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

• Which of the following best expresses the main idea of the second paragraph of the passage?

• Throughout the passage, the author implies that the Mandans _____.

• In order to operate a computer, a student does not have to be _______

ZUNIA12

• The best way to protect children from improper material is .............

• What is the zone of volcanoes and seismic activity in the world called?

• The word "their" in line 12 refers to ......

• The Beatles stopped their live performances because .............

• Which of the following is NOT mentioned in the passage?

• What can be inferred about railroads in 1849 from the information in the last paragraph?

• The phrase ‘as well as’ in paragraph 4 is closest in meaning to ___.

• The word “laundry” in paragraph 2 is closest in meaning to ___________.

• How many orchid seeds are typically pollinated at one time?

• The word "symptoms" is closest in meaning to ___________.

• The author mentions the trail pheromone of the leafcutter ant in line 11 to point out ........

• What is NOT true about the passage?

• What is the main idea of the passage?

• The word “minute” in the second paragraph is closest in meaning to _______.

• The passage is mainly about ..................

• According to the passage, copyright law is _______

• The word "vulnerable" in paragraph 4 is closest in meaning to .............