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Sampling Procedures for Tentiform Leafminer and its Parasites

Pest Control Note 93-01.

Introduction

This Pest Control Note describes procedures for determining the abundance of tentiform leafminer and some of the parasitic insects that feed on it as part of a tentiform leafminer pest management program. Knowing the abundance of the leafminer and its parasites is essential in deciding if the number of leafminers represents a potential economic threat to the crop and if the level of the parasite activity is sufficient to prevent a leafminer outbreak. The life cycles of the leafminer and its main parasite, Pnigalio flavipes, are described in the publication "Field Guide to Harmful and Beneficial Insects and Mites of Tree Fruits" available from BCMAFF offices.

Besides leafminer abundance, assessment of tree vigor is also important in determining the potential for leafminer damage. Trees must be maintained in optimum condition to minimize the impact of leafminer feeding (leaf injury) on fruit quality. In many cases, leafminers are "the straw" that breaks the camel's back" - their feeding damage is worse as a result of the trees suffering from some other disorder.

Determining Parasite Activity

Determining the level of parasite activity involves calculating the proportion (percentage) of first-generation tissue-feeder leafminer larvae (they make the tent-shaped mines) killed by the parasites. To carry out this procedure you will need a hand lens with at least 16X magnification in order to see any very small parasite larvae and pupae in the mines.

When the tent-shaped mines appear on the leaves (usually May or early June), collect 100 leaves showing these mines from 10 trees per block (10 leaves per tree). Carefully open each mine and record which life cycle stage of leafminer or parasite is present using the list below:

  1. live leafminer larva - larva is greenish in colour and wriggles when disturbed;
  2. dead leafminer larva - larva is yellow to pale coloured and moves very little if at all when disturbed;
  3. leafminer pupa - brownish coloured, tapered shape, wriggles when disturbed;
    leafminer pupa Leafminer pupa
  4. empty leafminer pupa - brownish coloured case, sticks out underside of mine;
  5. parasite larva - very small, shiny white to cream-coloured; maggot-like, no legs or visible head; usually found feeding on leafminer larva;
    leafminer larva with parasite larva Leafminer larva with parasite larva
  6. parasite pupa - very small, shiny black, flattened tapered body (empty case may be present if adult emerged; then look for small, circular hole cut through bottom or top of mine).
    parasite pupa Parasite pupa

    This sampling should be done before leafminer pupal cases are found sticking out of the mines (described above). The granular black material in the mines is leafminer excreta and can be confused with parasite pupae.

    Consult "Field Guide to Harmful and Beneficial Insects and Mites" for additional help in identifying the leafminer and parasite life stages. Also see Tentiform Leafminer factsheet on this website.

    To calculate the percentage of leafminers killed by parasites, subsitute the values you recorded for each of the stages for the corresponding letters (a to f) in the following formula: 

    % parasitism = 100 X b + e + f
                                   a+b+c+d+e+f

    Assistance in determining percent leafminer mortality is available from your packinghouse field service, pest management consultant or horticultural advisor.

    Determining Leafminer Abundance

    Based on WSU Cooperative Extension Service Recommendations

    Thirty percent mortality of the first-generation tissue-feeder leafminer larvae is considered adequate to prevent a problem from developing over the rest of the growing season. If the percentage is less than 30%, follow the procedures below to determine whether the leafminer is at or building up to potentially harmful levels.

    1. Determine the average number of second-generation sap-feeder mines per leaf in June or early July by randomly selecting 10 later developing leaves per fruit spur and 10 leaves from the bottom part of vegetative shoots per tree (one tree per acre and up to 10 trees per block). Do not look for infested leaves or the results will be biased and invalid. To check the leaves for sap-feeder mines, hold the leaves up to the sun and record the number of lighter coloured blotch mines found on each leaf. To calculate the average number of sap-feeder mines per leaf, divide the total number of mines found by the number of leaves examined.

    2. If the average number of second generation sap-feeder mines is two or more per leaf, control is recommended. If the second generation is developing early (late May - early June) and parasitism was low in the first generation, treatment action is recommended if one sap-feeder mine per leaf is present.

    3. If the third generation develops early (August), follow the sampling plan as per #2 in early and mid-August. If no treatments were applied against second-generation sap-feeders and if the average number of third-generation sap-feeders exceeds five per leaf, then control is recommended. Be aware of treatment preharvest intervals.

    4. If earlier treatments are made to control leafminer or if the third-generation sap feeders do not appear until September (normal situation), sampling the third generation should not be necessary.

    The information generated from the above sampling procedures is essential in making a sound decision whether or not and when to control tentiform leafminer. It is recommended that you consult with your packinghouse field service or pest management consultant to discuss your sampling results along with other information important to making a sound control decision.

Tentiform Leafminer factsheet
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