Temperature and relative humidity influence Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) infestation and larval parasitoid activity in the tomato agroecosystems of the Niayes area, Senegal

Imagine a bustling city where the residents are tiny, hungry creatures, and the buildings are delicious, juicy tomatoes. In this city, there's a constant tug-of-war between the "invaders" (pests that want to eat the city) and the "guardians" (tiny wasps and flies that hunt the invaders). This isn't a fantasy novel; it's the real-life drama happening in the vegetable gardens of Senegal. Scientists call this field entomology (the study of insects) and agroecology (how farming and nature interact). The big question researchers ask is: "What makes the invaders win, and what helps the guardians win?" The answer often lies in the weather. Just like humans get grumpy in the heat or sluggish in the rain, insects have their own favorite temperatures and humidity levels. If the weather changes, the balance of power in the garden shifts. Understanding this helps farmers protect their crops without spraying too many chemicals, keeping the garden healthy and the food safe to eat.

Now, let's zoom in on a specific story from the Niayes area of Senegal, where a team of scientists decided to play detective with the tomato fruitworm (Helicoverpa armigera). This pest is a notorious troublemaker; its larvae are like tiny drills that bore right into tomato flowers and fruits, ruining the harvest. But nature has its own police force: parasitoids. These are special insects that lay their eggs inside the pest larvae. When the eggs hatch, the baby parasitoids eat the pest from the inside out, turning the pest into a biological snack. The scientists wanted to know: Does the weather act like a referee, deciding whether the fruitworms or the parasitoids get the upper hand?

To find out, the researchers set up a massive game of "follow the worm" across 20 different tomato plots in the Niayes area, splitting their time between the southern and central zones. From April to July 2025, they played the role of garden detectives. Every 10 days, they checked 25 tomato plants in each plot, counting how many fruitworms were there and how many plants were being attacked. They also set up mini-weather stations to record the temperature and relative humidity (how much water is in the air) like a constant diary of the sky's mood. When they found a fruitworm larva, they didn't just leave it; they brought it back to the lab, put it in a little box with fresh tomato leaves, and watched what happened. Did the worm grow up? Did it die? Or did a tiny wasp or fly burst out of it?

Here is what the data revealed, and it's a bit like a weather report for a war zone. The scientists found that the fruitworms and another pest called Tuta absoluta were everywhere, showing up in 100% of the plots. However, the fruitworms were a bit less common than Tuta absoluta, attacking about 7.4% of plants in the south and 9.1% in the center. But the real story was in the weather. The researchers discovered that relative humidity was the fruitworm's worst enemy. When the air was damp and humid, the fruitworm infestations dropped. It's as if the wet air made the worms feel too heavy or uncomfortable to thrive. On the other hand, temperature played a more complex role. While warmer weather didn't directly cause a massive spike in worm numbers on its own, it seemed to be the key that unlocked the power of the guardians.

The guardians, or parasitoids, were incredibly effective. Out of all the fruitworm larvae they collected, a staggering 75.2% were killed by parasitoids. That's like having a security team that stops three out of every four burglars! But not all guardians are the same. The team found two main "superheroes" in the fight: Meteorus laphygmarum and Cotesia vestalis. These two species were the stars of the show, but they had different favorite weather conditions.

The study showed a fascinating split in how these heroes reacted to the climate. Cotesia vestalis seemed to love the heat. The warmer it got, the more active this wasp became. It was like a sunbather who gets more energetic as the temperature rises. In contrast, Meteorus laphygmarum was the champion of the damp days. This species was more closely linked to higher humidity. The data suggested that when the air was moist, this wasp was more likely to do its job. In the southern zone, the damp-loving Meteorus ruled the show, making up 85.2% of the parasitoids found. In the central zone, the heat-loving Cotesia took the lead, accounting for 88.2% of the action.

The scientists also looked at the bigger picture using a special map called a Principal Component Analysis (PCA), which helps visualize how all these factors dance together. The map showed that temperature and humidity are like two opposing forces. High humidity pushed the fruitworms down but boosted the Meteorus wasps. High temperatures boosted the Cotesia wasps. The study suggests that these different weather preferences create a sort of "teamwork" between the wasps. When it's hot, one team steps up; when it's humid, the other takes over. This means that nature has a built-in backup system to keep the fruitworms in check, provided the weather doesn't get too extreme.

So, what does this mean for the future of tomatoes in Senegal? The paper suggests that farmers don't need to rely solely on chemical sprays, which can hurt the good bugs. Instead, by watching the weather, they can predict when the fruitworms might be weak (like during humid spells) and when their natural guardians will be strongest. If the weather is hot, the Cotesia wasps will likely be working hard, so farmers might want to hold off on spraying to let them do their job. If it's humid, the Meteorus wasps are on the case. The study concludes that understanding these climate-pest-parasitoid relationships is the secret sauce for a smarter, more natural way to grow tomatoes, keeping the city of the garden safe and the harvest plentiful.