5.2 Insect & Related Arthropod Biology
Key Takeaways
- Insects have three body regions, six legs, and one pair of antennae; arachnids such as mites and ticks have two body regions, eight legs, and no antennae.
- Complete metamorphosis passes through egg, larva, pupa, and adult, and the damaging stage is usually the larva; gradual metamorphosis passes through egg, nymph, and adult with both stages feeding alike.
- Chewing mouthparts remove tissue and respond to stomach poisons and contact sprays, while piercing-sucking mouthparts remove fluids and respond best to systemic products.
- Degree-day accumulation and plant phenology predict vulnerable stages better than the calendar in Ohio's variable spring.
- Correct staging matters because early instars are far easier to control than mature larvae or armored adults.
5.2 Insect & Related Arthropod Biology
Insects are the largest pest group an Ohio commercial applicator meets, and nearly every insecticide failure traces back to treating the wrong stage or misreading the mouthparts.
Telling Insects From Their Relatives
| Group | Body regions | Legs (adult) | Antennae | Examples |
|---|---|---|---|---|
| Insects (Insecta) | 3 - head, thorax, abdomen | 6 | 1 pair | Beetles, flies, ants, termites, aphids, caterpillars |
| Arachnids (Arachnida) | 2 - cephalothorax, abdomen | 8 | none | Spiders, mites, ticks, scorpions |
| Crustaceans | 2 | 10 or more | 2 pairs | Sowbugs, pillbugs |
| Myriapods | Head + many segments | Many | 1 pair | Centipedes, millipedes |
The distinction is practical: miticides are often distinct from insecticides because mites are not insects, and a product effective on aphids may have no activity on twospotted spider mite.
Metamorphosis: The Timing Key
Complete metamorphosis (holometabolous)
Egg -> larva -> pupa -> adult. The larva and adult look nothing alike, live in different places, and often eat different things.
- Beetles (grubs, wireworms), flies (maggots), moths and butterflies (caterpillars), bees, wasps, ants, fleas.
- The damaging stage is usually the larva. White grubs feeding on turf roots are beetle larvae; the adult Japanese beetle is a foliage feeder on a completely different plant.
- The pupa is not feeding and is usually not controllable. Timing an application into the pupal window wastes the product.
Gradual (incomplete) metamorphosis (hemimetabolous)
Egg -> nymph -> adult. The nymph resembles a smaller, wingless adult, occupies the same habitat, and does the same damage.
- True bugs, aphids, leafhoppers, scale crawlers, grasshoppers, cockroaches, termites.
- Both nymphs and adults feed, so populations damage continuously rather than in a pulse.
- The most vulnerable stage is usually the young nymph or crawler, before a waxy covering or armored test develops. Scale crawler timing is the classic Ohio ornamental example.
Mouthparts Drive Product Choice
| Mouthpart type | How it damages | What controls it |
|---|---|---|
| Chewing | Removes tissue: notched or skeletonized leaves, holes, tunnels, frass | Stomach poisons and contact residues on the treated surface; coverage matters |
| Piercing-sucking | Inserts a stylet and removes fluids: stippling, leaf curl, honeydew and sooty mold, chlorotic flecking | Systemic insecticides that move in plant fluids; surface residues alone often fail |
| Sponging | Laps liquids and dissolved solids (house flies) | Baits and residual surfaces where they rest |
| Siphoning | Coiled tube for nectar (adult moths, butterflies) | Adults rarely the damaging stage; target larvae |
| Rasping-sucking | Rasps tissue and laps exudate (thrips) | Translaminar or systemic products; protected feeding sites |
A key exam inference: honeydew and sooty mold mean a piercing-sucking insect, so a contact-only spray on the leaf surface is the wrong tool.
Timing With Degree Days and Phenology
Ohio springs vary by weeks between years, so calendar timing fails. Two better tools:
- Growing degree days (GDD). Accumulated heat units above a base temperature - commonly 50 degrees Fahrenheit - predict development. Many Ohio ornamental pests have published GDD windows for egg hatch and crawler emergence.
- Plant phenological indicators. Bloom stages of common landscape plants track the same heat accumulation. Using bloom of a reliable indicator plant as a trigger is a low-tech GDD substitute.
Monitoring confirms what the model predicts: sticky cards for flying adults, pheromone traps for moth flights, double-sided tape on twigs for scale crawlers, soap flushes for turf caterpillars, and soil sampling with a cup cutter for grubs.
Ohio Pests Worth Knowing by Name
| Pest | Type | Damaging stage and clue |
|---|---|---|
| Eastern subterranean termite | Gradual | Workers; mud shelter tubes on foundations, blistered wood, spring swarmers with equal-length wings |
| Carpenter ant | Complete | Workers excavate galleries; coarse sawdust-like frass with insect parts, smooth clean galleries |
| Japanese beetle / masked chafer | Complete | Larvae (white grubs) sever turf roots; irregular wilted patches that roll back like carpet |
| Bed bug | Gradual | All stages feed; fecal spotting on seams, shed skins |
| German cockroach | Gradual | All stages; egg case carried by female until hatch, so residual sprays alone often fail |
| Emerald ash borer | Complete | Larvae girdle phloem; D-shaped exit holes, serpentine galleries, canopy dieback |
| Twospotted spider mite | Arachnid | All stages; fine stippling, webbing, populations explode in hot dry weather |
| Bagworm | Complete | Larvae; spindle-shaped bags on evergreens, best controlled on small early larvae |
Why Early Instars Matter
An early-instar caterpillar or a newly hatched scale crawler has a thin cuticle, a small body mass relative to the dose it contacts, and no protective covering. A mature larva, an armored scale, or an adult beetle may need several times the exposure for the same mortality. Scouting exists to find the population before it reaches the tolerant stage - which is also why an economic threshold is set below the injury level rather than at it.
An Ohio landscape technician finds sticky honeydew and black sooty mold on the leaves of a linden tree. What does this indicate about the pest and the product choice?
Which stage of an insect with complete metamorphosis is generally the poorest target for an insecticide application?
How can an applicator distinguish an arachnid such as a spider mite from an insect?