Will ALM pupate in the soil?

Allium leafminer (ALM) larvae feed on the leaf tissues of allium crops then will pupate directly in the plant. We found this in many of the leeks that we sampled from the field. The pupae can be found in between leaves towards the bottom of the plant.

 

Five pupae can be seen on the outside of a leek pulled from the field

 

A single pupa is found in the outer leaf of a leek plant

We suspected that some of the larvae may leave the plant to pupate in the soil immediately surrounding the leek plant. So we conducted a float test on the roots and soil from the leeks we collected from the field in early February. Since the pupae are filled with air along with the developing ALM, they should float when submerged in water or salt water. We tried both.

We filled two tubs with 10 L of water. The second tub also contained 3 kg of table salt. A salt water solution is commonly used to sample for insect larvae by encouraging them to exit the substrate (soil, plant or fruit material) and float to the surface. Since pupae do not move much, we suspected that regular water may be just as effective.

 

Leeks submerged in salt solution.

Next, we submerged our leeks and gently loosened the soil around the roots. After just a few minutes and some gentle agitation pupae began to float to the surface! As we suspected, pupae emerged in the water-only tub as well.

 

Small brown pupa (center of picture) floating on the surface.

The pupae were carefully removed but unfortunately many of them did not look viable. It is possible that what we found were empty puparia - the casings left behind after the adult fly ecloses (emerges). It is also possible that they became damaged during agitation. The flotation method appears to be a good method for counting pupae in soil but may be less effective for collecting viable pupae for our colony.

 

Riley carefully removing pupae from the solution and placing on filter paper.

 

Pupae found floating on the surface of the water. The larger pupae could be ALM.

We have found approximately 11 pupae so far, some of which may not be ALM, and have many more samples to go. In order to confirm whether the pupae we found were ALM, we would have to rear them to adult flies. We will be more gentle when loosening the soil around the roots to avoid potentially damaging the pupae so we can attempt to identify them. If we can confirm that the pupae we find are ALM then it would seem that ALM will pupate in the soil. We will provide an update as we continue processing the soil samples. Luckily, we also found that the water-only flotation was as effective as the salt solution so we can save the salt for french fries.