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Pollen Use in Bees and Other Insects

Session leader: Margrit McIntosh.

Main sources for this introduction: Burgess (1991), Gess (1996), Kevan and Baker (1983), Labandeira (1997)

Pollen use by bees (Hymenoptera: Apoidea) is described in detail in the up-to-date and comprehensive review by Wcislo and Cane (1996); hence, in this introduction I will focus on pollen use in other insects.

Pollen grains consist of a nutrient-rich inner protoplasm, surrounded by an exceptionably durable coat ("exine"), and the grains are often covered by oil-rich and fragrance-rich "pollenkit" (a substance derived from tapetal tissue in the anther). Pollen contains abundant protein and free amino acids, although the proportion of these nutrients can vary widely among plant taxa. The protein is intended for the growth of the pollen tube, and it is this nutrient in particular that makes pollen an appropriate food for the developmental stages of insects. Lipids are always present; starch may or may not be present.

There is abundant evidence for pollen- and spore-eating by insects in the fossil record. Pollen evolved from the microspores of seedless vascular plants, appearing in the fossil record during the Carboniferous. Spores and pollen ingested and excreted by insects during the Late Carboniferous to Miocene eras are sometimes even identifiable to source plant taxa (Labandeira 1997). Fossil insects associated with pollen feeding include thysanurans, diaphanopterodean nymphs, adult hemipteroids, protorthopterans, and grylloblattids (Labandeira 1997). Modern pollen-eating insects occur in at least 14 orders (Table 1).

Insects have evolved several different methods for ingesting pollen grain, and the mode of ingestion may or may not be associated with specialized mouthparts. The most common modes include:

  1. Swallowing the grains whole.
    Following ingestion, the grains may burst due to osmotic pressure. Or the contents may be digested and absorbed through the pores in the exine of the grains.
  2. Chewing up the grains.
    Pollen exine fragments are not digested, and may be identified in insect excretions.
    => Bee larvae either chew or swallow pollen grains.
  3. The punch-and-suck method (e.g., certain thrips)
    The insect mouthparts are used to pierce the grain through one of the pores, and the contents are extracted without swallowing the grains.

There appear to be two primary paths by which pollen-feeding has been arrived at, evolutionarily, by insects. One is via carnivory, and in particular, feeding on other insects. This seems to be the path taken by bees, who share a common ancestor with the exclusively carnivorous sphecid wasps; by masarine wasps in the family Vespidae; and by (e.g.) staphylinid beetles. Because it is a rich source of protein, pollen is nutritionally much more similar to insects themselves than it is to non-reproductive plant parts. The second path is that taken by phytophagous insects which come to feed on the flowers of plants, instead of on foliar tissue or other parts. Some insects are general florivores, while others are specialized to pollen or ovules.

A particularly interesting case is that of the masarine wasps (Hymenoptera: Vespidae: Masarinae), which have independently evolved the habit of provisioning their larvae with a mixture of pollen and nectar, as do bees. Some masarines carry the pollen in their crop and then regurgitate it to form the provisions, as do some bees (Colletidae: Hylaeinae). Others have developed brushes on their legs for carrying pollen, such as those possessed by bees -- but these masarine wasps have them on the forelegs, instead of the hindlegs! In many cases it is thought that masarine wasps are effective pollinators of the plants they visit. I find it particularly fascinating that most masarines seems to be specialists on their pollen hosts, as are most bees (and, of course, as are most herbivorous insects). There are only about 300 known species of masarines, compared to the >20,000 spp. of bees.

Just in the past year, a report has been published of a pollen-collecting sphecid wasp in Sri Lanka (Krombein & Norden 1997; Hymenoptera: Sphecidae: Crabroninae), so it would appear that the habit of collecting pollen instead of insects to provision the larvae has evolved three separate times in the Hymenoptera.

Table 1: Insects that eat pollen. Compiled from Burgess (1991), and Kevan & Baker (1983)

HEMIPTERA Anthocoridae
COLEOPTERA Alleculidae
DIPTERA Anthomyiidae
LEPIDOPTERA Micropterygidae
Vespidae (Masarinae)
Sphecidae (one species; Krombein & Norden 1997)

Required papers

Full references list

General References

Pollen Use in Bees

Pollen Use in Masarine Wasps

Pollen Use in Other Insects

Copyright 1998, Margrit Mcintosh.