Lupinus mutabilis is a species of lupin grown in the Andes, mainly for its edible bean. Vernacular names include tarwi, tarhui, chocho, altramuz, Andean lupin, South American lupin, Peruvian field lupin, or pearl lupin.[1] There are different aspects why L. mutabilis could become an important international crop. But until now, the high content of alkaloids in the seed is the main reason why the crop is not very known outside the Andes.

Like beetles, flies also undergo complete metamorphosis. The larvae, however, are commonly referred to as maggots. The fact that flies have only a single pair of wings distinguishes them from most other flying insects. A pair of small knoblike appendages, known as halters, is all that remains of what may have been a hind pair of wings. Adults have either piercing-sucking or sponging mouthparts, but larvae are endowed with chewing or rasping mouthparts.Anthomyiidae (root maggot flies). In this family, the larva is the injurious life stage. Although most larvae are root- and seed-destroying maggots, some are leafminers. The seedcorn maggot is a common pest belonging to this family. To distinguish the adults from similar flies, wing venation must be closely examined (see couplet 3 of Key to Adults).

Cecidomyiidae (midges). Although most injurious members of this family are gall formers, the agronomic pest species do not cause this type of damage. The adult midges are innocuous, gnatlike insects; the larvae feed in seedheads or between the leaf sheath and stem. Adults can often be identified by their long legs and antennae. The Hessian fly and the sorghum midge are notorious members of this family.

Which species do you think for? There is a steam borer fly, insect pest of Lupinus (and polyphagous) named Delia platura ? 

http://entnemdept.ufl.edu/creatures/FIELD/CORN/seedcorn_maggot.htm

Dear Catalina,

Considering the aspect of the nymph I think this is a Cecidomyiidae (not an Anthomyiidae), but without any indication about their size or pictures of adults,  this is hard to be certain.

Thanks for your answers, in the north of my country was identified Delia platura, but we discarded  this genus, because the damage is different. We are thinking in Cecidomyiidae too. We are breeding larvae in the lab. Now I´m looking for an identification key for this family

Dear Catalina,

Please check this paper for Delia pests identification.

http://actazool.nhmus.hu/49/2/darvas.pdf

Attila

(Also if you think you may write to Dr. Bela Darvas DSc., he is dipterologist and plant protection specialist)

Dear Catalina

I am agree with Ravi and Vicent about your insects photographs, they belong to Cecidomyiidae. But I think that stem damage was caused by other insect (probably Anthomyiidae or Agromyzidae) and you found the secondary pest. 

Javier

Thanks for your inputs. We are conducting a study of the behavior of the pest in the different phenological stages of the crop, when we get adults we can move forward more certainty. 

Thanks Attila for the contact and the information you gave me.