Forages: Alfalfa Blotch Leafminer

Excerpt from Agronomy Guide for Field Crops (Chapter 5)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Action Thresholds
6. Management Strategies
7. Updates on Forages: Alfalfa Blotch Leafminer
8. Related links...

Description

The adult is a very small, black, hump-backed fly. The larvae are small, pale yellow maggots found within tunnels in the leaf tissue.

Life History

In late May, the adult fly emerges from pupa overwintering on the soil surface. The female adult lays her eggs inside the leaves of new alfalfa plants. The larvae develop inside small tunnels in the leaves. Mature larvae drop to the ground when mature and pupate. A second generation of adults appear in approximately 1 week (mid-July) and a third generation appears in mid-August.

Damage

This pest of alfalfa is now a more serious problem in Northern Ontario. Small pinhole punctures are left in the leaves when the adult feeds and lays its eggs. The developing maggots feed inside the leaflet, creating tunnels or mines between the top and bottom layers of the leaf. These tunnels usually begin at the base of the leaflet and widen towards the leaf apex, creating a "blotch" appearance. Feeding damage primarily decreases forage quality and seldom causes yield loss.

Plate 91. Alfalfa blotch leafminer larvae feed inside the leaflet, creating tunnels or mines between the top and bottom layers of the leaf.

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Scouting Technique

Scout fields weekly to determine the percentage of leaves with pinhole feeding.

Action Thresholds

Control is only necessary if more than 40% of leaflets show adult pinhole feeding.

Management Strategies

A species of parasite successfully controls the alfalfa blotch leafminer in Southern Ontario. Insecticides are harmful to this parasite and, therefore, are not recommended unless leafminer populations are extremely high. Refer to the OMAFRA Publication 812, Field Crop Protection Guide, for recommended insecticide products. For insecticides to be effective, they should be applied no later than the pinhole stage of feeding. (Order OMAFRA Publication 812, Field Crop Protection Guide)

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Updates on Forages: Alfalfa Blotch Leafminer

• Field Crop Protection Guide - Publication 812
  • Forages: Alfalfa Blotch Leafminer

Related links...

• Insects
• Forages

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