Pollination in Watermelons

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 honeybee 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 honeybees are needed to set fruit.

First planted watermelons will be flowering in late May in Delaware and Maryland. Honeybees 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. Honeybee flights are reduced significantly in rain and when winds are 15 mph or greater. Cloudy weather also reduces bee activity. Honeybees 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 honeybees by:

  • Increasing the number of honeybee 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.

Honeybee visiting a male flower in a watermelon field. Photo credit Purdue University Entomology.

Honeybee visiting a male flower in a watermelon field. Photo credit Purdue University Entomology.

Bumble Bees
Compared to a honeybee, 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 honeybees. 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 honeybees 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 have an average of 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 wildflowers 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 honeybee hives to avoid honeybee pollen theft from bumble bee nests.

Insecticide Use

Using IPM practices to limit insecticide use will also preserve bees and increase pollination. Research at Purdue University showed that yields were lower in fields with more applied insecticides. This is because of non-target effect of insecticides on visiting pollinators. Researchers found that “during 5 weekly surveys during peak watermelon flowering, there were 653 pollinators in the IPM plots, as opposed to just 349 in the conventionally managed plots. These visitors included “managed” species such as honeybee and bumblebees along with numerous native sweat bees and syrphid flies. The total number of visits to flowers and number of pollinating events (traveling from a male to a female flower) were recorded for all of the surveyed insects. The IPM plots had twice as many total flower visits and three times as many pollinating events compared to the conventional watermelon plots. Since multiple insect visits are required for watermelon to produce healthy and marketable fruit, the reduced frequency of pollination could be leading to the differences in yield. These trends show that a “less is more” approach may be a superior management strategy when it comes to insecticides in watermelons and the other vegetable crops that rely on bees for pollination.” A factsheet on protecting pollinators can be found at https://pollinatorprotection.org/wp-content/themes/pollinator/assets/pdf/POL-2.pdf