1. A computer-implemented method performed by data processing apparatus comprising one or more computers in data communication, the method comprising: for each query submitted for a respective first search operation and for which first resources have been identified by the first search operation as being responsive the query, wherein each first resource is a web-page resource: determining a search probability ratio for the query that is at least based on a measure of a likelihood of the query being submitted for a second search operation being of search operation type that is different from a search operation type of the first search operation; for each query for which the search probability ratio for the query meets a threshold search probability ratio: initiating the second search operation and receiving data indicating second resources identified by the second search operation as being responsive the query, each second resource being a native application; determining, based at least on the search probability ratio, whether to insert a search result identifying the second resource in a set of search results identifying the first resources; and for each query for which a determination is made to insert a search result identifying the second resource in a set of search results identifying the first resources, inserting the search result identifying the second resource in the set of search results identifying the first resources.
4. The computer-implemented method of claim 1 , wherein inserting the search result identifying the second resource in the set of search results identifying the first resources comprises: determining an insertion score based, in part, on the search probability ratio, wherein the insertion score defines an ordinal insertion position at which a second resource search result referencing the second resource is to be inserted into a ranking of first resource search results referencing the first resources; and generating a search results resource for displaying the first resource search results according to their respective ordinal positions in the ranking and the second resource search results at the ordinal insertion position.