After years of hammering away at Apple's share of the smartphone market with cheap-to-free Android phones, Google has lately adopted a new tactic to win mobile.

Call it "the worm strategy"—because Google is attacking Apple from the inside out.

Over the past six months, Google has begun to systematically replace core, Apple-made iOS apps with Google-made iOS apps.

In July, Google launched Chrome for iPhone—a Safari replacement.

Then, in October came Google Search—which included a voice search feature to compete with Siri.

In December, Google launched Google Maps to replace Apple Maps, and a much-improved Gmail to replace Apple's core Mail app.

It also put out a new YouTube app, to replace the one that Apple removed during its last iOS upgrade.

Google doesn't plan to stop there.

In fact, it's throwing more resources at iOS development.

9to5Mac's Jake Smith notes that Google has launched an ad campaign recruiting iOS developers into the company, so that they can “do cool things that matter."

The ad links to a page on Google's internal jobs site—an interview with Google iOS developer Ken Bongort, headlined: "Wait, Google has iOS mobile apps teams?"

In the interview, Bongort talks about all the apps Google has launched on iOS of late, and then concludes: "Needless to say, there are similar opportunities to create and design new experiences for all Google apps on the iOS platform."

Google's tactic is working.

The anecdotal evidence: Business Insider gadget editor Steve Kovach says he's put almost all of his Apple-made iOS apps into a hidden folder, preferring to use apps from Google and others instead. Over on AllThingsD, Liz Gannes says 2012 was the year "I basically stopped using iOS apps."

She says Google apps are one big reason why:

More often than not lately, I find myself using a Google iOS app. I’m now using Google Maps instead of Apple Maps, Chrome instead of Safari, and Gmail instead of Mail. Those three have all made it to my home screen, where they replaced the in-house apps.

For the record, I wouldn’t consider myself a partisan of either Google or Apple — I’m praising Google’s mobile software here, but my primary phone is still an iPhone. I think iOS is a very nice operating system. But the Google utility apps increasingly fit my needs better, for some of the most important and basic things a smartphone does.

Then there's the statistical evidence that Google's inside-out tactic is working.

AppData, which monitors iTunes App Store rankings, reports that the numbers one and two free iPhone apps right now are YouTube and Google Maps.

None of this makes Apple executives very happy.

The world's universally positive reaction to Google Maps—and rush to replace Apple Maps—reportedly has Apple executives "seething." No surprise: Apple CEO Tim Cook fired iOS software boss Scott Forstall when Forstall refused to apologize for the product's deficiencies.

Meanwhile, this trend has to have Google executives elated.

When Apple launched the iPhone App Store in 2008, it threatened to permanently disrupt Google's most important business: search.

If the world became one where:

Businesses directed all their resources toward building apps to be discovered in the iTunes app store instead of building Web pages to be discovered by Google search.

And consumers went straight to non-Google apps for search problems like where they should and what they should buy.

Then Google search ads—still more than 95 percent of Google's revenues—would become much less valuable.

Instead, the world has become one where people there are two kinds of people.

There are Android users, surrounded by Google search, and there are iPhone users, downloading Google apps—all of which make Google search a prominent feature.