Pesticides not working? Agricultural supply store owners can respond to customers like this
Meitian Nongzi
People often say pesticides don't work! But here's the thing: whether a product works well is also related to many other factors.This article covers basically all the reasons, and I'd like to share it with all you readers.
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1. Factors related to the pesticide itself
Chemical composition of the pesticide
For example:
①Organochlorine insecticideshave poor efficacy against mites, but very high efficacy against Lepidoptera pests
②Chlordimeformhas good efficacy against both Lepidoptera larvae and mites
③Pirimicarbshows significant control efficacy against wheat aphids, but is almost ineffective against cotton aphids and similar aphid species
Physicochemical properties of the pesticide
The solubility, wettability, spreadability, dispersibility and stability of a pesticide all affect its efficacy. To improve the deposition, wettability and wet spreadability of the spray solution, surfactants or oily substances must be added to the solution,so as to reduce surface tension and improve the adhesion of the spray solution.
Mode of action of the pesticide
For example:
①Phenoxycarboxylic acid herbicidessuch as 2,4-D, 2,4-D butyl ester, MCPA, etc., are effective against broadleaf weeds but ineffective against monocotyledons.
They are used in cereal crop fields to control annual and perennial broadleaf weeds and sedges, and at low concentrations they promote growth while at high concentrations they inhibit growth (hormone-type herbicides);
②Systemicinsecticides are effective against pests with piercing-sucking mouthparts; stomach poisons are effective against pests with chewing mouthparts, but almost ineffective against piercing-sucking pests.
Whilecontact insecticidesare effective against both types of pests.
Concentration/dosage of the pesticide
① Generally, when applying, increasing the pesticide concentration will improve efficacy, but beyond a certain limit, increasing the concentration does not necessarily improve efficacy.
For example:
Phenoxycarboxylic acid herbicides are systemic translocated herbicides; if the dosage is too high during use, it will kill the plant's conductive tissue, so the pesticide cannot be translocated to the root system or growing point, and the efficacy actually becomes poor.
② Insects have a strong sense of taste; if the pesticide content in the food is too high, some insects will immediately show antifeeding behavior.
③ If the pesticide dosage is too much, it will cause runoff and cannot improve efficacy; moreover, it easily causes phytotoxicity to plants, prompts pests to rapidly develop resistance, kills natural enemies, pollutes the environment, etc.
2. Factors related to the control target
Feeding mechanism of insects
① Pests must come into contact with the crop whether they are active or feeding, thereforeinsecticides with contact action shouldhave the widest range of application.
In terms of pest activity habits, they are divided into: "leaf-surface pests", "leaf-mining pests (such as citrus leafminer)" and "boring pests". Contact-action pesticides can only be used against "boring pests" before the pests bore into the crop; pesticides with systemic/permeation action are suitable for controlling "boring pests" and "leaf-mining pests" that have already bored into the crop.
In terms of pest feeding methods, they are divided into: "chewing mouthpart pests" and "piercing-sucking mouthpart pests". Pesticides with contact/stomach action are suitable for controlling "chewing mouthpart pests"; pesticides with systemic/permeation action are suitable for controlling "piercing-sucking mouthpart pests".
②Stomach poisonssuch as trichlorfon and diflubenzuron, when sprayed onto the leaves, stems and fruits of crops, or made into poison bait or poison grain that pests like to eat and broadcast in the crop field, have very good control efficacy against chewing mouthpart pests such as armyworms, locusts, mole crickets and cutworms, but are ineffective against piercing-sucking mouthpart pests.
③Systemicinsecticides such as phorate and carbofuran are highly effective against piercing-sucking mouthpart pests such as aphids, mites and scale insects, and are convenient to use, do not necessarily require very thorough spraying, and can be used for seed treatment and soil root application.
Physiological state and developmental stage of the control target
① In the entire life history of a pest,the larval and adult stageshave relatively poor resistance to pesticides, while eggs are the most resistant. At the same time, larvae are the most damaging stage, so control should be carried out during the larval stage.
② The resistance of larvae also varies by instar: young larvae have poor resistance, while older larvae have strong resistance. Generally, control is most effective before the third instar. The best control period is 1-5 days after the peak of egg hatching.
③ When controlling diseases, attention should also be paid to the physiological state of the pathogen. Chlamydospores have relatively strong resistance; after spores germinate, the young germ tubes have reduced resistance, and applying pesticide at this time gives good control efficacy; once the germ tubes invade the plant body and grow and reproduce, applying pesticide becomes ineffective. Therefore, for plant diseases, control is best carried out at the early stage of disease onset.
④ "Timely application" of pesticides for field weeds in crop fields is also an important aspect of ensuring control efficacy: on one hand, the application period should be determined according to the properties and characteristics of the pesticide and the biological characteristics of the weeds; at the same time, the sensitivity of the crop must also be considered.
Generally speaking --
A: For annual weeds, application should be done at the germination stage, at the latest at the 3-5 leaf stage. After weeds become mature plants, besides the damage they have already caused, their resistance to herbicides increases, and it becomes difficult to achieve selective elimination at this point.
B: For perennial weeds, they must grow to a certain leaf area before application, so that an effective dose can be reached within the weed body to cause poisoning and death. However, for such weeds, application should also be done at the latest before flowering.
Once weeds enter the flowering stage, they shift from vegetative growth to reproductive growth, and changes occur in organ differentiation, physiological and biochemical reactions, etc. within their bodies, so that the herbicide cannot play its proper role.
C: For broadleaf weeds, control should be carried out at the germination stage using soil treatment, or after growing to a certain leaf area by relying on morphological selectivity, positional selectivity, biochemical selectivity or directional spraying. The most suitable application time is when the weeds are in a young and tender state and the crop is in a highly resistant period.
Surface structure and properties of the control target
① If the plant surface has fine hairs or a relatively thick waxy layer, and the leaf shape is upright, the spray solution generally does not deposit easily, as with monocotyledonous plants; dicotyledonous plants are flat and spreading, have few fine hairs and a thin waxy layer, so pesticides deposit easily.
② Different species of insects also differ greatly in the wettability of their body walls. Insects with a thin waxy layer on the body wall, few secretions and no scales are easily wetted and deposited by pesticides; insects with a thick waxy layer on the body wall, abundant secretions and many scales are not easily wetted and deposited by pesticides.
③ The physiological water absorption capacity of plant leaves (root system) also has a great influence on the absorption of the spray solution.
For example: in the evening or early morning, especially in the evening, the physiological water absorption capacity is strongest, so for pesticides with systemic action, the absorption of the pesticide is also highest.
3. Factors related to the environment
Changes in environmental conditions, on one hand, affect the physiological activities of organisms; on the other hand, they affect the physicochemical properties of pesticides, thereby affecting efficacy. The main environmental factors include temperature, humidity, rain, light, wind and soil properties, etc.
Temperature
① Temperature has a relatively large influence on pesticide efficacy, especially on insecticides.
Temperature coefficient of insecticides: the ratio of the median lethal dose (LD50) of a certain pesticide against a certain test insect at 20°C to the LD50 at 30°C.
② Positive temperature coefficient type
That is, within a certain range, the higher the temperature, the better the efficacy. Most insecticides currently in use belong to the positive temperature coefficient type. Temperature changes have a great influence on the exertion of pesticide efficacy. For example, when trichlorfon is used to control pine caterpillars, the efficacy at 20°C is 4 times higher than at 10°C; dimethoate has very poor control efficacy against aphids below 15°C.
③ Negative temperature coefficient type
That is, within a certain range, the lower the temperature, the better the efficacy. There are not many insecticide varieties belonging to the negative temperature coefficient type, mainly DDT, pyrethrins and synthetic pyrethroids such as allethrin, etc.
④ Generally speaking, at higher temperatures, pesticides show higher biological activity and stronger and faster penetration.
High temperature can also reduce the surface tension of objects and increase the spreading performance of the spray solution; under high temperature conditions, harmful organisms have high respiratory intensity, vigorous metabolism and strong activity. All these conditions are conducive to the exertion of the pesticide's killing efficacy. However, applying pesticides under high temperature also increases the phytotoxicity of the pesticide to crops and the chance of poisoning to humans and livestock.
Humidity
① Humidity has a relativelysmall influence on pesticide efficacy, but it is very complex.
Generally speaking, high humidity can accelerate the hydrolysis of certain pesticides, leading to pesticide failure or phytotoxicity. In particular, powders easily aggregate or cake, affecting efficacy and causing phytotoxicity.
Rainfall will wash away applied pesticides, therefore, re-spraying should be done promptly after rain.
② Foremulsifiable concentrates, wettable powdersand other formulations,
after the sprayed pesticide dries, high humidity or dew helps the pesticide to re-disperse and spread, thereby facilitating the exertion of efficacy.
③Insecticideshave a relatively low humidity requirement range
Generally speaking, under conditions of first high temperature and low humidity, then low temperature and high humidity, that is, spraying after 4 PM until evening, or dusting in the evening or at night, gives good control efficacy.
Most areas of Northwest China in summer and early autumn belong to this type of climate, so the above application timing pattern can be adopted in production.
④ Forfungicides, within a certain range, humidity and efficacy are positively correlated.
When humidity is high, protective fungicides easily kill invading and infecting pathogens; during soil treatment, high humidity also easily allows the pesticide to exert its efficacy. However, if humidity is too high, it is unfavorable for the systemic absorption, penetration and translocation of systemic fungicides.
Herbicides are mainly applied by soil treatment, so the relationship between humidity and efficacy is very close. Within a certain range, the higher the humidity, the better the effect. For foliar treatment, however, the pattern is the same as that of insecticides and fungicides.
Light
Light and temperature are closely related. The influence of temperature on efficacy has already been described. As for light alone, different pesticides respond differently.
①Most pesticidescan be decomposed by light to varying degrees, and some can even be rapidly degraded under light to the point of becoming ineffective.
For example, phoxim can be photodegraded by more than half in 4 hours under light, so it can only be used for soil treatment to control underground pests. However, if sprayed in the evening, it can exert strong efficacy for nearly 20 hours. Nicotine for controlling aphids and other pests is also only suitable for evening application. Pyrethrins, also because they are easily photodegraded, can only be used for controlling indoor sanitary pests and stored-product pests.
However, the easy photodegradation of these pesticides gives them the advantages of a short residual period and low residual toxicity, and makes them advantageous in terms of not polluting the environment, protecting natural enemies and being pollution-free.
②HerbicidesTrifluralin is extremely prone to photodegradation and must be harrowed into the soil immediately after application.
On the contrary, many pesticides have better efficacy under light. For example, when parathion is used to control pests, the effect is better under light, so it can be sprayed in the morning after the dew on crops has dried.
In particular, many herbicides (mainly photosynthesis inhibitors, such as substituted ureas, triazines, etc.) have better effects under light. Nitrofen, as a photodynamic herbicide, can only kill weeds under light.
Wind
Wind affects the conduct of pesticide application, mainly affecting spraying and dusting methods. When conventionally spraying pesticides, if the wind force exceeds level 3, it is not suitable to proceed.
① Wind first of all can cause drift loss of dust or spray solution, and secondly, because pesticide particles all gather on one side of the wind direction, it affects uniformity, thereby indirectly affecting the control efficacy of the pesticide.
In addition, spraying pesticide under windy conditions also easily causes poisoning of operators or causes phytotoxicity by blowing the pesticide onto sensitive crops in adjacent plots.
For example, when spraying dichlorvos in a cotton field, if sorghum is planted within about 5 m downwind, there is a risk of causing phytotoxicity; when spraying herbicides such as 2,4-D in a wheat field, it easily causes phytotoxicity to nearby goji berries.
② For low-volume and ultra-low-volume spraying, a certain amount of wind force is necessary for normal operation. Motorized mist dusters can also only rely on the wind force they generate themselves to send out and blow pesticide particles onto crops.
Rain
Rain generally affects the efficacy of stem and leaf treatment agents. The degree of influence is related to the type and properties of the pesticide, the time between application and rainfall, and the amount of rainfall.
①Systemic agentscan be absorbed by plants within a certain time after use, so rainfall has a relatively smaller impact on their efficacy.
For example, sethoxydim is a highly selective systemic translocated stem and leaf treatment agent; rainfall 3 h after spraying has no effect on its efficacy. Contact and stomach poisons and pesticides applied by dusting are generally basically ineffective after rain, and re-spraying should be considered.
② Suitable rainfall can significantly improve the control efficacy of pesticides applied by soil treatment.
Especially for herbicides, after soil treatment, moderate rainfall gives the most ideal effect; however, high rainfall or pesticides with strong leaching properties can have reduced efficacy due to rainfall, and there is a possibility of increased phytotoxicity, and they may even pollute groundwater.
③ Since things have two sides.
Therefore, in practice, it is necessary to comprehensively consider the variety and characteristics of the pesticide, the basic characteristics of diseases, pests and weeds, and various environmental factors, make full use of all favorable factors, control or avoid unfavorable factors, and apply pesticides timely and reasonably in order to obtain satisfactory control efficacy.
4. Factors related to pesticide mixing and spraying operations
Spraying methods
The degree of atomization of the spray liquid is mainly achieved through the equipment; the size of the droplets is directly related to the performance of the spraying equipment, and the spraying method also directly affects the amount of pesticide deposited on the target.
During the spraying process, the concentration of the spray must be kept consistent from beginning to end.
Water for mixing the pesticide
①Water hardness refers to the amount of dissolved calcium and magnesium salts in the water. The effects of hard water on the performance of the spray liquid are mainly as follows:
A: It affects the stability of emulsions and suspensions; it can cause the particles of wettable powders to aggregate and form larger clumps;
B: Some pesticides can be converted into sparingly soluble substances in hard water and lose their efficacy.
②Electrolytes in water can also affect emulsification, such as the content of sodium chloride
③The pH of water can also affect pesticides. The greater the hardness of the water, the greater the alkalinity, and some pesticides can decompose in alkaline conditions. For example: organophosphates, carbamates, and pyrethroids are sensitive to alkaline water.
Therefore, natural water sources with low hardness should be selected for spraying, and non-ionic surfactants with strong hard-water resistance should be used as emulsifiers.
(Source: Compilation of Plant Protection Knowledge for Sales Staff)
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