Pollinator Strength

David Owens, Extension Entomologist; owensd@udel.edu and Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Honeybees are used extensively to ensure adequate pollination for vine crop vegetables (cucurbits) and for many fruit crops (apples, berries, etc.). Without good pollination, poor fruit set or misshapen fruit can occur. Most of the honeybees used for pollination are rented from beekeepers. Questions have come up how to know if a colony is strong enough to provide adequate pollination service. A good resource on pollinators, colony strength, and farmer best management practices for pollinator health (including water sources can be found in the MidAtlantic Vegetable Production Recommendations, Section A, pages 21 – 27.

It is important to ensure having enough bees (managed and wild) to avoid having problems with fruit set and misshapen fruit. There are two ways to check the strength of a colony: in-hive inspection and assessing hive traffic at the entrance. In the hive, bees should cover 6 to 8 frames, have 4 to 6 frames of brood and (eggs, larvae, and capped) fill 1.5 to 2 boxes. This is considered a ‘minimum standard.’

An easier, but less accurate method of assessing colony strength is to watch colony entrances in late morning to early afternoon on a calm day. During a 1 minute interval, 50 – 100 bees should be arriving and leaving the colony. While counting bees, be sure to note the presence of bees carrying pollen. They will have large yellow ‘sacs’ on both back legs.

Farmers should work with their beekeeper to ensure that only strong colonies are placed in fields. This has become more difficult in recent years due to higher winter mortality caused by bee pests and pathogens. Stronger colonies provide much more pollination service than one or two weaker colonies. Beekeepers should work with the state apiarist, Meghan McConnell to assess colonies. On the farm side, farmers should read labels carefully and avoid making applications when bees are active in fields. Several insecticides and miticides have pollinator advisory language on them. The fastest way to find it is to download the label from a website such as cdms.net and search the label for ‘bee’ or ‘pollinator’ using Ctrl + F. Insecticides of special concern have a bee in a red diamond to indicate pollinator protection language. Bees can also be affected by fungicide applications. Bees feed their larvae fermenting pollen, and bees rely on the microbes living with them to fend off diseases; fungicides can disrupt the beneficial microbes in the colony. Thus, even fungicides should be timed for periods when bees are not active in the crop. On warm days, bees also forage for water to cool the colony. Having a clean water source within a ¼ mile will benefit the bees. This doesn’t necessarily mean flowing water; large puddles should suffice.

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.

 

 

 

 

 

Misshapen Strawberry Fruits

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

Most commonly, misshapen strawberries during spring result from poor pollination. Strawberries are aggregate fruits. They have multiple ovules per receptacle where the fruit is formed. The strawberry receptacle may have up to 500 ovules per berry. You will see these as “seeds” on the outside of the strawberry fruit which are called achenes. To have the largest berry possible, you need as many of these ovules to be successfully pollinated as possible. To avoid misshapen fruits the achenes need to be pollinated evenly and fully. With pollination, the receptacle tissue around the achenes will develop to form the strawberry fruit.

Strawberries have both male and female flower parts on the same flower and can self-pollinate. Wind and rain can move pollen within the flower. However this usually does not allow for full pollination of all the ovules. Bees, such as honey bees or bumblebees, are usually necessary to allow for complete pollination. Some flowers actually produce bigger berries when cross pollinated with pollen from other flowers. Incomplete pollination will often result in smaller or misshapen berries.

Strawberry flowers are not heavy nectar producers. However, bees do visit the flowers and studies have shown that where native bees are limited, adding hives of honey bees or bumble bees increased productivity. It is recommended that each flower receive 16-25 bee visits. This is particularly true of the king berries, which form from the first flower to open on a fruiting truss.

You can distinguish poor pollination from other types of damage because fruit will have variable achene (seed) size. Large seeds received pollination, while small seeds did not. Poor pollination is common when plants have been under row covers during bloom and when the bloom period has been rainy, stormy, or cold. Frost damage that does not kill the whole flower will also cause berry deformities because some achenes have been damaged.

Lygus bugs (Tarnished Plant Bugs) can also cause misshapen fruit by feeding on the flower. To distinguish between Lygus bug damage and poor pollination look at the seed size on the fruit – seeds on fruit affected by Lygus will be similar in size.

Boron deficiencies are another potential cause of misshapen strawberries.

Strawberry deformities caused by poor pollination and cold injury.

Reduced Fruit Size in Strawberries

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

Several growers have commented that fruit size in strawberries is smaller than normal in 2017. This poses the question “what affects strawberry fruit size?”

In plasticulture strawberries, one critical factor with varieties such as Chandler is the number of branch crowns that develop in the fall. Early planting or extended warm weather in the fall may cause plants to produce excess crowns leading to too many buds, flowers and fruits per plant in the spring and, consequently, small berries. This is also a common problem with carry-over plasticulture strawberries where crown thinning was not done or was inadequate.

Another cause of smaller sized strawberries is related to pollination. Strawberries are aggregate fruits. That is, they have multiple ovules per receptacle where the fruit is formed. The strawberry receptacle may have up to 500 ovules per berry. You will see these as “seeds” on the outside of the strawberry fruit which are called achenes. To have the largest berry possible, you need as many of these ovules to be successfully pollinated as possible. With pollination the receptacle tissue around the achenes will develop to form the strawberry fruit.

Strawberries have both male and female flower parts on the same flower and can self-pollinate. Wind and rain can move pollen within the flower. However this usually does not allow for full pollination of all the ovules. Bees such as honey bees or bumblebees are usually necessary to allow for complete pollination. Some flowers actually produce bigger berries with cross pollination with pollen from other flowers. Incomplete pollination will often result in smaller or misshapen berries.

Strawberry flowers are not heavy nectar producers. However, bees do visit the flowers and studies have shown that where native bees are limited, adding hives of honey bees or bumble bees increased productivity. It is recommended that each flower receive 16-25 bee visits. This is particularly true of the king berries, which form from the first flower to open on a fruiting truss.

This additional pollination by insects is limited when row covers are placed over fields for extended periods during flowering, by poor weather for honey bee flights (rainy, windy, cold), or by other actions affecting pollinator performance.

Improving Early Fruit Set in Seedless Watermelons

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

The first watermelon plantings have gone in across the region. Markets for early watermelons are normally the strongest so early planting is often more profitable. However, fruit set is often below desired levels in the earliest plantings and crown sets in early plantings often have quality issues such as higher levels of hollow heart.

The following are some considerations for managing watermelons to maximize early fruit set:

1) Get plants off to a good start with a minimum of stress. In early plantings always plant on a warming trend where temperatures are expected to increase and skies are mostly clear. Black plastic mulch will then allow soils to accumulate heat and roots will be able to establish more quickly. Use every row rye windbreaks (or clear row covers if windbreaks have not been planted) to reduce heat losses and protect plants. Plant well hardened off plants and train transplanting crews to handle plants carefully with a minimum of damage. Provide adequate water at planting and avoid putting excess starter fertilizers in transplant water which can cause salt stress on plants. Manage early fields more intensively by monitoring irrigation and fertigation programs so that stress is reduced throughout the growing period. Extra nitrogen can delay flowering so there is a fine balance between promoting growth and initiating flowering. Avoid practices that put extra stress on plants and be careful of phytotoxicities with misapplication of foliar fertilizers, fungicides such as copper products, and herbicides (proper shielding when spraying row middles, follow label guidelines for herbicides). Manage windbreaks so that mites do not infest watermelons when they are terminated. Manage insecticide applications so that bees are not affected during flowering (see pollinator protection information on labels).

2) Manage pollinizer-seedless combinations for maximum pollination potential. Loss of pollenizers after planting will reduce fruit set. This has been a problem in the past when pollenizers were not hardened off properly because they were seeded later in the greenhouse. In-row pollenizers should be used to achieve best early fruit set. Pollenizers should be chosen so that they are flowering adequately as the seedless come into flower. Pollen is the key for early fruit set and earlier flowering pollenizers should be used to improve crown sets. A case can be made also for increasing the number of pollenizer plants for the earliest plantings. A 1:3 ratio of pollenizer to seedless should be the minimum used and extra pollenizers that flower early could be planted at intervals to provide additional pollen. Another issue is the vigor of pollenizers. Make sure that pollenizers have good disease packages. In fields with a history of Fusarium wilt, Fusarium resistance in both pollenizers and seedless is needed. If at all possible, place early plantings in fields with little or no history of watermelon production to avoid soil borne disease stress.

3) Manage pollinators so that pollen is transferred effectively and in adequate quantity. Consider placing extra hives in early plantings. Have hives set when pollenizers are 10% in bloom so bees start to work fields immediately. If there are not enough bees when first female flowers open, you will lose much of the crown set. Avoid having flowering crops nearby that are more attractive to bees and could siphon off bee activity. Fruit set is often reduced when weather conditions at first flowering is rainy and windy or night temperatures are cold. Honey bees rarely work when the temperature is below 57°F and don’t fly when the temperature is below 55°F. They do not forage in rain or in wind stronger than 12 mph. Cloudiness also reduces flight activity, especially near threshold temperatures. A cold spell in June can reduce fruit set significantly because of reduced bee flights. While honey bees can work over a 2 mile distance, a case can be made for placing honey bee hives at more than one location in or around the field in early plantings to address shorter flights in bad weather. Bumblebees are stronger fliers that can fly in heavier winds and are active at lower temperatures. Placing bumblebee hives throughout the field may improve early fruit set. Growers should be cautioned not to place bumblebee hives near honeybees because the honeybees will place stress on and rob from the bumblebee colonies if both honey bees and bumblebees are used.

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

BeeBox