Poisonous plants can be a problem under certain conditions in limited areas. If a plant's nectar is poisonous, the symptoms of plant poisoning are limited to the blooming period of the plant. However, if the poison is in the pollen, the symptoms may linger as long as the pollen remains in the combs. There is no clear-cut method for differentiating between plant poisoning and pesticide poisoning. The effects of plant poisoning are usually more gradual and last longer than the effects of pesticide poisoning. Plant poisoning usually occurs in the same geographical area at the same time each year, whereas pesticide poisoning is indiscriminate. For a good review of poisonous plants, see Barker (1978). Some examples of plant poisoning are listed below and in table 3.

Purple Brood
Purple brood occurs when adult bees collect and use the pollen and nectar from Cyrilla racemiflora (titi, southern leatherwood). This "disease" is characterized by the blue or purple color of the affected larvae.

Paralysis
Aesculus californica (California buckeye) is probably the best known of the poisonous plants in the United States. Field bees exhibit symptoms similar to those of chronic bee paralysis; i.e., the bees are black and shiny from loss of hair and they tremble. Also, either the eggs do not hatch or the larvae die soon after hatching.

Milkweed Pollinia

The pollen of milkweed (Asciepias species) is produced in pollinia (coherent pollen grains) that are attached in pairs by a slender filament. When removed from a flower, the pollinia resemble a wishbone with pollen masses hanging from the ends. Honey bees become ensnared in the thin pollinia attachment and free themselves by pulling the pollinia from the flower. Honey bees often become seriously encumbered and unable to effectively fly or crawl because of the structures that remain attached to their body parts.

Table 3. Comparative symptoms in honey bees poisoned by toxic chemicals and selected plants

Source of poison	Stages most affected	Effect on adult	Effect on brood	Effect on colony
Toxic Chemicals	Adult	Field bees die in or near hive. Nurse bees may also die. Queens usually not affected.	Usually few larvae killed. symptoms of starvation may be evident if adult population reduced severely.	Weakened or killed. Many dead bees near hive.
California buckeye (Aesculus californica)	Young brood	Emerging young workers often deformed, pale, Some hairless and tremble. Queens lay eggs at reduced rate, cease, or become drone layers.	Eggs normal at outset; later fail to hatch or all are drone eggs. Larvae die soon after hatching and disappear. Little or no capped brood; if present, scattered.	Weakened or killed. May be many dead bees near entrance. Supersedure of queen may fail.
Yellow jessamine (Gelsemium sempervirens)	Larva, pupa, and young adult	Young workers affected and soon die. Old adults appear normal.	Pupae die in cells and become mummified.	Slightly to severely weakened.
Loco plants (Astragalus spp.)	Adult and pupa	Field bees die. Some become black and tremble. Queen may die.	Many cells contain dried pupae.	Population dwindles. Colony may die.
False hellebore (Veratrum californicum)	Adult	Many field bees die between plants and hive. Adults die in curled state. Queens not affected.	No effect	Field population lost.
Southern leatherwood (Cyrilla racemiflora)	Larva	No effect.	Many blue or purple larvae die in cells when nearly mature.	Slight to severe weakening.

Source: Modified from Burnside and Vansell (1936).