Sweet Deal: How Plants Invite Helpful Insects to Dine on Harmful Ones

I'm Shep O'Neal with the VOA Special English AGRICULTURE REPORT.

Students are taught at an early age about how insects help plants reproduce.  Schoolchildren learn that plants release sweet juices, or nectar, through small openings inside the flower.  These small openings are called nectaries.

The sweet-smelling nectar appeals to bees and other insects so they go to the plant.  While the bee is drinking the sweet nectar, the hairs on its legs become covered with pollen.  The bee flies to another flower and drops some of the pollen there.

During a flight, the bee usually visits several male and several female flowers.  In this way, flowers are able to reproduce.

Many plants, however, release nectar for another purpose.  Scientists have known about it for more than one hundred years.

The second way plants release nectar does not involve flowers.  The nectar is contained in extrafloral nectaries.  They are found on the tops of leaves, where the leaf and stem come together.

Trees that have extrafloral nectaries include the peach, poplar, viburnum, black locust and wild cherry.

Extrafloral nectaries are often smaller than a grain of salt. Researchers say the sweet juices released in them are not used to help the plant reproduce.  Instead, they are used to get some insects to come to the plant to help control harmful insects.

Scientists have known for many years that tiny ants feed on the sweet juices released by the extrafloral nectaries.  But only in more recent years did they make discoveries involving other insects.

Back in nineteen ninety-four there was a report about extrafloral nectaries in Agricultural Research magazine.  The magazine is published by the United States Agriculture Department.  It said two government scientists, Robert Pemberton and Jang-hoon Lee, had studied extrafloral nectaries for two years.  They looked for ways to control gypsy moths.

They did their research in forests near the South Korean capital, Seoul.  During their research, they discovered that two helpful insects liked the extrafloral nectaries.

In fact, the insects killed two times more gypsy moths on trees that had the extrafloral nectaries than on those that did not.  The insects are the Cotesia melanoscelus wasp and the Parasetigena silvestris fly.

A third insect, the Blepharipa schineri fly, also improved its control of gypsy moths on trees with extrafloral nectaries.

This VOA Special English AGRICULTURE REPORT was written by Bob Bowen.  Our reports are online at voaspecialenglish.com.  I'm Shep O'Neal.