Summer Humidity and Temperature Swings: Three Major Crop Pests to Watch
Sharp temperature fluctuations and high humidity have created favourable conditions for several agricultural pests to reproduce rapidly this summer. As a result, the risk of crop losses has increased across legumes, cereals, oilseeds and vegetable crops.
Three pests require particular attention: the pea aphid, cutworms and the cotton bollworm. Effective protection depends on regular field monitoring, early detection and timely treatment before pest populations exceed economic thresholds.
1. Pea aphid
Pea aphids cause the greatest damage to legume crops during budding and flowering. Infested plants may become deformed, grow more slowly and produce lower yields. Aphids can also transmit harmful plant viruses.
Main host crops:
- peas;
- field beans;
- vetch;
- clover;
- alfalfa.
Economic threshold: approximately 20% of plants infested during budding and flowering, or 30–50 aphids collected in 10 sweep-net strokes.
A common mistake is starting control measures too late. Fields should therefore be inspected regularly, and registered systemic insecticides should be applied when the economic threshold is reached.
2. Cutworms
Black cutworm, turnip moth and related cutworm species have become increasingly damaging in recent years. Their larvae feed on roots and stems near the soil surface, often cutting off young seedlings and causing complete plant loss.
Crops commonly affected:
- winter wheat;
- winter rapeseed;
- sunflower;
- soybean;
- sugar beet;
- potato;
- tomato;
- carrot;
- maize.
Control is difficult because the larvae remain hidden in the soil during the day, often at a depth of around 2 centimetres. They usually emerge at night to feed, which means daytime foliar sprays may provide limited results.
The most effective treatment window is generally between 8:00 p.m. and midnight, when larvae are active near the soil surface.
Registered insecticides containing active ingredients such as indoxacarb, chlorantraniliprole, emamectin benzoate, chlorpyrifos, malathion or diazinon may be used where legally permitted. Insecticidal seed treatments can also help protect young seedlings.
3. Cotton bollworm
The cotton bollworm is a highly destructive, polyphagous pest. Its larvae feed on leaves, flowers, fruits, pods, maize ears and sunflower heads, leading to substantial yield losses.
Main host crops:
- maize;
- sunflower;
- chickpea;
- soybean;
- tomato.
Indicative treatment thresholds:
- sunflower — 2 larvae per flower head;
- soybean and maize — 1–2 larvae per plant when 10% of the crop is infested;
- pheromone traps — more than 20 moths per trap per day.
Timing is critical. Older larvae quickly enter maize ears, pods or flower heads, where they become difficult to reach with insecticides. Treatments should therefore target first- to third-instar larvae while they are still exposed on the plant surface.
Products based on indoxacarb, chlorantraniliprole and emamectin benzoate are commonly used. An adjuvant may improve spray coverage and penetration into hard-to-reach plant parts.
Conclusion
Successful pest management relies on three key actions:
- monitoring fields regularly;
- acting before pest populations become widespread;
- matching the treatment time and product to the pest’s biology.
Only locally registered crop-protection products should be used. Farmers must follow the label rate, pre-harvest interval, safety requirements and local regulations. The final control programme should be selected together with a qualified agronomist, taking into account crop type, growth stage, weather conditions and infestation level.
