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.

Corn Reproduction and High Temperatures

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu, Cory Whaley, Extension Agent – Agriculture, Sussex County; whaley@udel.edu, and Phillip Sylvester, Extension Agent – Agriculture, Kent County; philllip@udel.edu

Droughty conditions aren’t our only worry right now. Corn that survived the deluge of rain is tasseling in our earliest planted fields, and our current heat index can be an issue with pollination. While corn enjoys warmer weather, anything above 86°F will actually slow plant growth. A corn plant also prefers cooler nights, with temperatures in the 60s.

Pollination occurs during tasseling and silking stages, and high temperatures can adversely affect kernel formation either during pollination or grain fill. First of all, warmer weather this week may accelerate corn maturity earlier than we would like. High daytime temperatures (> 86°F) may limit photosynthesis which provides sugars for ear formation. A survey of worldwide research on corn growth and temperatures observed that the ideal temperature for growth as well as flowering was 86-87°F. Temperatures over 99°F severely affect pollen production during silking, but consecutive days in the 90s will at least reduce pollination. For grain fill, optimum temperatures are 80°F, with a maximum of 97°F. In the mid-west, high night-time temperatures (>72°F) have been observed to reduce grain fill, possibly due to the use sugars for respiration (energy) rather than kernel production.

This photo from last year shows a worst-case scenario when temperatures are high, with many kernels failing to pollinate. In most fields, it may just be a few kernels per ear that fail, but over several acres that can add up.

Over the week of June 28-July 4, when tasseling started in some fields, we have had temperatures at least 86°F, with four to six days in the 90s (Table 1). New Castle has seen two nights above 72°F, while both Kent and Sussex have had three. As of writing this, cooler days and nights are in the forecast, as more of our earlier planted fields will start pollination. That is good news for most fields that are just getting started, but for all the late-planted corn and replants, watch the temperatures in mid and late July. Temperature may be an additional factor to consider if this year’s yields are lower than expected.

Table 1: Number of days above threshold temperatures over the week of June 28-July 4

New Castle Kent Sussex
1 Week = 7 days possible
Daytime > 86°F 7 7 7
Daytime in the 90s 6 4 6
Nightime > 72°F 2 3 3


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 .

Lack of Netting in Muskmelons

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

Recently a muskmelon field was observed with a whole section of the field producing fruits without netting (bald).

Melons have several different potential rind patterns: smooth such as honeydews, wrinkled such as canary melons, or netted such as our eastern muskmelons and western cantaloupes. Netting is controlled genetically and is highly heritable. Breeders select for netting types in their programs when developing new melons.

Commonly we find lack of netting in muskmelons where fruits have not fully developed due to poor pollination and late in the season when nights are cool affecting fruit development. There is also an association with calcium levels and netting. Poor netting can be a result of calcium deficiencies under low pH soil conditions.

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.

Reduced Seed Set in Peas

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

There have been a number of early pea fields with reduced seed set. Pods developed but only one or two seeds were formed.

Reduced seed set is often related to flower development and pollination. Peas are self-pollinated. As the flower opens, the pollen from the anthers is released to the stigma of the pistil of the same flower. Once on the pollen is on the stigma, the pollen germinates and a pollen tube is formed and then grows down the style and when it reaches the ovule, the egg is fertilized by one of the two sperm cells, the other fuses with polar nuclei to become the seed endosperm. During the development of the pollen tube, plant hormones are released which are also essential for seed set.

Seed set problems therefore may be related to lack of pollen formation, pollen that does not release to the stigma, reduced pollen germination, abnormal pollen tube development, abnormalities in the stigma or stile, or abnormalities in the ovule. Lack of Gibberellin hormone release has also been shown to reduce seed set or lead to early seed abortion in peas. Stress to peas just after flowering has been shown to cause seed abortions.

What are the potential causes of reduced seed set in peas? Frost or freeze when flowers are opening has the potential to injure pollen or directly damage flower parts. Peas are very cold tolerant normally but are susceptible to injury at flowering. Our last freeze event on Delmarva was on April 9 where temperatures dropped to below 30°F at some locations. Fortunately, early peas were not in flower during that time.

Research has shown that peas under temperature and moisture stress produce fewer seeds. Experiments have shown that temperatures at 93°F or above can also reduce seed set in some varieties of peas. Dry soil conditions will magnify this effect. We had temperatures at 90°F in some locations on April 29 with upper canopies approaching critical temperatures during flowering.

Another factor to consider is timing of chemical applications to peas – applications near and at flowering may damage pea flowers under certain weather conditions and reduce seed set.

Pea pods with reduced seed set. Affected fields yielded 1000 lbs/a while nearby unaffected fields yielded near 4000 lbs/a.

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.