This summer we’ve experienced high levels of lygus infestation, especially in the Santa Maria and Oxnard growing districts. It’s a good time to review some basic lygus biology.
1. There are 19 species of Lygus in California; however, only two species can be regularly found in strawberries, and these are generally referred to as “lygus bugs.” Lygus bug refers to the western tarnished plant bug, Lygus hesperus, and the pale legume bug, Lygus elisus (Fig. 1).
2. Lygus bugs are around all year in the strawberry crop grown year-round on the Central Coast. The influx of lygus bugs into strawberry fields often occurs when the surrounding vegetation dries down and goes dormant.
3. Lygus bugs feed primarily on the reproductive parts of the strawberry plants, including the achenes (seed-like structures) and the flesh around the achenes. They have a serrated straw-like beak (proboscis) which they stick into the fruit and inject enzymes and other liquids to start digestion before sucking up the liquid (Fig. 2.). The feeding behavior will collapse the cells around the feeding sites when they feed on the fleshy part of the fruit. Lygus bugs will cause achenes to become hollowed out after feeding, leaving behind holes, which can be seen under high magnification (Fig. 3). Since the achene ceases further development after lygus feeding, the expanding flesh around the damaged achene distorts (catfacing) as the rest of the berry forms normally (Fig. 4). Lygus bug damage looks similar to fruit that did not receive adequate pollination due to environmental, nutritional deficiencies, etc (Fig. 5). The achenes on fruit deformed due to poor pollination will be variable in size, while lygus-damaged fruit will have uniform seeds size.
4. The movement of the nymphs and adults on and between plants varies greatly throughout the day. Wind, high or low temps, and potentially even human activity around the strawberry plants will alter the lygus bugs’ activity, causing inactivity and hiding. Likewise, the flight activity of the adults in strawberries varies during the season. Previous studies have shown adults are most active right after sunset, and then most movement slows in the dark, except for probing behaviors (and maybe oviposition).
5. Lygus bugs feed on over 200 plants, including many weeds and crops. Key hosts near strawberries include celery, artichoke, lettuce, cole crops (cabbage, cauliflower, broccoli, etc.), asters, russian thistle, mustards, goosefoot, and alfalfa. Preferred hosts shift based on flowering and dormancy periods. Alfalfa trap crops are useful in minimizing lygus bug movement, and cutting should be staggered to preserve the attractiveness of the habitat if planted in or near strawberry fields. Lygus bugs will also cannibalize younger instars and feed on other species of small insects when needed. When kept together in our lab cages, adults must be separated from the young to prevent cannibalization, even when providing sufficient food resources.
6. Temperatures up to 87.9°F will speed the development of all stages of lygus bugs. High relative humidity at ~75% has been shown to speed up egg development at 77F and nymphs at both 77°F and 93°F. At <50°F, lygus bug eggs will not hatch, and newly hatched lygus bugs will not properly develop. The mean optimum and lethal maximum temperatures are 92°F and 105°F for eggs and 93°F and 99°F for nymphs. Females lay the most eggs at 80°F, but above this temperature, the number of eggs laid per female decreases. Generations will double the fastest at 86°F. The average adult lifespan of males and females is temperature dependent. In lab studies, at 89°F, males live for ~11 days, and females live for ~22 days. At 70°F, males lived for ~21 days, and females lived for ~55 days.
7. Development time of the nymphal stages 1-4 takes 2 to 7 days, and the 5th nymphal instar takes 4 to 10 days. Up to ten eggs can be laid daily, and up to 400 eggs over their lifetime. The average number of eggs laid by females varies depending on food and climactic conditions but is generally between 50-200 in a single female’s lifetime. The preferred egg-laying site on strawberries is between the achenes on the developing green berry. This may contribute to catfacing as the berry develops and swells in all places except where the eggs were laid. It has been suggested that this could have evolved to escape parasitism as the lygus bug’s eggs are hidden amongst the achenes. Eggs may be laid in other parts of the plants as well.
Have questions about lygus bugs? Contact Sarah Zukoff at szukoff@calpoly.edu or give me a call at 805-540-4876.
1Handley, David T., and James E. Pollard. “Microscopic examination of tarnished plant bug (Heteroptera: Miridae) feeding damage to strawberry.” Journal of Economic Entomology 86.2 (1993): 505-510.
Authors: Sarah Zukoff and Kiley Jensen