INSECT PEST CONTROL
More detailed information on insect pest control can be found in the ‘Integrated Pest Management in Lettuce: Information Guide’ (see references page 15). Most pests of lettuce are common pests of other vegetable and field crops. The insects either damage the lettuce plants by physical damage or by transmitting diseases. Pests are typically managed by calendar spraying, strategic spraying or integrated pest management.
Calendar spraying
Calendar spraying is the term used for a set program of sprays, usually using older cheaper broad spectrum chemicals. Unfortunately resistance has developed to many of the older insecticides by some insect pests. Although calendar spraying has the advantage of being predictable, it often means sprays are applied when either not necessary or not at the optimal time.
This strategy is not recommended.
Strategic Spraying
Strategic spraying involves scouting the crop on a regular basis to determine the level of pest activity. When pests are detected and their numbers reach a level that will cause damage, insecticides are used for control. Broad spectrum insecticides are also more commonly used due to their cheaper price. Integrated Pest Management (IPM) This option is a pest management system where all the pest and beneficial insects are considered in management decisions. Crops are scouted on a regular basis to determine the number of both pests and beneficial insects. While trying to reduce insect pest numbers, every effort is made to try and preserve beneficial insects as they eat and kill the pest insects. All available tools for managing pests are considered including, cultural practices, beneficial insects, biological insecticides, new generation narrow spectrum insecticides and broad-spectrum insecticides. Where possible, new generation narrow spectrum insecticides are preferred over broad-spectrum insecticides to encourage beneficial insect activity.
Heliothis
Heliothis is the most common caterpillar pest of lettuce with two species found in Australia
Probably the most difficult caterpillar to control as it has developed resistance to a range of insecticides. Helicoverpa armigera are usually locally bred pests that emerge as moths in late spring from over-wintering pupae. There are usually 3 or 4 generations each year with each successive generation larger in number than theprevious. Helicoverpa armigera populations decrease in autumn as more of the mature larvae delay their development into a pupa over the winter months.
Helicoverpa punctigera
This native budworm is also prevalent on lettuce and has tended not to develop resistance to insecticides. Helicoverpa punctigera is the dominant species in spring but generally numbers diminish in summer. Helicoverpa punctigera are believed to over-winter in the arid regions, flying in early spring (usually during September), often with spring storm fronts. In temperate areas both Helicoverpa armigera and Helicoverpa punctigera moths have usually finished laying eggs by the end of April with only very low numbers of larvae seen by the end of May. In sub-tropical areas, Heliothis may continue breeding throughout the winter months. A female moth may deposit a thousand eggs, usually laid singly, on different leaves or plants. When the eggs hatch, small larvae emerge, about 1.5 mm long. The larvae begin feeding almost immediately and usually burrow into the heart. After 6 moults, the mature larvae finish eating, descend the plant and burrow into the soil to pupate. Spraying for control should be timed when grubs are just hatching and before they burrow into the heart. Even though Helicoverpa armigera have developed resistance to a number of chemicals, they are still susceptible at the 1st and 2nd instar stage. Timing sprays for egg hatch will mean that sprays can be less frequent during the spring and autumn compared to the hotter months of summer. After harvest, cultivate paddocks to a depth of 10 cm (‘Pupae Busting’) to kill any pupating larvae. Do this as soon as possible after harvest for maximum benefit.
Other caterpillars
Other caterpillar pests of lettuce include cutworms (Agrotis spp), loopers (Chripodeixis spp) and cluster caterpillars (Spodoptera spp). These can also be destructive pests in lettuce but are not usually seen in as high numbers as heliothis. Control measures for heliothis will usually also control loopers and cluster caterpillars. Cutworms are more of a problem at establishment and can be controlled by cultural methods and insecticides.
A number of aphid species infest lettuce including the Green peach aphid (Myzus persicae), Brown sowthistle aphid (Vroleucon sonchi), Potato aphid (Macrosiphum euphorbiae), Rose aphid (Macrosiphum rosae), and Sowthistle aphid (Hyperomyzus lactucae). These aphids are all vectors of the Lettuce Mosaic virus while only the sowthistle aphid is a known vector of the Necrotic Yellows virus. Cool dry weather favours aphids. Aphids have a wide range of natural enemies, and if narrow spectrum insecticides are used to control other pests, then aphid populations are usually kept below levels that cause damage from direct feeding. Aphids can be in either the winged or non-winged form. When in the winged form they are on the move and when they are in the non-winged form they are colonising and rapidly increasing in number. Aphids mostly reproduce asexually and are well adapted to rapidly increase in number. Damage from direct feeding results in leaf curling and wilting. Lettuce will tolerate a moderate level of damage depending at the growth stage. Aphids generally tend to feed on the lower side of outer leaves, which are mostly removed at harvest. Beneficial insects usually keep aphids in control. When aphid levels increase to damaging numbers use only narrow spectrum chemicals for control. A new and highly invasive aphid is soon expected to arrive in Australia. The Lettuce Aphid (Nasonovia ribisnigri) is presently found in Europe, North America, South Asia, and New Zealand. Unlike other aphids that colonize on the outer leaves, the lettuce aphid prefers to feed in the centre of lettuce hearts and is therefore difficult to control.
A number of thrips species infest lettuce including Western Flower thrips (Frankliniella occidentalis), Onion thrips (Thrips tabaci), Plague thrips (Thrips imaginis) and
Tomato thrips (Frankliniella schultzei)
Thrips are very small insects about 1 to 2 mm long. They vary in colour and size depending on species and generally have a wide host range. Thrips can cause mechanical damage to leaves of seedlings, resulting in curled leaves, silvering and wilting. Transmitting disease is usually the biggest concern with thrips. Some species of thrips can transmit Tomato Spotted Wilt virus which causes wilting and a distinct russetting of the leaves.
Yellow sticky traps are a good method of monitoring thrips. If using an IPM program, beneficial insects may keep their numbers down. When their numbers increase, and/or high levels of the virus are present, insecticides are needed for control.
Other sap sucking pests
Other sap sucking pests include leafhoppers and Rutherglen bugs. These are occasional pests and can migrate into lettuce crops when the surrounding crops and weeds dry off. The main concern is their presence causes a contamination problem. When these insects are in very high numbers throughout the hearts, the lettuce becomes undesirable for both fresh and processing markets.
YIELD AND HARVESTING
Iceberg lettuce is ready for harvesting once a firm heart is formed. This can be as quick as 8 weeks over summer, if sown as transplants, and as long as 16 weeks over winter, if sown by seed. The lettuce is cut close to the ground and excess leaves are trimmed off. The heads are washed if needed and packed in the field. Most iceberg lettuce are cut by hand and loaded onto harvest aids for packing in the field. Growers are moving towards mechanising this operation with a number of mechanical harvesters\ now operating around Australia. To supply the fresh market, the lettuce is packed into waxed cardboard cartons. A total of 12 heads are packed in each carton with the bottom (butt) facing the top. If supplying a processing market then the lettuce is loosely packed in large bins or crates. Lettuce for processing requires heavier trimming with more of the wrapper leaves removed at harvest. Yields can vary from 15,000 to 30,000 cartons per hectare with an average of about 2200 cartons per hectare. Lower yields are experienced when the climatic conditions become harsh. Yields in the Murrumbidgee district are higher in the warmer shoulder seasons than during the very cold winters. Unformed hearts, pest and disease are also reasons for reduced yields and returns. Prices returned to growers vary from season to season and currently range from $5 to $12 per carton.
COST OF PRODUCTION
Cost of lettuce production varies depending on the type of growing system used and the amount of variable inputs needed. The following table gives an approximate cost of production for a direct seeded crop yielding 2200 cartons per hectare. The cost is shown on a ‘per carton’ and ‘per hectare’ basis. These costs are a guide only but give a good indication of which operations are the most expensive. For this example 82% of the cost of production is incurred during and after harvest. This percentage will change depending on the distance from the markets. Transport cost will be much cheaper close to the markets but will increase to over $1.00 per carton if carting long distances. Chemical rates, fertiliser rates, labour costs and irrigation requirements are other examples where costs vary from area to area.
STORAGE AND TRANSPORT
Harvesting is usually conducted in the cool of the morning before the lettuce temperature becomes too warm. Once harvested and packed in the field, the lettuce should be cooled to 1ºC to ensure that it arrives at the market in a fresh condition. Prompt and thorough cooling is necessary to remove any field heat for a longer shelf life. Forced air cooling is preferred to a standard coolroom as it will bring the lettuce temperature down much quicker. Vacuum cooling is also very efficient on leafy products but not as effective on bulky vegetables. Lettuce held between 0ºC and 1ºC and in high relative humidity (95–97%), have a shelf life of 2 to 3 weeks. If harvesting close to markets, unrefrigerated trucks are used for transport. If harvested a long distance from the market then refrigerated trucks are needed. Harvesting over-mature lettuce or storing without cooling will hasten lettuce breakdown and deterioration.
