The Cosmic Monster (1958)

Starring Forrest Tucker, Gaby Andre, Martin Benson, Wyndham Goldie, Alec Mango
Directed by Gilbert Gunn
(actor & director credits courtesy IMDB.com)

Scientists experimenting with magnetic fields accidentally expose a nearby forest to dangerous cosmic rays, which transform the forest's insects into giant monsters. 

I like the premise of this British science fiction picture, and the casting, as I think Tucker and Andre provide sympathetic and engaging leads, but the promise of the film based on those elements is unfortunately squandered by static photography and weak special effects.  Giant projections of insects, although more finely rendered then in some other pictures of the era, just don't cut it as the movie's bogeymen.  If some stop-motion creatures or detailed puppets had been used, I think this would have been a much more memorable and enjoyable film, but that was likely beyond the production's limited budget.  There's a couple moments where the insects are well-integrated into shots with the actors, but for the most part the footage is uninspired and disappointing.  Similarly the presence of an alien character doesn't add much interest to the proceedings, as there's no creativity to his introduction, nor much emphasis on making him stand out visually.  The music score by Robert Sharples isn't bad and the story includes some clever ideas, but the film would have fared far better in different hands.