How to Manage Pests

UC Pest Management Guidelines



Monitoring for Potato Aphid and Tomato Fruitworm

(Reviewed 12/13, updated 12/13)

In this Guideline:

Start sampling for potato aphids 6 to 8 weeks before harvest and continue for 4 weeks. Start sampling for fruitworm eggs after adults are trapped in pheromone traps in late July or early August and continue through harvest.

How to Monitor     (View photos of potato aphids and fruitworm eggs)

  1. Pick the leaf below the highest open flower on 30 plants selected at random throughout the field. Use these leaves to monitor both potato aphid and fruitworms.
  2. Record presence or absence of potato aphid on each leaf. Also note natural enemies.
  3. Record observations on a monitoring form (PDF)
  4. Count fruitworm eggs on each leaf.
    • If less than 3 eggs are found, stop sampling.
    • If 3 or more eggs are found in a 30-leaf sample, sample 30 more leaves.
  5. Additional 30-leaf sample:
    • If less than 5 eggs are found in the second 30-leaf sample, stop sampling.
    • If 5 or more eggs are found in the second 30-leaf sample, assess egg parasitism to see if treatment is warranted.
  6. If warranted, assess egg parasitism for fruitworm by counting the number of black (parasitized) eggs and compare them to the number of white, gray, and brown eggs in the table below (eggs darken naturally before hatching). If no black eggs are observed, hold the leaves and assess them 48 hours later to see if the eggs turn black from parasitism.
  7. Treat if monitoring shows that thresholds have been exceeded as outlined below. The letter "T" indicates the ratio of black to white fruitworm eggs at which treatment is recommended.

Treatment Thresholds

Tomato fruitworm Potato aphids
No. of black eggs Number of white eggs
  • If 50% of leaves are infested, treatment may be warranted.
  • If the proportion of mummies is increasing or predators appear to be gaining control, and aphid populations are not yet damaging, avoid sprays that will disrupt these natural enemies.
4-8 9 10 11 12 13 14 15
0 T T T T T T T T
1   T T T T T T T
2       T T T T T
3         T T T T
4           T T T
5           T T T
6           T T T
7             T T
8             T T
9               T
10               T


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Tomato
UC ANR Publication 3470

General Information

  • F. G. Zalom, Entomology, UC Davis
  • E. M. Miyao, UC Cooperative Extension, Yolo County

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