According to an article on TheConversation.com the insect population is expected to increase over the summer as we experience a rise in the average temperature.
The reason insect populations seem higher during this season is a complex interaction of rainfall, food sources and increasing temperatures.
Because insects are ectothermic their body temperature depends on the external environment, in summer the increase in temperature correlates with an increase in insect activity.
Read more at http://theconversation.com/from-warm-to-swarm-why-insect-activity-increases-in-summer-69637