And doing it in the dark?
In a recent paper, David Pattemore and associates reveal some fairly different observations about avocado flowering. One, that the female stage can be open and potentially receptive to pollination at night. And Two, that moths and crane flies amongst other nocturnal insects are visiting the flowers and carrying pollen!!! These are two very new observations, made possible by the digital world we live in.
Of course, insect visitation doesn't mean fruit set. Is there pollination, transfer of pollen to the female stage? Is there enough pollen? Is it the right pollen? Is it the right temperature for fertilization to occur? Whatever else needs to happen for fruit set, is it happening?
But these observations are opening up new discussion topics for the avocado world.
LOW OVERNIGHT TEMPERATURES ASSOCIATED WITH A DELAY IN ‘HASS' AVOCADO (PERSEA AMERICANA) FEMALE FLOWER OPENING, LEADING TO NOCTURNAL FLOWERING
David Pattemore1,2*, Max N. Buxton1, Brian T. Cutting1, Heather McBrydie1, Mark Goodwin1, Arnon Dag3
1The New Zealand Institute for Plant & Food Research Limited, Ruakura Research Centre, Hamilton 3210, New Zealand
2School of Biological Sciences, University of Auckland, Auckland, New Zealand
3Gilat Research Center, Agricultural Research Organization, 85280, Israel
Abstract—Avocado (Persea americana) has synchronously protogynous flowers: flowers open first in female phase before closing and opening the next day in male phase. Cultivars are grouped based on whether the flowers typically first open in female phase in the morning (type A), or in the afternoon (type B). However, it is known that environmental factors can alter the timing of flower opening, with cold temperatures being shown to affect the timing of flowering. The aim of this study was to investigate how low spring temperatures in New Zealand affect the flowering cycle of commercial avocado cultivars, focusing primarily on the receptive female phase of ‘Hass', a type A cultivar. Time-lapse photography was used to assess flower opening times of ‘Hass' over three years. Decreasing minimum overnight temperatures were associated with a delay in the timing of ‘Hass' female flower phases and resulted in nocturnal flowering of both male and female phase flowers. We recorded insects visiting female flowers at night, and some nocturnal flower visitors collected were carrying avocado pollen. Our study suggests that nocturnal pollination needs to be considered for avocados grown in temperate regions. Furthermore, as the timing of the female phase of ‘Hass' varied significantly with overnight temperature, the activity patterns of potential pollinators need to be considered to ensure adequate pollinator activity across the range of times in which ‘Hass' flowers are receptive.
Eight different invertebrate orders were captured from avocado flowers at night. Coleoptera, Diptera and Lepidoptera were the most frequently caught floral visitors, but it was coleopteran, dipteran and neuropteran individuals that carried the greatest number of pollen grains on average. This is an important distinction to make, as not all floral visitors behave as pollinators: visitation does not necessarily infer pollination. Species such as Costelytra zealandica (Coleoptera), Micromus tasmaniae (Neuroptera), along with Tipulidae and Sylvicola species (Diptera) may be especially important, as these were both frequently caught and often carried a high number of pollen grains. Compared with diurnal pollination, nocturnal pollination is poorly understood and relatively little research in New Zealand has tested assumptions that nocturnal floral visitors can act as pollinators.
This is an interesting read and introduces further areas of pursuit to understanding what brings on fruiting in the wild avocado.
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Rhapsa scotosialis – a potential avocado pollinator?
Where sufficient habitat is available, wild native bee species often provide all of the pollination services needed for high crop yields and fruit quality. Depending on the species, native bees may nest in underground tunnels, hollow plant stems, and tunnels in wood. Bumble bee colonies favor small cavities under lodged grass, in abandoned rodent burrows, in trees, or old bird nests. These unmanaged pollinators are an on-site natural resource, and unlike honey bees, cannot be moved from the field when pesticides are used. In fact, many ground-nesting species, such as squash bees, long-horned bees, mining bees, and sweat bees, construct their nests in the midst of annual and perennial crop fields. To protect these bees, ensure that drifting pesticides never contact adjacent habitat, even when crops or wildflowers are not in bloom. Scout crop fields, and protect ground nests of solitary bees and bumble bees from insecticide spray. Learn more about managing pesticides and protecting bees at:
Oregon State University Extension: https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/pnw591.pdf
Xerces Society: www.xerces.org/
Bee Book: http://www.helpabee.org/
Imagine going to the supermarket to stock up on groceries but coming home empty-handed because you just couldn't figure out how to work the shopping cart or figure out how to get to the ice cream tubs in the freezer aisle.
Welcome to the life of a bumblebee.
Gathering sweet nectar from flowers, it turns out, is much more difficult than one might think, and it requires a lengthy learning process. By the time a bee has figured out how to efficiently pry open the lips of a snapdragon flower, for example, most likely it has made dozens, if not hundreds, of floral visits.
How does a bee in charge of shopping for food needed to raise dozens of hungry larvae back in the hive learn to navigate the multitude of floral architectures it may encounter during an average workday, let alone over the course of its life?
Mostly by what biologists call associative learning, more widely known as trial and error, researchers have found. But while extensive research -- starting with famous bee researcher and Nobel laureate Karl von Frisch a century ago -- has focused on uncovering how bees forage for nectar, much less is known about how bees go about collecting pollen, which constitutes the most important protein source for the developing brood in the hive.
Avery Russell, Stephen Buchman and Daniel Papaj in the Department of Ecology and Evolutionary Biology at the University of Arizona decided to take a closer look. In a new paper published in the journal Behavioral Ecology, they tell a fascinating story of what is involved in a seemingly simple process of a bumblebee visiting a flower to gather pollen. And for the first time, they have untangled the subtle cues that a bee looks for when she visits a flower in search for pollen.
"For a long time, we have known that bees can learn all kinds of cues -- tactile, visual and olfactory -- when going after nectar rewards," says Russell, the study's first author. "When you open a can, you have to use a can opener, then use your fingers to pry the lid open. A bee might have to pop open the flower's petals, and might have to try many times over multiple trips until they get good at it. But not much was known in the context of pollen rewards."
Specifically, Russell and his co-authors wanted to know if bees need to learn in order to collect pollen efficiently from flowers that vary in their form. The research suggests they don't, and they don't need to.
"Our findings suggest that unlike nectar foraging, which requires complex learning behavior, bumblebees already know how to collect pollen," says Russell, who did the research as a doctoral student in the UA's Graduate Interdisciplinary Program in Entomology and Insect Science, "and they do it by switching between two responses that are seemingly hardwired into their brains."
Once a bumblebee touches down on a flower, it wastes no time. If it senses that the anthers are laden with abundant pollen just waiting to be shaken off like ripened apples from a tree, the bee does the obvious: a behavior that bee researchers call "scrabbling." Using its mandibles and legs, the bee brushes the pollen grains onto its body, then combs them off into collection baskets located on each of its hind legs.
"If you picture a happy toddler in a play pit filled with plastic balls, you get the idea of scrabbling," Russell says.
However, some flowers make their pollen grains more difficult to access, or sport intricate anther designs that dispense only a little bit of pollen at a time.
"That way, the plant makes sure pollinators don't eat it all, but carry it to other flowers for pollination instead, and also leave some for other visitors as well, so the flowers aren't limited to a single pollinator," he says.
When visiting some of these trickier flowers, Russell's team found, bumblebees switch to a different behavior called sonication -- or, in more familiar terms, buzzing. Not unlike a sonicating toothbrush that vibrates to shake plaque from teeth, a sonicating bee vibrates vigorously to free pollen grains hidden inside the flower.
The team observed that the bees switched between these two motor regimes depending on chemical and mechanical cues: They scrabbled when pollen was abundant, and sonicated when pollen was scarce, either because the flower already had been depleted or because its pollen is less accessible by design.
To tease apart the cues that trigger each behavior, the researchers made artificial flowers and treated some of them with chemical extracts from natural anthers. Bees visiting a surrogate flower without extract didn't stick around and took off again in search of more rewarding offerings. When they encountered a foam flower without pollen but with the chemical cue, they buzzed them in a futile attempt to harvest the nonexistent pollen. And when they sensed pollen grains, even artificial ones, scrabbling ensued.
"Bumblebees tend to sonicate on pollen-concealing anthers right away, but they also buzz accessible anthers when they can't detect pollen by touch," Russell says. "We think they do that in an effort to collect the dregs from a flower after most of its pollen has been harvested."
Being able to switch between two programmed routines allows bees to effectively collect pollen from flowers in many different shapes and forms, the researchers conclude. This flexibility also may explain a fact that had evolutionary biologists stumped for a long time: Flowers with concealed pollen stores evolved many times independently, suggesting that pollinators must always have had a way to harvest pollen from them, or else the co-evolution between the two would have led to a dead end and not survived.
"Researchers used to think that floral sonication is a behavior only used to collect pollen from concealed pollen stores," Russell says, "but because we often observe bees buzzing on flowers with accessible pollen, we conclude that it's a behavior that has evolved as a general strategy to collect pollen from any type of flower."
To buzz or to scrabble? To foraging bees, that's the question
Presenting bumblebees with various combinations of natural and artificial flowers laced with chemical and mechanical cues, UA biologists have discovered that for a bumblebee, foraging for pollen versus nectar is very different.
photo: honeybee on avocado flower. Is it buzzing or scrabbling?
Diadasia bituberculata – Digger Bee
The hills are alive with the sound of BEE-EEZZE. And often they are found crawling on the ground, as is the case of Digger Bees. At this time of year, they might be seen along the margins of avocado orchards, near hiking trails or in undisturbed areas of citrus orchards. They are called Digger Bees commonly, but this is just a generic name for a large group of bees that nest in the ground. There are many genera and species and because of the general lack of study of these bees they are lumped under the name Digger for lack of any greater knowledge and naming of them. In the case of this bee find reported here, they are possibly Diadasia bituberculata – as suggested by Robin Thorpe, retired UCD entomologist. They are uncommon in the rest of the world, but found here in California and western US.
There are several kinds of small hairy or metallic bees that dig into the soil to nest, hence the common name, digger bees. They are a diverse group that comes from different families and the term digger bee can include the andrenid bees, halictid bees, and colletid bees such as the plasterer and yellow-faced bees. These are solitary bees and native pollinators that are active early in the season. Each female digs a cylindrical underground tunnel as a nest where she reproduces (as opposed to social bees such as honey bees where only the queen reproduces and maintains a colony with the help of sterile workers). Although solitary, they form colonies that may have several hundred nests in one spot, but all nests are independently owned.
The subterranean nest is provisioned with a mixture of nectar and pollen collected from nearby flowering plants. This "bee-bread" is food for the bee's offspring (larvae) that develop in the underground chamber and emerge as adults the following year.
Digger bees are 1/4 to 1/2-inch-long and variable in color (mostly shiny metallic or dark, but some with markings of white, yellow or reddish brown). There is one generation of digger bees per summer and once the adults finish perpetuating the species by laying eggs of the next generation there will be no activity till the following spring.
Digger bee nests are commonly located in areas where grass and mulch are scarse, either from too much shade, previous drought conditions or other stress. Most of them like to fly around their airspace at different times of the day, something to do with mating, air temperature or staking territory. They often travel great distances to forage.
The threat of being stung by digger bees is unlikely. The bees are docile and not likely to sting unless handled or threatened. There is no nest guarding behavior or attack behavior like there is with social insects such as honey bees and yellowjacket wasps.
Image: Digger Bee "Colony", Thanks to Pest Control Adviser Jane Delahoyde's friend.
Gordon Frankie a bee biologist at UC Berkeley and I are doing a study to ultimately identify what plants could grown in avocado orchards to attract more honeybees, as well as other pollinators and potential biocontrol agents. There are five orchards in Ventura and Santa Barbara where we have been monitoring flower visitation by different insects in order to get a baseline of what is there before introducing potential pollinator/biocontrol attractants. The numbers are finally in for last spring avocado bloom. Virtually no honeybees, but significant numbers of syrphid flies. They are also called flower flies or hover flies. They superficially resemble bees. They are predatory as larvae, primarily feeding on aphids. Then they become important pollinators as adults. They are not as efficient as honeybees because they lack all the hairs on their bodies where pollen gets stuck and carried to female flowers. The cause of the honeybee decline has many causes, but the most likely one is the drought in the avocado growing areas. There just aren't any plants in the foothill areas to provide pollen and nectar year round.
Images of egg, larvae (going after aphids) and adult syrphid flies