Shepard Ndlela and Susan Dimbi
Tobacco production is one of the most profitable enterprises in Zimbabwe. However, pests pose a serious threat to tobacco yield, quality and profitability if not properly managed.
Huge loses can occur in the seedbed, field and storage due to poor management of pests. Lack of proper effective insect management techniques as well as unnecessary applications of pesticides lead to reduced profits due to unnecessary expenses.
Most growers are unaware that calendar-based sprays; excessive use of pesticides and the use of rates above the recommended levels do not necessarily lead to effective control of pests but create even more problems for the grower in the long run.
Some of the problems associated with this include the development of insecticide resistance and phytotoxicity.
The solution to the effective management of pests in tobacco production is the adoption of an integrated pest management (IPM) approach.
This is an orderly, pest management system that relies on knowledge and common sense to combine several methods of lowering pest levels in the crop, with a strong emphasis on cost of application, environmental friendliness, personal health, effectiveness and ultimate profitability.
An effective IPM programme ensures the grower protects the crop at the lowest possible cost to increase profits.
Integrated pest management begins with using certified authentic tobacco seed or acquiring professionally produced seedlings.
For example, the Tobacco Research Board produces certified seed that is treated with an aphicide, thereby protecting the seedlings against tobacco virus-transmitting aphids for up to eight weeks.
Thus money is not spent on expensive pesticides meant to control the aphid menace during this period.
In addition, sanitation is important in lands earmarked for the seedbed, float-bed and transplanted seedlings to guard against the carry-over of pests from one season to the next.
Sanitation refers to activities aimed at preventing initial infestation or eradicating sources of infestation thus reducing the use of chemical insecticides.
For example, completely clearing lands of crop residues and re-growths from the previous season’s crop removes food sources required by the pest for survival and reproduction during the off-season.
Ratoon plants are also a source of pests such as cutworms (Figure 1) and are thus, strongly discouraged.
As a vital part of disease and pest control procedures, legislation exists that requires that all growers sow tobacco seed from the 1st of June and not before. Seedlings can only be transplanted into the field from the 1st of September each season and not before.
Additionally, the same legislation stipulates that seedbeds and living tobacco plants should be destroyed by December 31 and May 15 respectively.
This protects the Zimbabwean tobacco industry from carry-over of disease and pests from one season to the next.
Thus it is extremely important that legislated dates governing sowing, planting, seedbed destruction and destruction of tobacco in the lands be religiously adhered to (For more details refer to the Flue-cured Handbook of Recommendations; Flue-cured Tobacco production Field Guide and other publications available at Kutsaga).
Some pests can be avoided by timing of planting by ensuring the crop escapes infestations. For example, tobacco planted from September into mid-October can evade the tobacco aphid seasonal build-ups that usually occur in late November and peak in mid-December.
Growers should also desist from growing alternate hosts of problem pests and pathogens, for example potatoes, pepper, tomato and vegetables such as Chinese cabbage, once the tobacco growing season begins or in-between tobacco crops.
When seed treated with an aphicide is used, and planting is properly timed, an combined with a planting hole aphicide, the aphid threat can be greatly reduced. Additionally, if topping operations are well timed, this can result in significant reductions of aphid populations in the lands.
Topping results in thickening of tobacco leaves and an increase in chemical substances known as alkaloids that help fight off disease. The unfavourable conditions thus created result in aphid mortality and reproduction is severely curtailed.
Scouting refers to the gathering of information pertaining to pest presence and level of infestation in the seedbeds or lands, by conducting systematic and careful observations or inspections.
The ultimate aim of scouting is to correctly identify the pest, determine if infestation is below the economic threshold level and then develop the best situation-specific control measure.
When in doubt, the grower should collect insect samples and send to experts for advice. The Tobacco Research Board operates a pest and disease diagnosis clinic and growers can send in samples for identification and obtain advice on appropriate control strategies.
Applying pesticides without adequate information will lead to insecticide resistance, pest resurgence, environmental pollution and pose a health risk to those handling the pesticides.
It is also important to note that the scout should enter the field from different locations and vary scouting patterns as shown in (Figure 2).
A good sampling plan involves sampling at least 100 plants from different parts of the crop and always away from the edges. One should make careful observations of the upper and lower sides of the leaves, as well as the bud and the stem.
Wilting and poorly growing plants should be uprooted, and roots together with the soil around examined for soil borne pests such as cutworms and wire-worms.
Figure 2. Some of the effective scouting patterns that can be followed on a weekly basis; Left — option 1 and right — option 2.
In nature there exist a number of organisms that regulate the populations of other organisms. They are called natural enemies or “the farmer’s friends”.
These beneficial organisms are usually parasites, pathogens and predators occurring naturally in the agro-ecosystem.
Their action is insufficient to replace conventional methods of pest control, but complements existing strategies. Their importance is often enhanced when the use of broad spectrum residual pesticides such as carbonates and organophosphates is minimised or avoided completely.
The Tobacco Research Board has of late dropped some insecticides from the list of agrochemicals approved for use in tobacco and these include Acephate, Monocrotophos, Fenvalerate and Methamidophos.
The use of softer replacements has resulted in appreciable build-ups of natural enemies for example, in the control of the tobacco aphid. Observations have shown that non-persistent insecticides encourage the build-up of parasites and predators that keep some pests in check.
The TRB is in the process of testing new pesticides that are friendly to beneficial insects and in other work beneficial organisms such as entomopathogenic fungi (EPF), parasitoids and entomopathogenic nematodes (EPN) are being evaluated for possible widespread use on tobacco in Zimbabwe.
The grower should always establish the reason necessitating an intervention such as spraying with a pesticide. Pesticides are effective when the correct active ingredient is applied at the right time and using the right method and rate.
Each application should be accompanied by an economical justification based on recommendations from scouting operations, unless the treatment is preventative.
For example, treatment thresholds stipulate that curative measures for budworm should be applied when 10 out of a 100 plants are infested, and for cutworm when five out of a 100 plants sampled are infested.
A number of growers reduce their revenue by spending money on unnecessary calendar based sprays that do not add any value to crop quality and yield but may instead introduce chemical residues that could lead to extensive losses if the tobacco is deemed unacceptable.
Avoidable losses are also usually incurred when farmers apply pesticides at the wrong time, for example, spraying droughted seedlings or crop. This will lead to phytotoxicity and sometimes death of seedlings or plants depending on severity.
In conclusion, IPM reduces the reliance and overuse of pesticides by incorporating a battery of other pest management solutions, thereby widening the choices and options available to the grower.
Using several management options results in pest control from multiple modes of action and compensates for the ineffectiveness of other strategies adopted. Above all, it reduces unnecessary expenditure on pest management and increases revenue to the grower.
For more information, contact Kutsaga Research Station’s Plant Health Services Division on telephone numbers (04) 575289-94 or toll free, 0800 4511 or Email: [email protected] or visit Kutsaga Research Station, Airport Ring Road, Harare
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