Humans have struggled against weeds since the beginnings of agriculture. Marring our gardens is one of the milder effects of weeds - any plants that thrive where they are unwanted. They clog waterways, destroy wildlife habitats, and impede farming. Their spread eliminates grazing areas and accounts for one-third of all crop loss. They compete for sunlight, nutrients, and water with useful plants.

   The global need for weed control had been answered mainly by the chemical industry. Its herbicides are effective and sometimes necessary, but some pose serious problems, particularly if misused.

   Toxic compounds threaten animal and public health when they accumulate in food plants, groundwater, and drinking water. They also harm workers who apply them.

   In recent years, the chemical industry has introduced several herbicides that are more ecologically sound. Yet new chemicals alone cannot solve the world's weed problems. Hence, scientists are exploring the innate weed-killing powers of living organisms, primarily insects and microorganisms. The biological agents now in use are environmentally benign and are harmless to humans. They can be chosen for their ability to attack selected targets and leave crops and other plants untouched. In contrast, some of the most effective chemicals kill virtually all the plants they come in contact with, sparing only those that are naturally resistant or have been genetically modified for resistance. Furthermore, a number of biological agents can be administered only once, after which no added applications are needed. Chemicals typically must be used several times per growing season.

Which of the following terms does the author define in the first paragraph?

Câu hỏi liên quan

• According to the writer, students’ difficulties to cope with college life are partly due to ____________.

• What is Martin Luthur King well- known for?

• What is Martin Luthur King well- known for?

ZUNIA12

• With what topic does this passage primarily deal?

• According to the first paragraph, how is the life changed since our parents time?

• It can be inferred from the passage that .............

• According to the passage, which of the following factors might spoil the tourists’ fun at Mediterranean resorts and beaches?

• Which word is similar in meaning to the word “glue” ?

• Brown algae would most likely be found ...........

• What percent of the earth’s area do rain forests cover?

• The passage indicates that Harvard is ............

• The word "assigned" in the fourth paragraph mostly means ......

• According to paragraph 2, evidence from fossil suggests that ...............

• According to the passage, what does a constricted and harsh voice indicate?

• You can be happy if your surgeon can operate because it means  .................

• It can be inferred that the names of the three basic shapes mentioned in paragraph 2 reflect ______.

• Why does the author mention spaghetti?

• Which of the following helps to create the Earth’s magnetic fields?

• The topic of this passage is______________.

• What is the main idea of the passage?