Why people don't harvest Locusts when infestation happeds?
Locusts have been harvested in Thailand and Australia they call them sky-prawns. They deep-fry them and consume them happily.  

http://www.bugsfeed.com/locust

http://www.fao.org/docrep/017/i3246e/i3246e.pdf

In Thailand Orthoptera or
grasshoppers include Patanga succincta,
Locusta migratoria, Acrida sp.,
Cyrtacanthacris tatarica and Oxya
japonica japonica (Thunb.)
(Hanboonsong et al. 2001; RaƩ anapan
2000). All of them are pests of
economic crops such as maize and
rice. Grasshopper species, parƟ cularly
Patanga succincta and Locusta
migratoria, used to be one of the
major pests of maize and rice. Today
they have become one of the most
popular edible insects since they were
introduced for human consumpƟ on
by entomologists − a campaign to eat
grasshoppers had been launched
because control efforts had been
unsuccessful. For example, in 1983
this was launched by local offi cials and
villagers in Prachin Buri Province
collected more than 10 tonnes for use
as food (Lewvanich et al. 1999).*