MANAGING INSECT PESTS IN ORGANIC VEGETABLE PRODUCTION
INSECT PESTS are a major hindrance to profitable vegetable production, as these may cause more than a 50 percent reduction in yield. Damage caused by pests is irreversible; the plants may not be able to recover from the resulting infections to their shoots and other parts.
An important first step in managing insect pests is being able to recognize and identify those that attack vegetables. Farmers generally find it difficult to recognize the pests and the damage they cause, so they tend to immediately spray insecticide upon noticing the presence of insects without monitoring them.
The main physical characteristic of insects is their having three body regions: the head, thorax, and abdomen. Aside from this, insects have three pairs of legs in their adult stage, one or two pairs of wings, and a pair of antennae. Insects undergo two general types of development: complete metamorphosis from egg to larva (first to fifth instar) to pupa and to adult, and incomplete metamorphosis from egg to first instar nymph up to the fifth instar nymph to adult.
The two classifications of insects ¬¬¬are according to their taxonomic order and according to their mouth parts, which classify them as chewing (removing pieces of plant tissue and eating leaves), sucking (piercing plant tissue and removing plant sap), and boring (getting into stalks or fruits and eating plant tissue).
Insects are successful in destroying vegetables because of the short time that it takes for them to develop (only a month from egg to adult), their ability to produce many offspring. Many of them can fly and travel long distances. They are small and difficult to detect, and their population and the damage they cause are difficult to control. Finally, they have a wide range of hosts as they feed on a wide variety of crops.
COMMON INSECT PESTS ATTACKING MAJOR VEGETABLE CROPS
For eggplant, this is the fruit and shoot borer, the common cutworm, the corn semi-looper, whitefly, leafhopper, and lady beetle. A single adult borer lays up to 121 eggs at night; these hatch in four to six days. The larvae bore into and feed on the eggplant shoots, flowers, and fruits, with the total larval period of about 15 days with six instars, after which the mature larva spins a leathery cocoon within the plant. Pupation takes from 9.44 to 11.5 days. The male adults live for four days; the female, for 7.5 days. Borer damage causes eggplant flowers to fall, with surviving ones developing into small and irregularly-shaped fruit. Infested shoots wilt and yield reduction can be from 20 to 80 percent. The cutworm, on the other hand, feeds on the leaves and outer portions of fruit.
For tomato, these are the tomato fruit worm, whitefly, corn semilooper, common cutworm, leafhopper, lady beetle, and leafminer. The fruit worm bores into the petioles, flower buds and flowers, and fruits, which eventually fall off. Both the nymphs and adults of the whitefly suck plant sap and transmit the Tomato Leaf-Curl Virus.
For brassica or cabbage family vegetables, there are the diamondback moth (DBM), cabbage moth, common cutworm, cabbage webworm, beetle, and aphids (Aphis gossypii Glover). DBM damage appears as irregularly chewed patches in the leaves with their veins and upper epidermis still intact. Cabbage moth larva feed on leaves, often resulting in the non-formation of heads and leaf perforations in non-head-forming Brassicas.
For legumes, these are the beanfly, aphids, leafhopper, cutworm, bean pod borer, and earworm. Plants infected by aphids (Aphis craccivora Koch) stop growing, curl, crumple, and lose color. The sooty mold that grows on the insects’ excretions covers the leaves and interferes with the plants’ photosynthetic activities. The bean pod borer’s newlyhatched caterpillar feeds on the flower buds whose unconsumed outer petals subsequently dry up.
For cucurbits, there is the squash beetle, melon worm, aphids (Aphis gossypii Glover), and fruit fly. Squash beetle grubs bore into the roots and plant portions close to the ground and feed on their tissues. The adult beetle feeds on the epidermis of the leaves and flowers, leaving behind what look like shredded leaves. Melon fruit fly maggots feed on the stems and fruits, causing stem and fruit rot. Severe infestation results in many fruits dropping or immature ones decaying.
ORGANIC PEST MANAGEMENT (OPM) OPM is the intelligent manipulation of the pest population using a combination of techniques in cultural practices, legal control, and physical and other methods in consideration of natural regulatory factors to reduce economic damage. It involves a series of management decisions as to the right manner for the control or suppression measures.
The techniques to be used are: enhancing the presence of biological control agents; the use of resistant crop varieties; the application of compatible cultural farm practices; and the needbased application of botanical insecticides.
Biological control is the use of living organisms such as parasitoids ( Trichogramma, Diadegma semiclausum, Cotesia plutellae, Snellenius manilae, Telenomus, Trathala flavoorbitalis, Opius longicaudatus, etc.); predators (earwigs, stink bugs, Coccinellid beetles, flower bugs, lacewings, etc.); and microbial pathogens like nuclear polyhedrosis Virus (NPV),
Beaveria bassiana, Bacillus thuringiensis, etc. to suppress pest populations.
Parasitoids, which are parasitic during their immature stage, attack the eggs or larvae of insect pests. They need a single host to complete their life cycle, always kill their host, and are free-living as adults.
In using Trichogramma for tomato, release 50 T. chilonis cards per hectare twice a week two to three weeks after transplanting (WAT), and repeat four times at weekly intervals at 25 cards per hectare. For eggplant, starting at two to three WAT, release 25 T. chilonis cards per hectare twice and repeat four times at weekly intervals.
The parasitoids D. semiclausum and C. plutellae effectively control DBM larvae. The common cutworm’s eggs are parasitized by Telenomus, its larvae by S. manilae, or preyed on by earwigs or NPV. T. flavoorbitalis, which is all-female and reproduces without males, has potential as a parasitoid against the eggplant fruit and shoot borer, while O. longicaudatus adults lay their eggs on fruit fly larvae.
Predators, which are usually bigger than their prey, require several victims to complete their life cycle. Earwigs (chiefly Euborellia annulata and E. philippinensis), which are elongated and flattened insects whose mobile abdomens extend into a pair of forceps, feed on the egg masses, young larvae, and pupae of Lepidoptera, Coleoptera, and Diptera insect pests. More active at night, earwigs also prey on leafhoppers, aphids, and other soft-bodied insects. Stink bugs are predators of cutworms and the larvae of other lepidopterous pests, as Coccinellid beetles prey on the instar larvae of those pests, and flower bugs on those insects’ eggs and early instar larvae.
Mass-rear E. annulata in plastic trays for biological control purposes. Place 2.5 to three kilograms of a sterilized mixture of three parts soil and one part sand in each tray, wet the medium to 27 to 30 percent moisture content, and maintain this throughout the rearing period. Release 36 female and 12 adult earwigs in each tray and feed the insects weekly with 10 to 20 grams of fishmeal and a mixture of 20 grams each of corncobs and dog food.
Release the adult E. annulata in an eggplant field with mulched plots two to three WAT. Place 2,000 second to fourth instar nymphs in each plastic tray with moist soil among the plants. Release 20,000 individual earwigs per hectare at three WAT and another 20,000 at four WAT.
Microbial pathogens like the NPVs are highly specific insect pathogens used as microbial pesticides. Their infection route is through ingestion and they multiply in internal organ tissues, killing the host insects. They do not affect mammals, birds, fish, and non-target insects.
Another microbial pathogen is Bacillus thuringiensis (Bt), a bacterium isolated from the soil that can produce toxins for controlling specific insect pests. Ingestion by the insects severely affect their digestive system and lead to starvation and death. The University of the Philippines-Los Banos’ (UPLB) biotech department produces a commercial Bt formulation called Bactrolep which is effective against the DBM in brassicas and the Asian corn borer. It’s specific and kills only its target pests.
PLANT EXTRACTS AGAINST INSECT PESTS
Plant extracts have been used worldwide as botanical pesticides since the 1940s. Local plants used for that purpose include lemongrass, serpentina, oregano, marigold, langkauas, luyang dilaw, garlic, black pepper, citronella, sili, chichirica, and makahiya. Whole plants, stems, leaves, roots, flowers, fruits, pods, seeds, sap, bark, pods, bulbs, and tubers are used. Extraction processes use water, vinegar and wine. Naturallyoccurring toxins in the plant parts are effective against pests, although many reports on these have not been properly documented, extraction procedures are not valid, proper exposure methods are not followed, and target insect age is not specified.
Many plant extracts are less toxic to mammals than synthetic insecticides, although several are harmful to fish and other wildlife. Extracts tend to break down fast in the environment so they need frequent application.
Fresh juice extracts can be obtained from succulent plants like luyang dilaw (rhizomes), oregano (stems and leaves), pansitpansitan, and kamantigue. Wash the parts thoroughly before extracting their juice.
Extraction with a blender is also doable for non-succulent plants like langkauas (rhizomes), makabuhay (stems), and serpentina and lantana (leaves). Chop 500 grams of the material and grind in the blender. Soak the blended material overnight in a liter of water then press and filter in the morning for the stock solution. Refrigerate the solution when not using it.
In preparing extracts of oregano stems and leaves, and luyang dilaw and langkauas rhizomes, chop or macerate two kilos of the materials and soak in four liters of water with one-half bar crushed Perla soap for 24 hours for the rhizomes and 72 hours for oregano. After the required time, filter the solution and place in a 16-liter knapsack sprayer. Further macerate the residues in about 11 liters of water and add the solution to the one in the sprayer. Spray to the point of run-off in the afternoon, repeating this every seven to 10 days, depending on the pest population. A hectare generally requires 10 sprayer loads.
Evaluate the extract’s contact toxicity by topical application or leaf spraying. Record mortality data at 24 and 48 hours after treatment. An effective anti-feedant extract gives insects the opportunity to feed on the treated plants but gradually reduce their intake until they starve to death. Repellants, on the other hand, drive away the insects after exposure even without feeding on the treated plant parts. Feeding may also expose the insects to growth-inhibiting effects when they bear abnormal pupae and adults.
NON-PESTICIDE CONTROL METHODS
Cultural control techniques are farm operations that are favorable to crop production but not for pests. One is thorough land preparation in plowing and harrowing for exposing pest and disease inoculum and reducing weeds. Another is synchronized planting so that the pest-susceptible crop stage does not coincide with the peak of pest infestation.
Routine sanitation tasks like removing debris and old leaves rids the farm of pests’ breeding sites and food sources, while intercropping or multiple cropping, such as with mongo whose flowers provide nectar to beneficial insects, are also helpful.
The use of pest-resistant vegetable varieties is another nonpesticide control method. Selective weeding also helps in that some wild plants serve as a refuge for spiders and other predatory insects. Mechanical or physical control is also advisable, such as hand-picking when only a few plants are involved, and bagging fruits with plastic, newspaper, or net to prevent flies from laying eggs there—though this is a method requiring proper timing.
Planting trap crops is another control method. A trap plant should be more preferred by pests over the main vegetable crop and must be planted ahead of it. Kadyos and mustasa-mustasahan are examples of trap crops. Planting repellent herbs around vegetables is another method.
Another means of control is the use of traps, such as with sex pheromones which female insects secrete. Used to attract male fruit flies in cucurbits and fruit trees, the traps can greatly reduce fertilization in the female pests and result in very few offspring. Improvised traps like plastic bottles with basil leaves that attract fruit flies inside and holes through which the insects can enter are also effective.
Semi-ultraviolet lamps have been found to trap the adults of pests immediately after these emerge and prevent them from laying eggs, thus greatly reduce pest population.
Although many organic farmers now use botanical pesticides, the use of parasites, predators, and micro-organisms in the biological control of insect pests in general is not yet popular with farmers. The reasons for this are the natural enemies are highly specific and cannot control the wide array of pests that attack crops, and the difficulty of mass-producing biocon agents.
The enhanced use of bio-pesticides and biocon agents requires village-level mass production and the aggressive dissemination of the technologies through organized pest management programs for organic vegetable growers.
This article on how organic vegetable growers can manage insect pests in their gardens or farms was one of the lectures delivered during the well-attended Urban Agriculture Trade Show held recently by the Manila Bulletin’s Agriculture Monthly Magazine at the Rockwell Center in Makati City. Dr. Pio Javier is a research professor with the Crop Protection Cluster of the University of the Philippines at Los Baños (UPLB) College of Agriculture.