It is easier to pull one or two invasive plants when first detected before they spread than treat thousands of acres later.
Here is a story about the importance of averting costly weed control efforts through prevention.

Back in 1954, a mysterious plant emerged in an isolated county in central Utah. Government officials met to discuss this new invasive plant, squarrose knapweed, once it began to spread to cover a few hundred hectares near the rural town of Eureka. Not knowing much about the nature of this new invader, they decided to study it. While studying the weed, it continued to spread, and soon the infestation was too large to remove. As a result, by the beginning of 2002, squarrose knapweed had spread widely in the western United States, and covered about 60,000 ha in Utah alone. Control of this weed costs hundreds of thousands of dollars every year, plus a lot of time and effort.

Here’s another story:

In 1938, a professor from the University of Idaho first identified just two plants of yellow starthistle near Lewiston, Idaho. It now infests hundreds of thousands of hectares in the region. In 1993 in Oregon, more than 40,000 ha were reported in Jackson County and 80,000 ha in Umatilla county. By 2001, these populations had doubled! Read more about yellow starthistle in the field guide.

Yellow starthistle spreads rapidly if not caught early. A quick response to new infestations can prevent the spread of invaders. Here’s an example from Sheridan County, Wyoming, a beautiful area in northern Wyoming in the shadow of the Bighorn Mountains. The expansive grasslands and forests are bisected by Interstate 25, a prime pathway for invasive weeds to be introduced. In 1982, county officials noticed that spotted knapweed was beginning to show up along the Interstate. This Interstate connected Wyoming to Montana, where spotted knapweed was a very large problem. The Sheridan County officials did not want Montana’s knapweed problem to spread into Wyoming. So they acted quickly to determine the size of the infestation, control it, and educate the community about it so everyone could pitch in. They initially found 120 ha of spotted and Russian knapweed. They immediately got to work. The county assisted landowners with the cost and application of the herbicides needed to control this invading plant. As a result, this plant is now under control in Sheridan county. With help from local landowners, the county monitors all known sites and is on the lookout for new infestations of knapweed, which are quickly treated.

Experts estimate that without this quick intervention, knapweed might have spread to cover as many as 2,000 ha in two years, and with no action, the infestation would have continued to increase exponentially, doubling about every five years.

This spotted knapweed can threaten natural areas if not stopped.
Once an infestation of invasive plants is large enough to be noticeable, it is usually too late to control it effectively without a large investment of money, effort and time.

It is best to find infestations when they are small.

The problem is that these small infestations are difficult to find, and they don’t scream out, "I am a problem!"
That is why land mangers need more detectives on the ground looking for these small infestations – people who can identify these plants, so if they see a small grouping of them, they can report them or remove them while there’s still time.

WHEN THE INFESTATION IS LARGE
When early detection and prevention are not employed, large hard-to-control infestations of invasive plants can occur. In many cases, these infestations are too large to get rid of; the best hope is to control seed production and to prevent them from spreading to new areas.

This plant diversity supports the great animal diversity in the park, including 70 species of mammals and 260 species of birds. But this diversity is threatened by large invasions of a number of nonnative plants, such as spotted knapweed, leafy spurge, and oxeye daisy.To control the spread of these aggressive plants, managers use a combination of methods (integrated pest management). They remove them by cutting, pulling or mowing them (mechanical control), they introduce insects that feed on the plants (biological control), and they treat them with herbicides (chemical control). For each site, the manager determines which combination of methods will be the most effective in controlling the invader while causing the least impact to the native ecosystem.

For example, in the Big Prairie area of the North Fork subdistrict of Glacier National Park, managers are trying to control a large infestation of leafy spurge. They have found that mowing tends to stimulate growth of these particular plants, so they discontinued it. They introduced a beetle that feeds on the plant, and while this biological control agent keeps the number of total seed heads down, it is not enough to totally eradicate the entire population of leafy spurge in this area. Now managers spray the smaller satellite populations of leafy spurge with an appropriate herbicide and let the beetle control the larger populations. They also educate visitors about what they need to do to prevent spreading this plant to new areas. This is an integrated approach that will manage (but not eliminate) the large leafy spurge infestation while keeping it from spreading to new areas.

Volunteers can help prevent problems with invasives. Here, students work in the native plant nursery.

(Source: Westbrooks, R. 1998. Invasive Plants, Changing the Landscape of America: Fact Book. Federal Interagency Committee for the Management of Noxious and Exotic Weeds (FICMNEW), Washington, D.C. 109 pp.)

See also:
Combining Weed Management Efforts, by Roger Sheley, Range Weeds Specialist, Montana State University at http://weeds.montana.edu/range/iwm.htm