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Black egg trap for navel orangeworm.

Egg Traps for Navel Orangeworm

If a mummy removal program was not implemented, it may be necessary to treat for navel orangeworm at hullsplit. Use egg traps to follow navel orangeworm development and to determine when navel orangeworm eggs will hatch in relation to hull split so treatment can be timed precisely. Egg trap counts will not tell you if a treatment is needed; winter mummy counts (more than 2 per tree) are the best indication of need for treatment.


  • Set traps out the first week of April.
  • Check twice weekly to determine the biofix—this is the first of 2 dates in which egg laying increases in 75% of the traps in a given location.
  • Record the biofix date.


  • Use traps baited with almond meal and 10% (by weight) crude almond oil. Black traps work best, but their caps do not need to be black.
  • Place 1 trap per every 10 acres, for at least 4 traps per orchard.
  • Choose trees that are at least 5 trees in from the edge of the orchard.
  • Hang traps at head height on the north side of non-pareil trees, 1 to 3 feet inside the drip line of the tree. Avoid areas where traps will be hit with sprinkler irrigation.

For larger orchards, divide sampling blocks into portions that can be sprayed as a unit. Orchards that exceed 1000 acres can be divided into larger sampling blocks if conditions within each block are uniform.

Trap reading

Continue monitoring traps, counting and recording egg numbers on the egg trap monitoring form (110 KB, PDF). Remove eggs as you monitor.

  • Change baits every 4 weeks.
  • Look for flat eggs that are laid mostly on the ridges of the trap or on the raised lettering on the top and bottom of the trap. Eggs will be white when first laid but turn orange-red before hatching.
  • Graph numbers of eggs laid at each trap reading on the monitoring form. This will give you an idea of when new generations of navel orangeworm are laying eggs.
  • Use this information to verify degree-day calculations. If you wish to use this information for timing a hullsplit spray, continue monitoring for the entire season.

Degree-Day Calculations

  • Use the biofix determined by egg trap monitoring to start accumulating degree-days for following navel orangeworm development and to time hullsplit treatments.
  • Egg laying by the second flight of moths is predicted to begin 1056 DD after the biofix.
  • Shake trees before third generation egg laying takes place.
  • If treatments are planned and hullsplit begins before egg laying predicted, apply the hull split spray at the beginning of egg laying. If hull split begins after egg laying is predicted, apply the spray at the beginning of hull split. Back up degree-day predictions by checking egg traps.

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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