if it is more favorable than the intended crop. Place bees in the middle of the field to encourage in field foraging.

Allow time for bees to settle before opening units. Always follow instructions provided by the bumble bee supplier when placing bees within the field. Give the allotted time before opening up the colonies for the first time. Although bumble bees will need to chew out of the hole in order to begin foraging, colonies should be given at least 30 minutes to settle after being handled during shipment and placement. Also, be sure to check on each colony 2-3 hours later to make sure that the bees have successfully chewed out of the hole and exited the nest. On occasion, bees will not successfully chew out of the hole and will need to be cut out of the colony. Although this has been known to occur, it is not common and most colonies will successfully find their way out of their colony and into the crop, on their own.

Close bumble bee units before each pesticide application. During the season, change each bumble bee colony entrance to one open hole at least two hours before all pesticide applications. This will allow time for bumble bee foragers to return and be kept in the colony in order to limit forager exposure to pesticides, see the pesticide section below for more information.

Place bumble bees under shade, to increase their productivity and longevity. Bumble bee units placed in natural shade (along forest/field edges) or fitted with a shade structure last longer and are significantly more productive than units in the sun. Placing bees under shade is especially important during the warm summer months of Delaware’s watermelon bloom. Bumble bees constantly and actively strive to keep their colony temperature at around 86°F. Colonies exposed to direct sunlight have to work harder and use more energy to thermoregulate. Colonies placed in full sun without shade cannot maintain normal worker activity (pollen and nectar foraging and duties within the colony) for as long as the colonies with shade.

Keep bumble bees away from honey bees. Bumble bees should be placed as far from honey bee hives as possible. This is especially true when crops are not in bloom. When forage is low for the commercial pollinators they should be greater than 1 mile from each other. Honey bees are very resourceful and a bumble bee colony is a great source of pollen and nectar which honey bees are constantly seeking. If surrounding forage is low or not agreeable to honey bees, bumble bees will be susceptible to honey bee robbing causing a weakened colony and overall loss in productivity from both pollinator species.

Bumble bee units should be weighed or strapped down, especially when placed within a shade structure. These units may be susceptible to being flipped or carried by strong winds. Not only does this disrupt the normal orientation of the colony, causing helpless larvae, nectar and pollen to fall out of their individual waxen cells, but can cause blockage to the unit openings, trapping bees within the unit.

Bumble bees may be transferred to another field for additional pollination services throughout a season. Before moving, close the plastic opening tab to the one-hole open position. Allow forager bees at least two hours to return to the colony. The bumble bee colony may then be transferred to another site.

Close up colonies before each pesticide spray. Bumble bees very easily accumulate pesticides within the wax of their brood clump and their bodies by foraging in crops that have been treated with various chemistries. Although bumble bees will inevitably have some exposure to sprayed pesticides within the field, growers can limit exposure by using the plastic opening tab within each colony box. Growers are urged to close up the commercial nests at least two hours before spraying to decrease the exposure of bees to the pesticides.

Dispose of bumble bee colonies in a timely and humane fashion. There is a risk of commercial bees breeding with native populations. Commercial bumble bees are mass reared, and therefore the genetic diversity of the commercial bees does not mirror what is naturally found and occurring in the wild bees. The integrity of this wild genetic stock is important because it allows for the bees to be adapted to a wide variety of environmental conditions and exposure to various pathogens that they may encounter. If commercial bees mate with wild bees, the commercial bees will be diluting the genetic stock of the wild bee population.