The insects that are most destructive to lettuce are thrips, aphids, loopers and leafminers, according to Shimat Joseph, an Integrated Pest Management (IPM) advisor in the Entomology Department with the University of California’s Cooperative Extension. Currently, he said springtails seem to give growers a fit compared with lygus bug, the major pest last year. The following are a few insects to watch out for in lettuce.
Thrips cause scars on the leaves, transmit viruses and create issues with the quality of lettuce, Joseph said. According to the IPM center at University of California, “Western flower thrips feed on lettuce and vector plant viruses. Thrips cause injury to lettuce by puncturing leaves and sucking the plant sap.”
These thrips like to feed on weeds, ornamentals and other vegetation, then fly onto the lettuce field, where they transmit the virus, according to the IPM center. It is important to remove any weeds that can serve as hosts, and growers are advised to monitor their fields closely.
According to the IPM center, chemical treatments that are available include: spinetoram (Radiant SC), spinosad (Entrust), methomyl (Lannate SP, Lannate LV), zeta-cypermethrin (Mustang Max) and lambda-cyhalothrin (Warrior).
The IPM center noted that insecticidal control is about the only viable control; however, natural enemies are predaceous mites, minute pirate bugs and lacewings. Yet, they are susceptible to insecticides.
Aphids present can include green, lettuce and fox glove. They are the worst offenders in lettuce, Joseph said. Not only do they cause quality issues but they encourage thrips populations.
The lettuce aphid “feeds deep inside the plant toward the center on younger leaves,” the IPM center noted. “In head lettuce, it is found almost exclusively at the heart of the plant.”
According to the University of California’s IPM center, chemical treatments used are: imidacloprid (Admire Pro, Provado 1.6F), flonicamid (Beleaf 50SG), spirotetramat (Movento), oxydemeton-methyl (MSR Spray), dimethoate (Dimethoate 267), acetamiprid (Assail 70WP) and pymetrozine (Fulfill).
Biological controls include syrphid fly larvae and green lacewing larvae. The IPM center warns that syrphid fly larvae are susceptible to insecticides.
In a paper written by University of California entomologist Kent Daane and colleagues Erik Nelson, Brian Hogg and Nicholas Mills, Daane said syrphid flies can suppress lettuce aphids because the larvae are known as key predators to aphids. Over a two-year period, they surveyed the fields for syrphid flies to see if they were capable of colonizing in those fields and capable of suppressing aphid populations. They found that syrphids females left more eggs at places where aphids were present. As a result, the larger syrphid numbers lead to fewer increases in aphid populations.
“Sweet alyssum plants [which harbor syrphid flies] are used for aphid control,” Joseph said. The IPM center adds that baby’s breath, common coriander and Persian clover are also used.
Green peach aphid
If green peach aphid populations become too large, they can stunt seedlings or transplants, according to the IPM center. They also can contaminate lettuce heads and encourage viruses.
The IPM center indicates that several chemical treatments work to control the green peach aphid. They include: diazinon (various products), imidacloprid (Admire Pro, Provado 1.6F), azadirachtin (Aza-Direct 1.2%, Agroneem), flonicamid (Beleaf 50SG), spirotetramat (Movento), insecticidal soap (M-Pede), acetamiprid (Assail 70WP) and pymetrozine (Fulfill).
The IPM center indicates that biological controls that work with other aphids do so with green peach aphids, too. In addition, the parasites Lysiphlebus testaceipes, Aphidius matricariae and Aphelinus semiflavus are natural enemies. The fungus Entomophthora aphidis can kill some green peach aphids.
According to the IPM center at the University of California, “foxglove aphids feed deep inside the plant on younger leaves. They also can vector viruses. Management is difficult because these aphids can increase in numbers so rapidly. The center strongly advises growers to carefully monitor for these pests early.
Chemical treatments for the foxglove aphid can include: imidacloprid (Admire Pro, Provado 1.6F), oxydemeton-methyl (MSR Spray), dimethoate (Dimethoate 267), acetamiprid (Assail 70WP), pymetrozine (Fulfill), flonicamid (Beleaf 50SG) and spirotetramat (Movento).
Loopers and other worms may bore into the head of lettuce, Joseph said. The IPM center noted that young larvae may feed on the underside of the lower leaves so much that the lettuce leaves may be stripped. If looper populations become too large, it may affect the seedlings by either killing them or slowing their growth.
The IPM center recommends checking “at least 25 plants for caterpillars in each quadrant of a 40- to 80-acre field twice a week.” Growers may just need to take some samples of smaller fields, the center indicated. The center also recommends treating seedlings or small plants when loopers become large enough to stunt their growth. More mature plants need treating when an average of more than one-half larvae per plant is present.
According to the IPM center, chemical products that may be available are: bacillus thuringiensis (various products), chlorantraniliprole (Coragen), acephate (Orthene), permethrin (Pounce 3.2 EC, Pounce 25W, Ambush 25WP), zeta-cypermethrin (Mustang 1.5EW), methomyl (Lannate SP Lannate LV), spinosad (Entrust, Success), methoxyfenozide (Intrepid 2F), thiodicarb (Larvin 3.2), indoxacarb (Avaunt) and emamectin benzoate (Proclaim).
Natural enemies of loopers are nuclear polyhedrosis virus, parasitic wasp, tachinid fly, hyposoter exiguae wasp, copidosoma truncatellum wasp and microplitis brassicae wasp.
Leafminers create speckles and mines that prevent lettuce from being marketable to customers. The IPM center at the University California indicated that “female adult flies puncture leaves with their ovipositors, both to create feeding sites and to oviposit.” Where the eggs have sat for a while, these punctures create whitish tunnels. The larvae may feed between the upper and lower leaf surfaces. Eventually, larvae pupate on the leaf or in cracks in the soil and contaminate the lettuce head.
The IPM center recommends applying chemical treatments if populations reach high levels, or if lettuce seedlings grow four to five leaves. It is best to treat in the larval stage if growers spot more than average of one mine per leaf in an overall field sample. Chemicals that may be available are: abamectin (Agri-Mek 0.15EC), cyromazine (Trigard WSP), azadirachtin (Neemix 4.5, Aza-Direct 1.2%, Agroneem 0.15%) and spinosad (Entrust, Success).
Parasitic wasps are an effective biological control unless they are killed in some way.
Symphylans can affect the plant development, he said. The IPM center noted that symphylans can damage sprouting seeds, seedlings or older lettuce plants. They can feed on root hairs and rootlets. Their destruction may lessen as plants mature, but the pitting of older roots can create entryways for pathogens. Feeding may stunt transplants.
According to the IPM center, chemical treatments include: zeta-cypermethrin (Mustang Max), lambda-cyhalothrin (Warrior) and cyfluthrin (Baythroid XL).
Centipedes, predatory mites, predaceous ground beetles and fungi can offer biological control.
Springtails can affect seed germination and crop stand, Joseph said. According to the IPM center at the University of California, springtails can feed on young and tender areas of the plant that may be close to the ground. An increase of populations can occur in a cover crop or when compost is added. The center advises waiting until populations decrease some before growers plant.
Lygus bugs leave dead tissue on the midrib, he said. In his blog, Joseph mentioned that lygus bugs can move from field to field, looking for a food source, moisture or refuge. Growers must be aware that higher populations of these bugs can form in ditches and sides of roadways. Alfalfa and beans also can serve as hosts.
Joseph said female lygus can lay on average 150 eggs in its lifetime so they can form colonies in lettuce.
To control lygus bugs, he suggests using Lannate and the pyrethroid insecticides such as zeta-cypermethrin, permethrins and lambda-cyhalothrin. Joseph warnsed that repeated use or overexposure of insecticides to same generation of lygus bugs to pyrethroids can lead to resistance.
Joseph said insecticides are the best method to control all of the aforementioned pests.
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