38.3: Animals That Protect Plants

Plants are food and there are myriad animals ranging from small insects to large mammals that eat them. There are also other animals that eat the ones eating the plants. In some cases when plants are attacked they emit chemicals which attract their protectors.

It has long been noted that plants respond to leaf devouring insect attacks by releasing volatile chemicals, a response that not only leads other plants to beef up their own leaf level of insect-repellents but that sometimes draws in specific insect predators and parasitizing wasps (Pare & Tumlinson, 1999). The timing and intensity of release can vary in accordance with a multiplicity of environmental factors, and blends of different odor-producing volatiles can be produced in response to different leaf-eaters, possibly summoning particular carnivorous insects specialized to feast on each kind of herbivore, making it a highly sophisticated response that has been considered, according to a ‘behavioural ecological approach’ that speaks in terms of plant ‘decisions,’ and a ‘crying for help’ within the larger ecological community (Dicke, 2009).

In other cases insects that prey on plant eating insects were purposefully introduced. Three of these include (Freedman, 1995)

• The cottony-cushion scale (Icyera purchasi) is a sap-sucking insect that was accidentally introduced to the United States, where it became a threat to citrus agriculture. Research in its native Australia discovered that the pest was naturally controlled by certain insect predators and parasites. In 1888, two of its predators, a lady beetle and a parasitic fly, were introduced to California. This allowed almost total control over this potentially disastrous pest. Unfortunately, this biological control was disrupted when DDT and other broad-spectrum insecticides were used to deal with other orchard pests beginning in the late 1940s.
• St. John’s wort (Hypericum perforatum), a common weed, is toxic to cattle. It became a serious pest in pastures after it was introduced from Europe. In 1943, two leaf beetles that feed on this plant were released to North America, and this pest is no longer an important problem.
• The prickly pear cactus (Opuntia stricta) was imported to Australia from North America and grown as an ornamental plant and a “living fence.” It escaped and became a serious weed in rangelands. This pest was controlled by the introduction of one of its herbivores, a moth whose larvae feed on the cactus.

Birds also are great consumers of insects (as well as themselves eating seeds and grain) but on balance they protect plants against insects which eat more. The Four Pests campaign was one of the first actions taken in the Great Leap Forward in China from 1958 to 1962. Four pests were targeted for elimination: rats, flies, mosquitoes, and sparrows. The extermination of sparrows resulted in severe ecological imbalance, being one of the causes of the Great Chinese Famine of 1959–1961. With no sparrows to eat them, locust populations ballooned, swarming the country and compounding ecological problems.

Below is a powerpoint video on this chapter

References

Dicke, M. (2009). Behavioural and community ecology of plants that cry for help. Planet, Cell, and Environment, 32(6), 654–665. https://doi.org/10.1111/j.1365-3040.2008.01913.x

Freedman, B., R. Morash, and D. MacKinnon. 1993. Short-term changes in vegetation after the silvicultural spraying of glyphosate herbicide onto regenerating clear-cuts in Nova Scotia, Canada. Canadian Journal of Forest Research, 23: 2300-2311.

Paré, P. W., & Tumlinson, J. H. (1999). Plant volatiles as a defense against insect herbivores. Plant Physiology, 121(2), 325–332. https://doi.org/10.1104/pp.121.2.325